ATPL

Radio Navigation

ATPL Radio Navigation 28 October 2003 ii

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CHAPTER 1

Basic Radio Theory

Wave Motion ......................................................................................................................................1-1 Introduction ........................................................................................................................................1-1 Electro-Magnetic Waves ....................................................................................................................1-2 Properties of Radio Waves ................................................................................................................1-2 Refraction, diffraction and reflection...................................................................................................1-3 Relationship Between Frequency, Wavelength and Velocity .............................................................1-3 Phase Difference ...............................................................................................................................1-5 Radio Spectrum .................................................................................................................................1-7 Wave Propagation .............................................................................................................................1-8 Surface Wave ....................................................................................................................................1-9 Type of Surface..................................................................................................................................1-9 Sky Wave.........................................................................................................................................1-10 Critical Angle....................................................................................................................................1-11 Dead Space .....................................................................................................................................1-11 The Ionosphere................................................................................................................................1-11 Frequency and Skip Distance ..........................................................................................................1-12 Ionization and Skip Distance............................................................................................................1-12 Space Wave.....................................................................................................................................1-12 Duct Propagation .............................................................................................................................1-14 Aerials ..............................................................................................................................................1-15 Aerial Characteristics .......................................................................................................................1-15 Aerial Length....................................................................................................................................1-16 Polar Diagrams ................................................................................................................................1-16 Omni-Directional Aerials ..................................................................................................................1-16 Simple Half-Wave Dipole .................................................................................................................1-16 Marconi Quarter Wave Aerial...........................................................................................................1-17 Aerial Feeders..................................................................................................................................1-18 Aerial Directivity ...............................................................................................................................1-18 Modulation .......................................................................................................................................1-20 Keying ..............................................................................................................................................1-20 Amplitude Modulation (AM)..............................................................................................................1-21 Frequency Modulation (FM) .............................................................................................................1-22 Pulse Modulation (PM).....................................................................................................................1-23 Classification of Emissions...............................................................................................................1-23 Basic Radar Theory .........................................................................................................................1-24 Introduction ......................................................................................................................................1-24 Radar Frequencies ..........................................................................................................................1-24 Principles .........................................................................................................................................1-25 Pulse Radars ...................................................................................................................................1-25 Radar Direction Finding ...................................................................................................................1-26 Lobe Comparison.............................................................................................................................1-26 Beam Direction Finding....................................................................................................................1-27 Radar Terminology...........................................................................................................................1-27 Choice of Frequency........................................................................................................................1-29

CHAPTER 2

VHF Direction Finding Introduction ........................................................................................................................................2-1 Principles of Operation.......................................................................................................................2-1 Frequency ..........................................................................................................................................2-1 Emission Characteristics....................................................................................................................2-1 Operation ...........................................................................................................................................2-2 Services .............................................................................................................................................2-2

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Siting ..................................................................................................................................................2-3 Receiving a Bearing...........................................................................................................................2-3 Asking For a Bearing..........................................................................................................................2-3 VDF Approach ...................................................................................................................................2-4 Decoding the Chart ............................................................................................................................2-6 Part 1 – Administration.......................................................................................................................2-6 Part 2 – The Plan View ......................................................................................................................2-7 Part 3 – The Elevation View...............................................................................................................2-8 Refusal of Service..............................................................................................................................2-8 Automatic VDF...................................................................................................................................2-8 Range and Errors...............................................................................................................................2-9

CHAPTER 3

Non Directional Beacons and Automatic Direction Finding Introduction ........................................................................................................................................3-1 Principles of Operation.......................................................................................................................3-1 Frequency ..........................................................................................................................................3-1 Emission Characteristics....................................................................................................................3-1 Loop Theory.......................................................................................................................................3-2 Sensing ..............................................................................................................................................3-3 NDB Operation...................................................................................................................................3-4 ADF Operation ...................................................................................................................................3-4 Bearing Determination .......................................................................................................................3-5 Types .................................................................................................................................................3-5 Control Panels and Indicators ............................................................................................................3-6 Control Panel .....................................................................................................................................3-6 TEST Switch ......................................................................................................................................3-7 Bearing Indicators ..............................................................................................................................3-7 Relative Bearing Indicator (RBI).........................................................................................................3-8 Radio Magnetic Indicator (RMI) .........................................................................................................3-9 Direct Wave Limitations ...................................................................................................................3-10 Sky Wave Limitations.......................................................................................................................3-11 Night Effect ......................................................................................................................................3-11 Errors of the ADF .............................................................................................................................3-12 Quadrantal Error ..............................................................................................................................3-12 Dip (Bank) Error ...............................................................................................................................3-13 Coastal Refraction ...........................................................................................................................3-13 Multipath Signals..............................................................................................................................3-13 Noise................................................................................................................................................3-14 Synchronous Transmission..............................................................................................................3-15 Promulgated Range .........................................................................................................................3-15 Absence of Failure Warning.............................................................................................................3-15 Accuracy ..........................................................................................................................................3-16

CHAPTER 4

NDB Navigation Introduction ........................................................................................................................................4-1 ADF Bearing ......................................................................................................................................4-1 Line of Position (LOP) using the RBI .................................................................................................4-1 Line of Position (LOP) using the RMI .................................................................................................4-2 Homing...............................................................................................................................................4-2 Intercepting a Course.........................................................................................................................4-3 Inbound to the Beacon.......................................................................................................................4-4 Outbound From The Beacon..............................................................................................................4-5 Tracking .............................................................................................................................................4-8 NDB Approach .................................................................................................................................4-10 Part 1 - Administration......................................................................................................................4-11

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Part 2 - The Plan View .....................................................................................................................4-12 Part 3 - The Elevation View..............................................................................................................4-13 Part 4 - Limits and Other Information ...............................................................................................4-13

CHAPTER 5

VHF Omnidirectional Radio Range (VOR) Introduction ........................................................................................................................................5-1 Principle of Operation.........................................................................................................................5-1 Frequency ..........................................................................................................................................5-1 Polarisation ........................................................................................................................................5-1 Emission Characteristics....................................................................................................................5-2 Conventional VOR .............................................................................................................................5-2 Reference Signal ...............................................................................................................................5-2 Variable Signal...................................................................................................................................5-2 Aircraft Receiver ................................................................................................................................5-2 Bearing Measurement........................................................................................................................5-2 Aircraft Equipment..............................................................................................................................5-4 Aerial..................................................................................................................................................5-4 Receiver.............................................................................................................................................5-4 Frequency Selector ............................................................................................................................5-4 Indicators ...........................................................................................................................................5-4 Monitoring ..........................................................................................................................................5-5 Terrain................................................................................................................................................5-5 Designated Operational Coverage (DOC)..........................................................................................5-5 Cone of Confusion .............................................................................................................................5-5 Accuracy ............................................................................................................................................5-6 Airway Navigation ..............................................................................................................................5-7 Test VOR ...........................................................................................................................................5-7 Doppler VOR......................................................................................................................................5-8

CHAPTER 6

VOR Navigation Introduction ........................................................................................................................................6-1 Radio Magnetic Indicator ...................................................................................................................6-1 Omni-Bearing Selector.......................................................................................................................6-2 Using The OBS ..................................................................................................................................6-3 Horizontal Situation Indicator (HSI) ....................................................................................................6-6 VOR Navigation .................................................................................................................................6-8 Establishing position ..........................................................................................................................6-8 Tracking a radial inbound from a present position .............................................................................6-8 Intercepting a radial ...........................................................................................................................6-9 VOR Approaches ...............................................................................................................................6-9 Part 1 – Administration.....................................................................................................................6-11 Part 2 – Plan View ...........................................................................................................................6-11 Part 3 – Elevation View....................................................................................................................6-12 Part 4 – Notes ..................................................................................................................................6-12

CHAPTER 7

Distance Measuring Equipment (DME) Introduction ........................................................................................................................................7-1 Principle of Operation.........................................................................................................................7-1 Frequency ..........................................................................................................................................7-1 Emission Characteristics....................................................................................................................7-1 Aircraft Equipment..............................................................................................................................7-2

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Transponder.......................................................................................................................................7-2 Frequency Allocation..........................................................................................................................7-2 Jittered PRF .......................................................................................................................................7-3 Reflected Transmissions....................................................................................................................7-4 Memory ..............................................................................................................................................7-4 Beacon Saturation .............................................................................................................................7-5 Co-location of Beacons ......................................................................................................................7-5 Slant Range .......................................................................................................................................7-6 DME Navigation .................................................................................................................................7-6 DME Procedures................................................................................................................................7-7 Slant Range .......................................................................................................................................7-7 Flight Overhead the DME...................................................................................................................7-8 Failure Indications..............................................................................................................................7-8 Accuracy ............................................................................................................................................7-9

CHAPTER 8

Instrument Landing System (ILS) Introduction ........................................................................................................................................8-1 Principle of Operation.........................................................................................................................8-1 Frequency ..........................................................................................................................................8-2 Localiser.............................................................................................................................................8-2 Localiser Coverage ............................................................................................................................8-4 Glidepath............................................................................................................................................8-5 Glidepath Coverage ...........................................................................................................................8-6 Airborne Equipment ...........................................................................................................................8-7 Frequency Pairing..............................................................................................................................8-7 Localiser and Glidepath Receivers ....................................................................................................8-7 ILS Indicator.......................................................................................................................................8-7 Horizontal Situation Indicator (HSI) ....................................................................................................8-9 ILS accuracy ....................................................................................................................................8-10 False Beams ....................................................................................................................................8-11 Localiser Back Beam .......................................................................................................................8-12 ILS Performance Categories............................................................................................................8-13 ILS Operational Performance Categories ........................................................................................8-14 Protection Range and Monitoring.....................................................................................................8-15 Use of ILS ........................................................................................................................................8-16 ILS Identification ..............................................................................................................................8-16 Flying the Localiser ..........................................................................................................................8-16 Flying the Glidepath .........................................................................................................................8-16 ILS Without Glidepath ......................................................................................................................8-17 Distance Measuring Equipment .......................................................................................................8-20 Rate of Descent (ROD) ....................................................................................................................8-20 Height Passing on the Approach......................................................................................................8-20

CHAPTER 9

Marker Beacons Introduction ........................................................................................................................................9-1 Principle of Operation.........................................................................................................................9-1 Frequency ..........................................................................................................................................9-1 Emission Characteristics....................................................................................................................9-1 Airborne Equipment ...........................................................................................................................9-2 Airway Marker ....................................................................................................................................9-3 Ground Installation.............................................................................................................................9-3

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CHAPTER 10

Microwave Landing System Introduction ......................................................................................................................................10-1 Principle of Operation.......................................................................................................................10-1 Frequency ........................................................................................................................................10-2 Polarisation ......................................................................................................................................10-2 Ground Installation...........................................................................................................................10-2 Azimuth Coverage ...........................................................................................................................10-2 Elevation Coverage..........................................................................................................................10-3 DME/P..............................................................................................................................................10-4 Back Azimuth ...................................................................................................................................10-5 Signal Transmission Format ............................................................................................................10-5 Time Reference Scanning Beam .....................................................................................................10-6 Angular Measurement in Azimuth and Elevation .............................................................................10-6 Airborne Equipment .........................................................................................................................10-7 Accuracy ..........................................................................................................................................10-8

CHAPTER 11

Radar Principles and the Cathode Ray Tube Pulse Techniques and Associated Terms ........................................................................................11-1 The Components of a Radar Unit ....................................................................................................11-1 The Timebase ..................................................................................................................................11-2 The Display ......................................................................................................................................11-2 The Transmitter................................................................................................................................11-2 Choice of Frequency........................................................................................................................11-3 The Aerial.........................................................................................................................................11-4 Beamwidth .......................................................................................................................................11-4 The Receiver....................................................................................................................................11-6 Timer – Cathode Ray Tube..............................................................................................................11-6 Cathode ...........................................................................................................................................11-7 Grid ..................................................................................................................................................11-7 Focussing Assembly ........................................................................................................................11-7 Screen..............................................................................................................................................11-8 Time Base........................................................................................................................................11-8 Saw Tooth Voltage...........................................................................................................................11-9 Radar Performance..........................................................................................................................11-9 Secondary Radar ...........................................................................................................................11-10

CHAPTER 12

Ground Radar Introduction ......................................................................................................................................12-1 Long Range Surveillance Radar ......................................................................................................12-1 Terminal Surveillance Radar............................................................................................................12-2 Aerodrome Surveillance (Approach) Radar .....................................................................................12-2 Range, Accuracy and Limitations of Surveillance Radar..................................................................12-3 Surveillance Radar Procedures .......................................................................................................12-3 En-Route..........................................................................................................................................12-3 Approach..........................................................................................................................................12-4 Precision Approach Radar (PAR) ....................................................................................................12-4 PAR Procedure ................................................................................................................................12-6 Airfield Surface Movement Indicator (ASMI) ....................................................................................12-7 Weather Radar.................................................................................................................................12-8

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CHAPTER 13

Secondary Surveillance Radar (SSR) Introduction ......................................................................................................................................13-1 Principles of Operation.....................................................................................................................13-1 Pulse Spacing ..................................................................................................................................13-2 Side Lobe Suppression ....................................................................................................................13-3 Operation .........................................................................................................................................13-5 SPI Code..........................................................................................................................................13-6 Use of Transponder .........................................................................................................................13-6 Presentation and Interpretation........................................................................................................13-7 Limitations........................................................................................................................................13-7 Fruiting .............................................................................................................................................13-8 Garbling ...........................................................................................................................................13-8 Mode S.............................................................................................................................................13-8 Operation of Mode S........................................................................................................................13-8 ATC Services ...................................................................................................................................13-9

CHAPTER 14

Airborne Weather Radar (AWR) Introduction ......................................................................................................................................14-1 Principle of Operation.......................................................................................................................14-1 Frequency ........................................................................................................................................14-1 Frequency Range ............................................................................................................................14-2 AWR Aerial ......................................................................................................................................14-2 Control Panel ...................................................................................................................................14-2 Power Switch and Timebase Range Switch.....................................................................................14-3 Function Switch................................................................................................................................14-4 Weather Function (WEA) .................................................................................................................14-4 Contour Function (CON) ..................................................................................................................14-4 Mapping Function (MAP) .................................................................................................................14-5 Manual Function (MAN) ...................................................................................................................14-6 Contrast ...........................................................................................................................................14-6 Manual Gain.....................................................................................................................................14-6 Tilt Control........................................................................................................................................14-7 Colour Displays................................................................................................................................14-7 Cloud Height Determination .............................................................................................................14-8 Shadow............................................................................................................................................14-9 Test ................................................................................................................................................14-10 Hold................................................................................................................................................14-10 Target Alert ....................................................................................................................................14-10 Use of the Radar on the Ground ....................................................................................................14-10

CHAPTER 15

Doppler Introduction ......................................................................................................................................15-1 Frequency ........................................................................................................................................15-1 Doppler Effect ..................................................................................................................................15-1 Doppler Measurement of Groundspeed...........................................................................................15-3 Two Beam Janus Array....................................................................................................................15-5 Further Janus Arrays........................................................................................................................15-6 Doppler Aerial ..................................................................................................................................15-6 Signal characteristics .......................................................................................................................15-7 Output and Presentation ..................................................................................................................15-7 Accuracy and limitations ..................................................................................................................15-9 Sea Bias...........................................................................................................................................15-9

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Memory Mode ..................................................................................................................................15-9 Pitch and Roll Error ........................................................................................................................15-10 Height Hole Error ...........................................................................................................................15-10 Sea Movement Error ......................................................................................................................15-10 Computational Errors .....................................................................................................................15-10 Heading Error.................................................................................................................................15-10 Summary of Errors .........................................................................................................................15-11 Advantages ....................................................................................................................................15-11 Disadvantages ...............................................................................................................................15-11

CHAPTER 16

Hyperbolic Navigation Introduction ......................................................................................................................................16-1 Hyperbolic Family ............................................................................................................................16-1 Lines of Position (LOP) ....................................................................................................................16-4 Errors of Hyperbolic Navigation .......................................................................................................16-4 Propagation Errors...........................................................................................................................16-4 Height Error......................................................................................................................................16-5 Simple Hyperbolic Calculation .........................................................................................................16-5

CHAPTER 17

LORAN C Introduction ......................................................................................................................................17-1 Principle of Operation.......................................................................................................................17-1 Frequency ........................................................................................................................................17-1 Typical LORAN C Chain ..................................................................................................................17-2 LORAN C Transmission...................................................................................................................17-2 Operation .........................................................................................................................................17-2 Coverage, Limitations and Accuracy................................................................................................17-5 LORAN C Coverage.........................................................................................................................17-5 Sky Waves .......................................................................................................................................17-5 Static Disturbances ..........................................................................................................................17-5 Radio Propagation Speed ................................................................................................................17-6 Geometry of Crossing Angles ..........................................................................................................17-6 Use of LORAN C..............................................................................................................................17-6 LORAN C Navigation .......................................................................................................................17-7 Transmitter Fault Indication..............................................................................................................17-8 Range and Accuracy........................................................................................................................17-8 Accuracy Limits................................................................................................................................17-8

CHAPTER 18

Global Navigation Satellite Systems (GNSS) Introduction ......................................................................................................................................18-1 System Capability ............................................................................................................................18-1 Frequency ........................................................................................................................................18-2 Basic Principle of Operation.............................................................................................................18-2 The GPS System .............................................................................................................................18-4 The Space Segment ........................................................................................................................18-4 GPS Timing......................................................................................................................................18-6 Frequency and Coding.....................................................................................................................18-6 Navigation Message.........................................................................................................................18-7

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The Control Segment .......................................................................................................................18-8 The User Segment ...........................................................................................................................18-9 GPS Operating Principles ................................................................................................................18-9 Pseudo Range ...............................................................................................................................18-11 Velocity Measurement....................................................................................................................18-13 GPS Receiver ................................................................................................................................18-14 System Limitations.........................................................................................................................18-15 Number of Users............................................................................................................................18-16 Coverage .......................................................................................................................................18-16 Reliability/Integrity..........................................................................................................................18-16 Receiver Autonomous Integrity Monitoring (RAIM) ........................................................................18-16 GPS Integrity Broadcast (GIB) .......................................................................................................18-17 Coverage Problems .......................................................................................................................18-17 Accuracy and Error Sources ..........................................................................................................18-17 Accuracy for Civil Use ....................................................................................................................18-17 User Equivalent Range Errors .......................................................................................................18-18 Dilution of Precision (DOP) ............................................................................................................18-18 Error Predictions ............................................................................................................................18-19 Differential GPS (DGPS)................................................................................................................18-19 DGPS Principle of Operation .........................................................................................................18-20 Summary of GPS Error Sources ....................................................................................................18-20 Pseudolite/DGPS ...........................................................................................................................18-21 Satellite Based Augmentation Systems (SBAS) ............................................................................18-22 RAIM in the Wide Area Augmentation System...............................................................................18-24 GLONASS......................................................................................................................................18-25 Basic concepts of the GLONASS system ......................................................................................18-25 Integrated Navigation Systems ......................................................................................................18-26 GNSS Applications.........................................................................................................................18-27

CHAPTER 19

Area Navigation Systems Introduction ......................................................................................................................................19-1 Area Navigation Concepts ...............................................................................................................19-3 Accuracy of RNAV Equipment .........................................................................................................19-3 Basic RNAV .....................................................................................................................................19-4 Use of Basic RNAV..........................................................................................................................19-5 RNAV Limitations.............................................................................................................................19-7

CHAPTER 20

Introduction to the Flight Management System (FMS) Introduction ......................................................................................................................................20-1 The Role of FMS..............................................................................................................................20-1 Use of FMS ......................................................................................................................................20-4 Output Information ...........................................................................................................................20-6

CHAPTER 21

An Overview of CNS/ATM Introduction ......................................................................................................................................21-1 Communications ..............................................................................................................................21-1 Air Traffic Management and FANS 1 ...............................................................................................21-1 Controller/Pilot Data Link Communication (CPDLC) ........................................................................21-2 Aeronautical Telecommunications Network (ATN)...........................................................................21-3 Navigation ........................................................................................................................................21-3 Eurocontrol BRNAV and PRNAV .....................................................................................................21-3

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Required Navigation Performance (RNP) ........................................................................................21-3 Terminal Area Initiatives...................................................................................................................21-4 Surveillance .....................................................................................................................................21-4 Automatic Dependent Surveillance (ADS) .......................................................................................21-4 Automatic Dependent Surveillance - Broadcast (ADS - B)...............................................................21-4 Mode S Data Link ............................................................................................................................21-4

CHAPTER 22

Electronic Display Systems Glossary of Terms............................................................................................................................22-1 Introduction ......................................................................................................................................22-1 General ............................................................................................................................................22-2 General Certification Considerations................................................................................................22-2 Display Function Criticality ...............................................................................................................22-2 Loss of Display.................................................................................................................................22-3 Navigation Information .....................................................................................................................22-3 Propulsion System Parameter Displays ...........................................................................................22-4 Crew Alerting Display.......................................................................................................................22-4 Flight Crew Procedures....................................................................................................................22-4 Information Display ..........................................................................................................................22-4 Information Display Colours .............................................................................................................22-4

CHAPTER 23

Boeing 737 - Electronic Flight Instrument System (EFIS) Introduction ......................................................................................................................................23-1 System Architecture.........................................................................................................................23-1 EFIS Symbol Generator ...................................................................................................................23-2 EFIS Control Panel ..........................................................................................................................23-3 EADI Controls ..................................................................................................................................23-4 EHSI Controls ..................................................................................................................................23-5 Electronic Attitude Direction Indicator ..............................................................................................23-6 EADI General ...................................................................................................................................23-7 Attitude Display ................................................................................................................................23-7 Mode Annunciations.........................................................................................................................23-7 Flight Director (F/D) Commands ......................................................................................................23-7 Glideslope (G/S) and Localiser (LOC) Deviation Displays ...............................................................23-7 ILS Deviation Warning .....................................................................................................................23-8 Rising Runway Symbol ....................................................................................................................23-8 Attitude Comparator .........................................................................................................................23-9 Digital Radio Altitude and Decision Height.......................................................................................23-9 Mach Display .................................................................................................................................23-10 Groundspeed Display.....................................................................................................................23-10 Pitch Limit Symbol .........................................................................................................................23-10 Speed Tape Scale .........................................................................................................................23-10 Digital Airspeed Readout ...............................................................................................................23-10 Airspeed Trend Arrow ....................................................................................................................23-11 Command Speed ...........................................................................................................................23-11 Max Operating Speed (VMO/MMO or Gear/Flap Placards)...............................................................23-11 High Speed Buffet Margin ..............................................................................................................23-11 Next Flap Placard Speed ...............................................................................................................23-11 Flaps Up Manoeuvring Speed........................................................................................................23-12 Decision Speed..............................................................................................................................23-12 VR (Rotation) Speed......................................................................................................................23-12 VREF (Reference Speed) ..............................................................................................................23-12 Minimum Flap Retraction Speed....................................................................................................23-13 Minimum Manoeuvring Speed .......................................................................................................23-13

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Stick Shaker Speed .......................................................................................................................23-13 EADI Symbols................................................................................................................................23-14 Radio Altitude Dial .........................................................................................................................23-16 Speed Tapes..................................................................................................................................23-17 EFIS – EADI Fault Displays ...........................................................................................................23-21 Operation .......................................................................................................................................23-21 EADI Failure Flags and Annunciations...........................................................................................23-21 EFIS Typical EHSI Centre Map, Map and Plan Displays ...............................................................23-22 General ..........................................................................................................................................23-22 EHSI Display Orientation ...............................................................................................................23-23 PLAN Mode....................................................................................................................................23-23 Features of PLAN Mode.................................................................................................................23-24 MAP and CTR MAP Modes ...........................................................................................................23-24 Features of MAP Mode ..................................................................................................................23-25 Features of CENTER MAP Mode ..................................................................................................23-26 NAV Mode Displays .......................................................................................................................23-26 Features of EXPANDED NAVIGATION Mode ...............................................................................23-27 Features of FULL NAVIGATION Mode ..........................................................................................23-28 VOR and ILS Displays ...................................................................................................................23-28 Expanded VOR Mode ....................................................................................................................23-29 Full Rose VOR Mode .....................................................................................................................23-29 Expanded ILS Mode.......................................................................................................................23-30 Full Rose ILS Mode........................................................................................................................23-31 EHSI Symbology............................................................................................................................23-31 EHSI System Failure Flags and Annunciation ...............................................................................23-40 Range Disagreement Annunciations..............................................................................................23-41 Weather Annunciations ..................................................................................................................23-41 Instrument Transfer Switching .......................................................................................................23-44 Light Sensing and Brightness Controls ..........................................................................................23-44

CHAPTER 24

Boeing 737 - Flight Management Computer (FMC) General ............................................................................................................................................24-1 Operation Overview .........................................................................................................................24-3 CDU Function ..................................................................................................................................24-5 CDU Page Display General .............................................................................................................24-5 Function and Mode Keys .................................................................................................................24-6 Line Select Keys ..............................................................................................................................24-6 Keyboard..........................................................................................................................................24-7 CDU Page Display ...........................................................................................................................24-8 Page Status .....................................................................................................................................24-9 Function and Mode Keys .................................................................................................................24-9 Lights .............................................................................................................................................24-12 System Components......................................................................................................................24-13 Data Bases ....................................................................................................................................24-13 Operation .......................................................................................................................................24-14 Lateral Navigation ..........................................................................................................................24-15 Vertical Navigation .........................................................................................................................24-16 Required Time of Arrival (RTA) Navigation ....................................................................................24-16 Radio Tuning..................................................................................................................................24-16 Electrical Power .............................................................................................................................24-18 Terminology ...................................................................................................................................24-18 Executing .......................................................................................................................................24-18 Inactive...........................................................................................................................................24-18 Active .............................................................................................................................................24-19 Page Status ...................................................................................................................................24-19 Modification....................................................................................................................................24-19 Initialization ....................................................................................................................................24-19 Line Select .....................................................................................................................................24-19

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Enter ..............................................................................................................................................24-19 Access ...........................................................................................................................................24-20 Propagate ......................................................................................................................................24-20 Page Concepts ..............................................................................................................................24-20 General ..........................................................................................................................................24-20 Page Sequence Logic ....................................................................................................................24-21 CDU Messages..............................................................................................................................24-23 Waypoint ........................................................................................................................................24-23 Waypoint Identifiers .......................................................................................................................24-24 Created Waypoints ........................................................................................................................24-24 Conditional Waypoints ...................................................................................................................24-25

CHAPTER 25

Boeing 737 - Navigation Equipment and Flight Management Inertial Reference System................................................................................................................25-1 IRS Mode Selector ...........................................................................................................................25-1 Align Light (White)............................................................................................................................25-2 Fault Light (Amber) ..........................................................................................................................25-2 On DC Light (Amber) .......................................................................................................................25-2 DC Fail Light (Amber) ......................................................................................................................25-3 Inertial Reference System................................................................................................................25-3 General ............................................................................................................................................25-3 Alignment .........................................................................................................................................25-3 Alignment on the Ground .................................................................................................................25-3 Fast Realignment on the Ground .....................................................................................................25-4 Loss of Alignment ............................................................................................................................25-4 IRS Display Unit ...............................................................................................................................25-5 Display Selector ...............................................................................................................................25-5 Brightness Control ...........................................................................................................................25-6 Data Displays...................................................................................................................................25-6 Enter Key .........................................................................................................................................25-6 Instrument Transfer Switch/Instrument Transfer Switch Light..........................................................25-7 DME System ....................................................................................................................................25-7 Radio Distance Magnetic Indicator ..................................................................................................25-8 VOR/ILS Navigation .........................................................................................................................25-9 Marker Beacon Switch ...................................................................................................................25-11 ADF Navigation..............................................................................................................................25-11 SSR Transponder Selections.........................................................................................................25-12 Weather Radar...............................................................................................................................25-14 Radio Altimeters.............................................................................................................................25-16

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ATPL Radio Navigation ©Atlantic Flight Training 1-1

Chapter 1.

Basic Radio Theory

Wave Motion Introduction A “wave” is a progressive disturbance in a medium, formed by alternating pressures and tensions, without any permanent displacement of the medium itself in the direction in which these stresses are propagated. This condition is readily observed on the surface of a pond. If a stone is thrown into the water then a series of waves is produced which radiate out until the bank is reached. If a plastic duck is placed in the pond, it will rise and fall as the wave passes underneath. There is no movement in the direction of wave travel. While the wave moves towards the bank, the water does not. The wave can be said to possess the following characteristics:

The form of the wave moves outwards although the water itself does not The wave possesses energy obtained from the stone. With the passage of the wave,

energy is lost due to friction and the further away from the source the smaller the wave

The wave travels at a constant speed The wave is sinusoidal, it travels as a sine wave

The radio wave is an alternating waveform and as such the following terms are used:

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Cycle A complete sequence of positive and negative values (AB) Period The duration of one cycle (T). In the figure above T = 1/100 seconds Velocity The speed with which a wave travels through a given medium. For a radio wave this is 300 000 000 metres per second better expressed as 300 X 106 m/s Frequency The number of complete waves passing a fixed point in one second,

denoted by the symbol “f”. Usually expressed as Hertz (Hz). It is obvious that f = 1/T

Wavelength The distance between similar points on successive waves or the

distance occupied by one complete cycle when travelling in free space (AB), denoted by the symbol λ (Lambda)

Amplitude The maximum height of the wave. This can be positive or negative.

The positive amplitude is represented by “b” Electro-Magnetic Waves The atmosphere carries light, heat and radio waves. These waves differ only in their frequency and wavelength and the effects they have on different materials. Termed “Electro-magnetic” because of their electrical and magnetic nature. All these waves travel at the same velocity, denoted by the letter c. For the Radio Navigation syllabus this velocity is:

300 000 000 metres per second for simplicity of calculation this is usually written as

300 X 106 metres per second Properties of Radio Waves A radio wave that leaves a transmitter has the following properties:

They consist of oscillating electric and magnetic fields that are at right angles to each other and at right angles to the direction of propagation

They require no supporting medium They can be reflected, refracted and diffracted as discussed below. They are subject to interference and Doppler effect They can pass through an opaque object such as a building although they do

suffer attenuation in doing so

Attenuation of an RF signal is the reduction in signal strength due to absorption, scattering or dispersion and diffraction.

ATPL Radio Navigation ©Atlantic Flight Training 1-3

Refraction, diffraction and reflection All radio waves can be subjected to refraction, diffraction and reflection, and even in combinations of these phenomena. The process of refraction occurs when the speed of the radio wave is altered offset from the centreline of the ray. This causes a difference in the propagation speed either side of the radio wave centreline which will bend the wave. The most common cause of refraction is when the wave travels obliquely through mediums which have differing effects on the speed of propagation. Diffraction occurs principally when a radio wave travels close to the Earths surface. Here the Earth has a very small voltage induced in it from the travelling wave, and during such induction the wave tends to stick to the Earths surface, giving up power and slowing down on the side of the wave on which the induction is occurring. As the inducing side of a vertical wave is the lower side the direction of propagation will curve around in the same direction as the Earths’ surface. Some surfaces have electrical properties which act like mirrors to radio energy, accepting the incoming signal and then re-radiating it, much like an aircraft reflects energy back to a radar system. Unlike energy reflected straight back to a radar head, where the reflection is at a very oblique angle of just a few degrees then very little power is lost, and goes on to recombine with the original unreflected signal reducing the accuracy of any derived bearing. Relationship Between Frequency, Wavelength and Velocity The frequency of a sinusoidal wave is the number of cycles occurring in one second. Conversion of frequency to wavelength and vice versa is needed for the JAR exam. Frequency is given the symbol f and the unit is the Hertz, wavelength is λ the unit used is metres.

c=f λ Because of the large figures used in frequency the following prefixes have to be used:

Prefix Magnitude

Kilo Mega Giga

103 106 109

Example 1000 Hertz = 1 Kilohertz = 1Khz

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1 000 000 Hz = 1 MHz 1 000 000 000 Hz = 1 GHz

Because of the large figures used in frequency the wavelength units can be small. The following prefixes are used:

Prefix Magnitude Milli

Micro or µ Nano

10-3 10-6 10-9

Example Convert 2 MHz to a wavelength

λ = c = 300x106 = 150 metres f 2 x 106

Example Convert 60 metres to a frequency

f = c = 300x106 = 5 MHz λ 60

Remember:

The higher the frequency the shorter the wavelength and vice versa Examples Convert the following wavelengths into the corresponding frequencies (The answers are given below):

1. 1500 m 2. 20 cm 3. 3500 m

Convert the following frequencies into the corresponding wavelength:

1. 300 KHz 2. 75 MHz 3. 600 MHz 4. 8800 MHz 5. How many wavelengths, to the nearest whole number, of frequency 200 MHz are

equivalent to 35 feet?

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Answers Wavelength to Frequency

1. 200 KHz 2. 1500 MHz 3. 85.7 KHz

Frequency to Wavelength

1. 1000 m 2. 4 m 3. 50 cm 4. 3.41 cm 5. Number of wavelengths in 35 feet for a frequency of 200 MHz

λ = c = 300 x 106 = 1.50 metres f 200 x 106

1 m is equivalent to 3.28 feet

1.5m = 4.92feet

The number of wavelengths in 35 feet = 35 4.92

Which is approximately 7

Phase Difference In the diagram below the two waves are said to be in phase. The waves pass the same point of their cycle at the same time.

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In the diagram below the waves are said to 90° out of phase:

Wave B leads wave A by 90°, or Wave A lags wave B by 90°

A B

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Where two waves have a phase difference of 180°, then they are said to be in anti-phase.

Radio Spectrum The electromagnetic spectrum is shown in the diagram below. The different effects brought about by electro-magnetic waves are determined by their frequency. The lower limit is determined by the size and efficiency of the aerials required and the upper limit by the attenuation and absorption of the radio waves by the atmosphere.

The part of the frequency spectrum which is of interest to the pilot is further sub-divided below.

Wavelength

100 km 1 mm 3 KHz 300 GHz

Radio Waves

Infra Red L

i g h t

Ultra Violet X-Rays Gamma

Rays Cosmic

Rays

VLF LF MF HF VHF UHF SHF EHF

3 30 300 3 30 300 3 30 300

KHz MHz GHz

Radio Spectrum

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Wave Propagation There are three principle paths which radio waves may follow over the earth between the transmitter and the receiver:

Surface Wave A wave which follows the contours of the earth’s surface Sky Wave A wave that is refracted by the Ionosphere and returned to earth Space Wave A wave which is line of sight

A combination of the surface and space waves is called a ground wave. The radio energy reaching a receiver may be made up of components due to any one or more of these mechanisms but, depending on the part of the radio spectrum concerned one of the three will predominate. In general:

Low frequencies are propagated by surface wave Middle range frequencies by sky wave, and

Surface Wave

Sky Wave

Space Wave

Ionosphere

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Upper range frequencies by space wave. Surface Wave The surface wave follows the curvature of the Earth, a process known as diffraction. The process is helped by the Earth’s attenuation of the radio energy. The wave is slowed as it touches the Earth’s surface. Therefore, the wave front in the direction of motion will lag at the surface.

The wave front is tilted and diffractive bending occurs. The stability of this type of propagation makes the low frequency surface wave suited to systems requiring consistency of signal over long distances. The propagation does require large aerials and the cost of transmission can be considerable. Type of Surface High conductivity favours the passage of a radio signal. So passage over the sea is better than over rock or desert.

Transmitter Power Surface absorption and free space loss reduce the signal strength of a radio wave. If there is no restriction in the available transmitter power then global ranges can be achieved by VLF radio waves.

Wavefront falls towards the Earth as it progresses

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Noise and Interference Noise affects the lower frequencies so affecting the signal/noise ratio. This can limit the usable range.

For maximum ground wave range:

Use low frequency — for maximum diffraction and least attenuation Use vertical polarization (see polarization)

Sky Wave The sky wave ascends into the upper atmosphere and encounters a region containing electrically charged particles (the Ionosphere) where it is refracted sufficiently to return to Earth.

β Is the Critical Angle. Note that it is measured from the vertical down.

When the wave enters the Ionosphere it changes direction due to a change in velocity. If the wave penetrates halfway through the layer before being bent parallel to the layer it will bend back in the opposite direction to emerge from the top of the layer as an escape ray. This is likely if:

The Ionization is insufficiently intense. The frequency is too high The angle of entry is too acute

β

Dead Space

Ground Wave

Skip Distance

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Critical Angle For a particular frequency and degree of ionization, it is possible to define a critical angle below which total refraction will not take place. Defining the critical angle also establishes the minimum range - the skip distance. Any ray travelling away from the aerial at greater than the critical angle will be freely refracted down to about 5° above the horizon. Dead Space Because of the high frequencies used in sky wave transmission the groundwave travel is not as far as the returning sky wave. The distance between the limit of the groundwave and the first returning sky wave is called the dead space. The Ionosphere The Ionosphere consists of a series of conducting layers between heights of 50 to 400 kilometres. It exists because of the transmission of ultra-violet radiation from the sun. Because of this dependence upon radiation from the sun the heights and densities of the layers vary according to the:

Time of day Season of the year

There is also a connection between the 11 year sunspot cycle. Short term effects occur in a random fashion and these result in the ionized layers being in a state of constant turbulence. Three main layers have been identified and are designated D, E and F. The F layer splits into two separate layers during the day, the time of highest ionization. The D layer is a region of low ionization that only persists during the day. The E layer is more marked and remains weakly ionized by night with little change in height. The F layer is the most strongly ionized and has the greatest diurnal change in height.

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Frequency and Skip Distance At a fixed level of ionization an increase in frequency will cause the ray, previously the critical ray, to become an escape ray. This will cause an increase in skip distance. Ionization and Skip Distance At a fixed frequency if ionization decreases the effect will be the same as above. The critical ray becomes an escape ray. This will cause an increase in skip distance. Space Wave Transmissions at VHF and above cannot propagate by either surface or sky wave. Attenuation is so severe that the surface wave is virtually non-existent. These frequencies are too high to be refracted by the ionized layers aloft. Transmission is therefore by straight line -

400 km 200 km 100 km

F2 F1 E D

Day Night

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the direct wave. In addition to the direct wave there can also be a reflected wave. The two components make up the space wave.

Because of the different emission paths the direct and reflected wave will sometimes be in phase and sometimes out of phase. This will produce lobes and nulls particularly when the receiver is close to the station.

The range of a space wave appears to be line of sight. In practice it is termed quasi-optical:

The lower atmosphere causes some refraction of the wave which bends it beyond the optical horizon, and

A further small increase is gained from diffraction when the wave becomes tangential to the earth’s surface

This range can be approximated by the following formula:

R = 1.25(√HT + √HR)

Where: R Range in nautical miles HT Height of the transmitter in feet HR Height of the receiver in feet

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Example An aircraft flying at 10 000 ft receives a transmission from a station at 400 ft. What is the maximum distance communications can be made between the two stations?

R = 1.25(√10 000 + √400) R = 1.25(100 + 20) R = 150 nm Duct Propagation Under certain abnormal climatic conditions transmissions on a frequency greater than 50 MHz can be received at ranges in excess of the quasi-optical expected.

VERY DRY AND RELATIVELY WARM

COOL AND MOIST

-40°C

-3°C+7°C

LOW CLOUD TRAPPEDBELOW INVERSION

INVERSION BASE

SUBSIDINGAIR

AIR TEMPERATURE DEW POINT TEMPERATURE

25 JANUARY 1989 - 0001 GMT. ST.HUBERT, BELGIUM

PRESSURE TEMPERATURE DEW POINT MODIFIED REFRACTIVE INDEX900mb 920mb 945mb

+ 7.1 + 7.5 - 3.5

- 40.0- 28.0- 3.9

250.8257.3295.8

The conditions that cause this abnormal propagation are:

A temperature inversion A rapid decrease in humidity with height

This forms a duct between the earth and a few hundred feet above the surface.

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Radio waves have a wavelength that is small compared with the duct height. This allows the duct to refract the wave back to earth. The wave is then reflected by the earth’s surface back to the duct ceiling. A series of these refraction/reflection hops occur and thus the wave can be received well in excess of the quasi-optical range. The same conditions can occur when there is an inversion aloft.

These conditions are normally associated with large high pressure systems; a condition which is a regular feature in the tropics. Aerials A transmitter/receiver is only as good as the aerial. An aerial can be defined as a device used for the efficient transmission and reception of electromagnetic energy. Generally we look at aerials that radiate, however, the properties of a transmitting aerial apply equally to the receiving aerial. Aerial Characteristics When an aerial radiates an electro-magnetic wave two radio frequency fields are transmitted;

E Field Electric or electrostatic field H Field The magnetic field.

These fields are transmitted at right angles to each other.

TRANSMITTER

TRANSMITTER

ELEVATED DUCT

SURFACE DUCT

EARTH

LAYER OF HIGHDIELECTRICCONSTANT

LAYER OF HIGHHUMIDITY

LAPSERATE ANDTEMPERATURE

INVERSION

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Note that the H field is always at right angles to the aerial and the E field is always parallel to the plane of the aerial. By convention the wave is said to be vertically polarized if the E Field Is perpendicular to the earth and horizontally polarized if parallel to it. A vertically polarized wave is produced by a vertical aerial: a horizontally polarized wave by a horizontal aerial. A standard dipole receiver aerial must be aligned in the same polarity as the signal to be received for maximum transfer of energy. For instance, a transmitted signal from a horizontal aerial will need to be received by an aerial which is itself horizontal for optimum reception. Aerial Length The aerial is manufactured to a specific length dependent on the frequency to be used. Polar Diagrams The effective radiation or reception of an aerial is shown by a polar diagram. These can be shown as: Horizontal Looking down on the aerial from above

Vertical Looking at the aerial from the side Omni-Directional Aerials Simple Half-Wave Dipole In its simplest form a dipole consists of a metal rod or a wire cut to a specified length. The aerial is cut to a half wave-length.

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Example For a frequency of 30 MHz λ = 10 m

The aerial for this wavelength will be λ /2 or 5 m

This is called the Electrical Length. In an ideal world the Electrical Length would be the length of aerial required for a given frequency. The speed of electro-magnetic radiation through a vacuum is constant. When an “aerial feeder” is used the speed of the radiation is slower. This slower speed is approximately 5% less than the in-vacuo speed and we must take this into account by factoring the Electrical Length to 95% of its value. This is the Physical Length of the aerial for a given frequency.

Example For a frequency of 100 MHz λ = 3 m

Electrical Length of the aerial = λ /2 = 150cm Physical Length = 95% of λ /2 = 142.5cm

Marconi Quarter Wave Aerial Most practical aerials are cut to λ /4. By using the reflective properties of electro-magnetic waves the aerial compensates for the missing half of the dipole.

The Marconi aerial is particularly suitable for fitting into aircraft structures. To ensure that the aerial can be used over a range of frequencies an aerial loading unit (ALU) is fitted. This unit electronically matches the aerial to the frequency selected.

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Aerial Feeders There needs to be a connection between the transmitter/receiver and the aerial, this is known as a feeder. The type of feeder used depends upon the frequency to be used. The most common feeder in use in aircraft communications is the co-axial cable (better known to us as the TV aerial wire). Higher frequencies need a more sophisticated feeder, such as radar where a wave guide is required. Aerial Directivity The dipole radiates power evenly in all directions or omni-directionally. The plan and side views show the radiating pattern.

Note that the radiating polar diagram is from the centre of the aerial not the tip. To modify the omni-directional properties and give the aerial directivity parasitic elements have to be added. The most common directional aerial in everyday use is the TV antenna - The Yagi. The directional properties are derived by adding parasitic elements in front and behind the dipole.

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To change the omni-directional properties a parasitic reflector, 5% larger than the dipole, is placed at a distance of λ /4 from the dipole. The normally circular polar diagram is now changed into an elongated heart shape. The reflector reflecting the power back towards the aerial. Note that the dipole is the only part of the aerial that has any power.

To enhance the directional properties parasitic directors are added on the opposite side to the parasitic reflector. These elements are 5% shorter than the dipole.

The resulting polar diagram is narrow in beam width and gives excellent directional properties. One disadvantage with the directivity achieved is that unwanted side lobes are produced. The side lobes are approximately 50% of the power of the main beam and can give spurious indications if not dealt with. Methods of suppression or removal of the side lobes are discussed in individual chapters on equipment.

Different polar diagrams can be achieved for different aerial combinations. An example of this being the figure of eight’ produced by two dipoles.

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The significance of changing the polar diagram will become apparent as each piece of equipment is discussed in detail. Modulation Modulation is the superimposing of intelligence, such as speech or Morse identification, onto a carrier wave. Varying a parameter of the carrier, such as its amplitude or frequency does this. When electro-magnetic energy is radiated as a sinusoidal wave no intelligence is transmitted. The frequency is beyond the scope of human hearing and the wave itself would be meaningless. Keying By interrupting the wave, a process known as keying, Morse Code can be transmitted.

The frequency may be identifiable as Morse code, but is still outside the audible range. To help with audible reception the carrier frequency has to be converted into a signal within the audio range. This is achieved by mixing the received frequency with another known radio frequency; this produces a signal in the audio range. The mixing of two radio frequencies is known as heterodyning, as opposed to modulation which is the mixing of a radio frequency and an audio frequency.

Example Received frequency 500 KHz Known frequency 501 KHz

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Four frequencies are produced:

500 KHz} 501 KHz} outside the audible range 1001 KHz} 1 kHz an audible frequency

Heterodyning is enabled by selecting the BFO function (Beat Frequency Oscillator). The incoming signal is received and mixed with the BFO frequency and the resulting audio frequency is fed to the intercom system. An audio tone therefore will only be heard when the two frequencies are present.

Note that the only piece of equipment that uses a BFO in the aircraft is the ADF. Normally electromagnetic radiation is modulated by one of the three methods listed below: Amplitude Modulation (AM) AM is where the modulating frequency alters the amplitude of the wave.

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Where a carrier is amplitude modulated by a single tone the resultant waveform consists of three components:

The carrier wave fc The upper sideband (fc + fs) The lower sideband (fc - fs), where fs is the modulating signal

The AM signal will consist of:

1 500 KHz the carrier 2. 501 KHz the upper sideband 3. 499 KHz the lower sideband

Intelligence is carried by both sidebands. The spread of the side frequencies is known as the bandwidth. For an amplitude modulated signal the bandwidth is 2fs. Both sidebands carry the same information, if one of the bands is suppressed (eg the upper sideband) then the only frequencies that need transmitting are 500 KHz and 499 KHz. This type of transmission will have two main advantages:

Less power is required to transmit one sideband and the carrier The signal occupies less of the radio spectrum. This means that a more efficient

use can be made of the frequency band the signal is in. HF transmissions make use of the single sideband transmission. Frequency Modulation (FM) FM is where the modulating frequency alters the frequency of the wave.

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The frequency of the carrier varies by an amount proportional to the instantaneous amplitude of the modulating signal. The rate of change of the carrier frequency is proportional to the frequency of the modulating signal, the amplitude of the modulated carrier remaining constant throughout. FM signals are relatively noise free. Unfortunately this type of broadcast uses a much wider bandwidth than AM and so FM has limited application in commercial aviation but is used in:

VOR Radio Altimeters Doppler

Pulse Modulation (PM) PM is where the carrier is transmitted in short pulses. These pulses can be coded to carry information. Two types of PM need consideration:

Pulse Amplitude Modulation (PAM) In a similar way to AM it is possible for an audio waveform to modify the amplitude of a fixed train of pulses.

Pulse Code Modulation (PCM) A system where each pulse amplitude is assigned a binary number.

Classification of Emissions Radio regulatory agencies have designed a coding system that fully describes the form that a radio transmission may take. The table below details the coding system.

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First Character

Type of Modulation Second Character

Nature of the Modulating Signal

Third Character Type of Information Being

Transmitted N – Unmodulated carrier 0 – No modulation N – No information

transmitted A - Amplitude 1 – Interrupted carrier A – Telegraphy – for aural

reception J – Single sideband (no carrier)

2 – Keyed or digital audio modulation

E – Telephony – including sound broadcasting

F – Frequency 3 – Telephony (voice or music)

W – Combination of the above

P – Unmodulated pulses 8 – Two or more channels of analogue information

X – Cases not otherwise covered

9 – Composite systems comprising of 1 & 2 above with 3 or 8

X – Cases not otherwise covered

The emission characteristics for civil aviation use that you need to know are: ADF N0N A1A N0N A2A HF J3E VHF A3E VDF A3E ILS A8W VOR A9W DME P0N Basic Radar Theory Introduction Radar is derived from the expression radio detection and ranging. It may be defined as any system employing radio to detect the presence of objects and to determine their position and movement. Radar Frequencies Radar occupies the frequency bands from VHF upwards. Higher frequencies are used because:

They are free from external noise Narrow beams operate more efficiently with a short wavelength

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Primary radar use pulses, high frequencies produce short pulses The efficiency of reflection depends upon the size of the target in relation to

wavelength. High frequencies are reflected more efficiently Principles A transmitter sends out, via the aerial, a brief pulse of radio energy. Every 6.2 microseconds (µs) this pulse will travel 1 nautical mile. If this pulse strikes a target, a small proportion of the radio energy will be reflected back to the aerial. The aerial picks up this reflected energy and passes it to the receiver. If the time of travel is known then the range can be calculated.

Pulse Radars Pulse radars are employed as:

Primary radars - ATC surveillance radars, Airborne weather radars Secondary radars - DME and SSR Doppler

The radar transmits energy in very short bursts of high energy. Timing the pulse yields a direct measurement of the range and requires a sensitive receiver. The transmission, travel and reception of the pulse must be achieved before the next pulse is transmitted. This will then ensure that we have an unambiguous target.

Primary Radar A primary radar relies on the weak reflections from a passive target. The effectiveness of the radar depends upon the transmitter power and the receiver sensitivity.

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Secondary Radar Relies on the target co-operating with the transmitter. The target transmits a reply signal to an interrogatory signal such as in SSR and DME. The interrogation and reply are usually on different frequencies.

Secondary radar has both advantages and disadvantages over primary radar:

Advantages

Primary radars require much more power to achieve the same range Target size and aspect are irrelevant because the target transmits the response Responses on the secondary radar are much more reliable Information can be encoded to give the transmitter and receiver information Clutter on the radar screen can be eliminated

Disadvantages

The radar requires the co-operation of the target Bearing resolution can be inferior Side lobes can be a problem at short range Beacon saturation can be a problem

Radar Direction Finding There are two principle means:

Lobe Comparison Beam Direction Finding

Lobe Comparison Mainly used by secondary radars two aerials are used to define direction. The aerial is rotated till an equal strength signal is received.

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Beam Direction Finding By using a parabolic aerial a near parallel beam can be achieved. Because the direction of the aerial is known and the pulse is transmitted and received before a second pulse is transmitted the azimuth of the target can be calculated.

The beamwidth of a parabolic aerial can be calculated by the formula:

Beamwidth = 70λ /d

Where: λ = wavelength of the radar d = diameter of the parabolic aerial Remember with this calculation that λ and d must be in the same units. Radar Terminology Certain terms are used in radar and these need to be understood.

Pulse Recurrence Frequency (PRF) This is the rate at which pulses are transmitted by the radar. The units used are pulses per second (pps). The maximum

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PRF is determined by the fact that each pulse must be able to reach the most distant target and return before the next pulse is transmitted. Otherwise there is a possibility of ambiguous range measurement. Pulse Recurrence Interval (PRI) The time interval between pulses. The units are normally microseconds. The PRI is used to determine the maximum range of the radar. The relationship between PRI and PRF is simple.

PRI = 1 ÷ PRF

Example For a radar with a PRF of 250 pps find the maximum range PRI = 1 / PRF = 1 / 250 = .004 seconds = 4000 µseconds = 4000 X 10-6 seconds (to convert seconds into microseconds multiply by 1 000 000) Distance = speed X time The total time of travel out and back for the pulse is 4000 µseconds The time of travel one way, so that the range can be calculated = 2000 µseconds or 2000 X 10-6 seconds Distance = (2000 X 10-6) seconds X (300 X 106) metres per second Distance = 600 000 metres = 600 kilometres This is the maximum unambiguous range of the radar

Pulse Width (PW) The duration of the pulse. This determines the minimum range of a radar. The pulse must travel half its distance before it hits a target and returns to the radar. Otherwise the radar will still be transmitting the same pulse.

Example A radar has a PW of 2 µseconds, what is its minimum range

The minimum range must be half the time of travel, which is 1 µsecond

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Distance = speed X time = (1 X 10-6) seconds X (300 X 106) metres per second Distance = 300 metres Choice of Frequency To produce a narrow beam a high frequency must be used. The advantages of using a narrow beam are obvious:

Bearing accuracy will be greater There is an increase in effective power The radar will be able to resolve closely spaced targets High frequencies also generate a squarer pulse shape Wavelength has to be shorter than the target size

All the above have to be taken into consideration

The basic radar has seven elements:

Master Timer This is the trigger unit and has two functions:

It generates the basic PRF Synchronizes the firing of the transmitter

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Modulator The output from the modulator switches the transmitter on and off

and so controls the pulse length of the transmitter output Transmitter Delivers the pulse to the aerial Receiver A sensitive superhet that can amplify the very weak returning echoes.

These are then processed for display. T/R switch The same aerial is used for both transmission and reception. The

receiver must be protected from the high power transmitter. This is achieved by electronically isolating the waveguides for both. A duplexer which in real terms is the brains of the radar does this isolation.

Indicator Radar information is usually displayed on a Cathode Ray Tube Aerial A parabolic dish on older aerials. Now a flat bed array which

electronically simulates a parabolic dish.

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Chapter 2.

VHF Direction Finding

Introduction Certain aerodromes are fitted with VHF Direction Finding (VDF) equipment. VDF provides an ATC Controller with the means of determining the direction of the VHF signal from an aircraft. The only onboard equipment required to receive this service is a standard aircraft VHF radio. This is one of the main advantages of the VDF service. Principles of Operation The basic VDF uses a phased array system based on the Doppler Principle. The modern VDF aerial is shown below.

Frequency VHF – 118 to 137 MHz Emission Characteristics A3E

ATPL Radio Navigation 28 October 2003 2-2

Operation If the communications transmitter on an aircraft is tuned to the VDF frequency and the transmitter is activated, the aerials at the VDF unit will detect the incoming transmission and each aerial element will feed a signal to the VDF receiver

The aerial elements are all at slightly different distances from the source of the signal

Each will detect a slightly different phase of the signal from the aircraft at the same moment in time

The value of these detected phase differences are directly related to the direction of the incoming signal

The phase differences are used to drive the bearing indicator. On some VDF units a simple digital read out gives the bearing. The ground VDF station can give true or magnetic bearings. Services It is common to use the Q codes to represent the bearings from a VDF station. The codes originate from the days of telegraphy where it was convenient to transmit short codes instead of full text messages. The Q codes are not abbreviations. Listed below are the codes that are relevant to direction finding.

QTE True track from the station QDR Magnetic bearing from the station QUJ True track to the station QDM Magnetic bearing to the station QGH DF controlled approach QDL DF steer

In practice, only QDM and QDR are normally used. The accuracy of the bearing is measured in degrees. Bearings are categorised, in accordance with the ICAO defined classifications, as given in the following list:

Class A accurate within ± 2° Class B accurate within ± 5° Class C accurate within ± 10° Class D accuracy less than Class C

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Siting Due to topography, some VDF stations are approved for use within certain sectors only. Specific information for an aerodrome is given in the AIP. Generally the class of bearing is no worse than class B (Many states will not permit Class C & D bearings to be provided). Ground VDF stations should not be used as en route navigation aids, as they are not providing the aircraft with continuous tracking information. However, in case of emergency or where other essential navigation aids have failed, their service is available. When flying VFR under marginal weather conditions, a VDF station is a useful navigation tool for any pilot. Receiving a Bearing To request a bearing or a heading to steer, the pilot should call the aeronautical station on the listed frequency. The pilot should then specify the type of service that is desired by using the appropriate Q code. Automatic VDF stations only require a short transmission, older manual stations may require a short preparatory period before being able to provide the service. After the bearing or heading to steer has been requested, the VDF station will advise the aircraft station the following way:

The appropriate Q code The bearing or heading to steer The class of bearing and time of observation (if necessary)

The pilot will read back the bearing or heading to steer as soon as the message has been received. Asking For a Bearing Assume that the pilot of an aircraft G-BOBA needs to confirm his position.

Aircraft Cranfield Tower, G-BOBA, request QDM Tower G-BOBA, QDM I80°, Class B” Aircraft QDM 180°, G-BOBA

Initially the pilot asks for the magnetic bearing from his aircraft to the airport (QDM). For the pilot to home to Cranfield he should fly a magnetic heading of 180°M. Remember that the bearing is for the still air condition. If there is any wind the initial QDM must be corrected for drift. Note that a QDM only gives a position line, not a fix position.

ATPL Radio Navigation 28 October 2003 2-4

Position Fix When a position fix is required the request must be made to a station with the capability for triangulation, such as the Distress and Diversion Cell on the international emergency frequency of 121.5. It should be appreciated that a bearing derived from one station is a line of position (LOP), so two LOP’s are required to get a position fix. VDF Approach The pilot may request a QGH procedure provided one is listed for the airfield. This is a procedure in which the controller will provide the pilot with a series of QDM’s so that the pilot can follow a laid down approach path. This approach path involves a number of steps described in the appropriate approach chart, and are summarised as follows:

Guiding the pilot into a position over the airfield Establishing the pilot on an outbound leg, which is about 20° off the reciprocal of

the desired final approach path. This path is flown at a safe level above the airfield and the surrounding

obstructions. The pilot times the outbound leg (allowing for wind effect and using the distances

given in the approach plate) and, using the QDM information supplied, establishes the drift being experienced.

At the end of the appropriate time a rate one turn is carried out to intercept the inbound final approach path.

On intercepting the final approach the course is maintained by using the drift established outbound (with the sign changed) as an inbound wind correction angle. This can be adjusted as the controller provides new QDM information.

At the prescribed time the ‘let down’ is commenced. Below is the VDF chart for Cranfield.

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ATPL Radio Navigation 28 October 2003 2-6

Decoding the Chart The chart is split into three parts: Part 1 – Administration

Right Side

The airfield the chart refers to The procedure “VDF 210° to Aerodrome” The VDF frequency in MHz

Left Side

Shows the Minimum Safe Altitude circle valid for the 4 quadrants shown out to 25 nm from the aerodrome reference point (ARP)

Centre

Gives the four letter ICAO code for Cranfield (EGTC) The categories of aircraft that can fly this approach (A & B) The airfield data The airfield frequencies. Note that:

The approach frequency is also the primary VDF frequency The tower frequency is the secondary VDF frequency

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Part 2 – The Plan View

The plan view shows a range circle out to 10 nm from the aerodrome which is the

centre point Inside the circle are marked airspace restrictions such as D206 Significant obstacles are also marked

The Approach

The aircraft will initially be homed to the Cranfield overhead, the initial approach fix (IAF) which is co-located with the VDF

The outbound leg is 016° and the inbound leg is 210° Overhead the VDF is the missed approach point (MAPt)

The above shows the course of the aircraft over the ground but shows no elevation information. This is given in the bottom part of the chart.

ATPL Radio Navigation 28 October 2003 2-8

Part 3 – The Elevation View The elevation view is as if you are a bystander on the ground looking at the aeroplane from the side.

Initially the aircraft is homed to the IAF at an altitude of 2200 ft. This will be QNH, the QFE figures are given in the brackets.

On the outbound leg the aircraft is descended to 1664 ft QNH (1300 ft QFE) After the inbound turn the aircraft can then be descended to the appropriate MDA

for the category of aircraft shown in the boxes to the bottom right of the diagram If the airfield is not seen by the time the aircraft is over the VDF then a missed approach must be carried out. Refusal of Service DF stations have the authority to refuse to give bearings when conditions are unsatisfactory or when the bearings do not fall within the calibrated limits of the station. The station will state the reason at the time of refusal. Full R/T procedures to be used, when requiring VDF assistance, are contained in the Communications section of your notes. Automatic VDF Automatic VDF stations assist in the radar identification for ATC procedures. They do not provide a normal VDF service to aircraft.

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Range and Errors Being a VHF transmission, the range is line of sight and the maximum range formula applies:

Range = 1.25(√HT + √ HR)

Where: R The maximum range between the stations HT The height of the transmitter HR The height of the receiver

Other factors that will affect the expected range are:

Intervening Terrain This can screen the transmitter/receiver path (remember that VHF is a line of site transmission) Atmospheric Refraction An increased refractive index (resultant from the inversions of temperature and/or humidity) can cause super refraction and increased ranges. Sub-refraction will reduce the expected range. Transmitter Power

The bearing signal measured may be in error. The major sources of error are: Ground Reflections These can cause VHF and UHF signals to reach the DF station aerial from multiple paths. Additional phase differences are detected which deflect the bearing indication. Synchronous Transmissions Signals from other aircraft communications equipment are detected at the DF station at the same time as the desired signal. This causes a deflection of the measured bearing. It should be noted that this is a problem in congested airspace when atmospheric conditions favour super refraction and cause transmissions from beyond the ‘radio horizon’ to be detected.

The signal quality may be reduced if an aircraft is not flying straight and level. VHF radio signals are vertically polarised and reception is only optimal when the aircraft has a small amount of pitch and bank. To ensure good reception avoid asking for bearings or headings to steer during steep turns.

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Intentionally Left Blank

ATPL Radio Navigation ©Atlantic Flight Training 3-1

Chapter 3.

Non Directional Beacons and Automatic Direction Finding

Introduction Non Directional Beacons (NDB) are ground-based transmitters which transmit radio energy equally in all directions. The airborne system in the aircraft is called the Automatic Direction Finder (ADF). The indicator in the aircraft always points towards the tuned NDB (exceptions to this will be discussed later in this chapter). Principles of Operation The NDB transmitter is very simple. A RF oscillator provides a carrier wave. This carrier wave is the NDB signal that is used by the airborne equipment (ADF) to determine the direction of the transmitting station. A low frequency oscillator provides the identification signal of the transmitting station or “ident”. The low frequency signal modulates the carrier wave in the modulator. Frequency LF/MF – 190 to 1750 KHz. In Europe the frequencies are normally between 225 to 455 KHz. Emission Characteristics

Long Range Beacons N0N A1A Short Range Beacons N0N A2A

It is important to bear in mind that although the airborne equipment only needs the bare carrier signal to indicate the direction to the transmitter, there has to be a way of identifying the selected station. In the above emission characteristics, both the long and short range beacons transmit N0N. This is the unmodulated carrier wave on which the indication relies. It is the A1A or A2A which provides the identification. The A1A emission keys the carrier wave. Remember from the paragraph on heterodyning in chapter 1 that to have an audio frequency input to the headphones there must be two radio frequencies, therefore the BFO must be selected on to provide the second frequency. This means that the audio tone will be heard during all of the N0N phase of the incoming signal and during the active part of the keyed A1A signal. The A2A emission modulates the audio tone frequency directly onto the carrier and therefore the BFO should be selected off. It is the demodulator within the ADF receiver which will feed the audio ident tone to the headphones.

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The ICAO recommended emission characteristic is A2A, unless operational or environmental considerations dictate the use of A1A, such as long range coastal installations. Loop Theory To understand direction finding an understanding of a loop aerial is necessary. Below is a representation of a loop aerial. This loop has two vertical members A and B.

If a vertically polarised signal is received by the aerial then voltages will be induced in the two vertical members A and B, Va and Vb. Consider a wave that has a wavefront BC. The distance BC is insignificant with regard to the distance that the wave has travelled, so BC can be considered a straight line. The wavefront arrives at B a short time before A. During this short distance of travel it can be assumed that there is no difference in the received signal strength at A or B. Because the wave travels the extra distance to A there will be a difference in phase difference equivalent to AB Cosβ. AB is a constant length and so the signal voltage induced in the loop aerial will be proportional to the value of Cosβ. If we plot all values of Cosβ then we get a polar diagram as shown below.

The polar diagram shows two ill defined maxima (90° and 270°) and two well defined minima at (0° and 180°).

A B

β

A B

Phase Difference (AC) = AB Cosβ

C

β

+ -

A B

ATPL Radio Navigation ©Atlantic Flight Training 3-3

It is usually the minima that are used in direction finding. However, with two well defined minima there is no indication as to which side of the loop the transmitter is sited. The ambiguity is solved by a sensing aerial. Sensing To resolve the ambiguity of the polar diagram above a vertical di-pole is inserted into the loop as shown in the diagram below.

The polar diagram of the sensing di-pole is shown below.

By combining the polar diagram for the loop aerial and the sensing aerial a cardioid is formed.

A cardioid diagram has only one null position, and the 180º ambiguity is now resolved. The principle of the ADF is that the loop is turned to the position for minimum which corresponds to the null position of the cardioid. The instrument’s needle indications are also relative to the

+ - +

ATPL Radio Navigation 28 October 2003 3-4

position of the loop aerial. The system is called the Automatic Direction Finder because the aerial rotation and the interpretation of its relative signal strength are done automatically. The indicator information is such that, if we lay the instrument panel down flat, the ADF needle points directly at the transmitting station. NDB Operation An amplified signal is radiated omni-directionally. The transmission mast may be either a single mast or a large T-aerial strung between two masts. These aerial arrangements produce a vertically polarised signal. The polar diagram for the aerial is omni- directional in the horizontal plane but, as shown below, exhibits directional properties in the vertical plane.

Above the station, marked by the points at which the radiated power has fallen to 0.5 of its maximum value, is a conical area in which signal strength may be too low to be used. This volume of space is called the ‘cone of silence’. For a NDB this angle is 40° from the vertical. ADF Operation The Automatic Direction Finder (ADF) consists of a receiver, a sense aerial, a loop aerial and an indicator. The receiver control panel and the indicator are located on the instrument panel, the loop and the sense aerial are normally combined in a single aerial unit, normally mounted under the fuselage. The pilot uses the receiver control panel to enter the frequency corresponding to the NDB for intended use.

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The ADF indicator consists of a needle, which indicates the direction from which the signals of the selected NDB ground station are being received. In its most basic form, the needle moves against a scale calibrated in degrees from 0° - 359°. This is known as a Radio Compass. The datum for the direction measurement is taken from the nose of the aircraft and therefore, the radio compass indications are relative bearings. Bearing Determination The loop, or directional, aerial is rotated electronically and, by combining information from the loop and sense aerials, the bearing to the station is internally derived. When a looped conductor, such as the loop aerial, is hit by electromagnetic waves, voltages are induced in the two halves of the loop. These voltages depend on the angular position of the loop relative to the incoming electromagnetic (EM) waves. The total voltage induced in the loop is the algebraic difference between the voltages from the two halves. This total voltage is the signal output from the loop aerial. Types Typical associated power outputs and uses are as follows:

Locator Beacon - 15 to 40 watts - Used for intermediate approach guidance towards establishing the final approach path of an ILS. These beacons are short range and are normally NON/A2A. Maximum range 15 – 25 nm. Homing Primarily an approach and holding aid in the vicinity of an aerodrome. Medium range beacon normally N0N A2A. Maximum range 50 nm. Airways/Route Beacons - up to 200 watts - Used for track guidance and general navigation. These beacons are normally NON/A2A Long-Range Beacons - up to 4 kilowatt -Generally located on islands or oceanic coastlines, these are intended to provide guidance and navigation resource to transoceanic flights. These beacons are normally NON/A1A.

It should be noted that different transmitters operate within the NDB band of frequencies and can be detected by the aircraft’s receiver. These include:

Broadcast stations (i.e. those carrying entertainment, news, etc.). Broadcast stations can have repeater transmitters at different locations causing synchronous transmission errors.

Marine Beacons. Stations must not be used if their details are not published in the AIP or appropriate Flight Guides. Where details of Marine Beacons are published, users should note that a number of beacons are grouped together on the same frequency. Each beacon transmitting for a period of 60 seconds in a cycle of six minutes.

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The use of signals from such published stations guarantees that, within the published range by day, the signal from the desired station will be at least three times stronger than any other signal on the same or near frequency. The use of transmissions from non-published sources may lead to errors, as they are not protected from such harmful interference. Control Panels and Indicators Control Panel There are different types of ADF control panels, but their operational use is almost the same and an example is shown below. The mode selector, or function switch, has several positions, enabling the pilot to select the function he wants to use. Typical markings are - OFF, ADF, ANT, and LOOP.

ADF The position when the pilot wants bearing information to be displayed automatically by the needle. ANT The abbreviation of antenna and, in this position, only the signal from the sense aerial is used. This results in no satisfactory directional information to the ADF needle.

There are two reasons for selecting the ANT position:

Easier identification of the NDB station, and Better understanding of voice transmissions

BFO stands for Beat Frequency Oscillator. This position can be labelled CW, the abbreviation for Carrier Wave. The BFO circuit imposes a tone onto the carrier wave signal to make it audible to the pilot, so that the NDB signal can be identified. The emission characteristics determine the position of the BFO switch:

Tuning Identification N0N A1A ON ON

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N0N A2A ON OFF Note: For N0N A2A the BFO is on for tuning to check the integrity of the incoming signal by providing an uninterrupted tone for an uninterrupted carrier signal. Once the station has been properly tuned and identified, the Mode Selector should be switched back to ADF. This is important, as no bearing information will be displayed unless the switch is in the ADF position. When BFO or ANT are selected some ADFs automatically default to the 180° position, others remain on the last bearing computed. So never leave the mode selector in ANT or BFO position if you are navigating using the ADF. In order to avoid the dangers of this problem, NDBs transmitting on A2A can be identified with the mode selector in the ADF position, so the ANT position can be avoided. There is no failure flag on an ADF receiver or indicator, the only way to be sure that the instrument is receiving a valid signal from the NDB is to continuously monitor the station’s identification. Each NDB is identifiable by a two or three lettered Morse code identification signal, which is transmitted together with its normal signal. This is known as its IDENT. When tuning an NDB it is absolutely essential that the facility be correctly identified before using. TEST Switch If a test switch is incorporated, pressing test swings the needle:

If the needle does not swing, the unit is not working properly. If the needle does swing but doesn’t return to its previous position, the signal is

too weak to be used for navigation. If it swings and returns to its previous position, then the system is working

properly and the received signal is good. Bearing Indicators Bearings to the station are displayed on an indicator consisting of a bearing scale (calibrated in degrees) and a pointer. There are four types of bearing scale with varying degrees of sophistication. They are:

The fixed card, The manually rotatable card, The radio magnetic indicator (RMI). Fixed card indicator or RBI

Only two systems will be discussed, the RBI and the RMI.

ATPL Radio Navigation 28 October 2003 3-8

Relative Bearing Indicator (RBI) The bearing displayed on a fixed card indicator is a relative bearing; thus the name Relative Bearing Indicator (RBI). Since the Card is fixed, zero is always at the top and 180° is always at the bottom.

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A relative bearing is always measured clockwise from the nose of the aircraft. In the diagram above the needle is pointing to 100°. This means that the station is 100° to the right of the aircraft nose. In the diagram below the relative bearing is 340°. The NDB is 340° right of the nose.

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A more convenient way of expressing this is that the station is 20° left of the nose. It is sometimes convenient to describe the bearing of the NDB in relation to the NOSE or TAIL of the aircraft. Since the card is fixed, the indicated relative bearing has to be combined with the magnetic heading of the aircraft in order to obtain the magnetic bearing to the station (QDM). If the result of this addition exceeds 360°, 360° has to be subtracted from the result in order to obtain a meaningful bearing.

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Example Assume for the diagram above that our aircraft is heading 230°M The bearing to the NDB is:

230° + 340° = 570° Because this is more than 360 then we must subtract 360 from 570 = 210° The QDM is 210° This means the QDR is 030°

The magnetic bearing of the aircraft from the station, the (QDR), is the reciprocal of the QDM. A quicker way to determine the QDM is to mentally superimpose the RBI needle onto the directional gyro. This is not very accurate, but it is a good double check on your calculations. The QDR can be visualised as the tail of the needle when it is mentally transferred from the RBI onto the directional gyro indicator. Radio Magnetic Indicator (RMI) This combines the Relative Bearing Indicator and Remote Indicating Gyro Compass into a single instrument, with the compass card being aligned automatically with Magnetic North. In the diagram below:

The heading is 332°M The VOR or ADF can be indicated by either pointer depending upon the switching The QDM is continuously indicated by the arrow head of the pointer The QDR is continuously indicated under the tail.

ATPL Radio Navigation 28 October 2003 3-10

VOR

VOR

ADF ADF

33 N3

6E

30W

24

21S 15 12

This is now the most common type of presentation. If the double pointer represents the ADF then:

The QDM is 300° The QDR is 120°

Direct Wave Limitations The Direct Wave follows the line of sight and its range can be determined from the line of sight formula. In most cases the direct wave range will be considerably less than that of the Ground Wave. Height may become significant when it is desirable to receive the direct wave, so as to minimise the risk of ADF error when flying in mountainous areas or when using coastal NDBs.

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Sky Wave Limitations

At some frequencies there will be a gap in coverage between the ground wave and the first return of the sky wave. The ground wave coverage might extend out to 300 miles, while the first skywave returns at 1000 miles. This gap is called the dead space. The exact length of the dead space depends on frequency and the state of ionisation of the atmosphere. At frequencies in the lower MF and the LF bands, intense ionisation by day attenuates (absorbs) RF signals and no sky wave return is noticeable. By night the ionisation levels fall and returning sky waves will be detected. Night Effect At short range (30 to 80 miles) the sky waves will mix with the ground wave signal (there is no dead space). Because the returning sky waves have travelled over a different path they have a different phase from the ground wave. This will have the effect of suppressing or displacing the aerial ‘null’ signal, in a random manner. The needle on the RMI or RBI will wander. This effect is at its most variable during twilight at dusk and dawn. A further effect is due to the design of the loop aerial system. The loop uses a vertically polarised signal. As the radio wave travels through the ionosphere the vertical polarisation is changed as the wave is refracted back towards the earth, so the returning wave has a horizontal polarisation component. A current is now induced in the horizontal members of the loop:

A B

ATPL Radio Navigation 28 October 2003 3-12

The horizontal member AB, and The two smaller feeds to the bottom of the aerial

The resultant current flow further degrades the null position and an accurate reading is impossible. At longer ranges the sky wave signal will become progressively stronger. However, ionospheric refraction may cause the plane of polarisation of the signal to be randomly shifted so that a horizontally polarised component may be randomly introduced into the loop aerial. This will cause the null signal to be displaced. In summary, the airborne ADF is designed and optimised to be used in conjunction with the more predictable ‘ground wave’ signal from the selected NDB. Errors of the ADF The ADF bearing is subject to a number of error sources including any or all of the following. Quadrantal Error The metal components of the aeroplane’s structure behave as an aerial. They absorb signals at all frequencies but more readily so at frequencies in the MF band. Once absorbed, these are then re-radiated as weak signals but, being close to the ADF aerial, are strong enough to be detected. The effect of this signal is to displace the measured null towards the major electrical axis of the aeroplane creating an error that is maximum on relative bearings 045°, 135°, 225°, 315° (the quadrantals). This error is minimised by calibration and electro-mechanical compensation at installation.

POSITIVECORRECTION

REQUIRED

MAJOR ELECTRICALAXIS OF AIRCRAFT

CORRECTBEARING OF

TRANSMITTER

INCOMINGRADIO WAVE

BEARINGACTUALLYMEASURED

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Dip (Bank) Error During turns, the horizontal member of the loop aerial will detect a signal. This will cause the null to be displaced and a ‘short-term’ erroneous bearing to be displayed. Coastal Refraction When flying over the sea and using a land based beacon, the changes in propagation properties of the signal as it passes from land to sea will cause the ‘wave front’ to be displaced. This will result in a bearing error.

CORRECT BEARINGOF BEACON

ACTUAL PATHOF RADIO WAVE

(INDICATED BEARING)

REFRACTIONTOWARDS COAST

WAVE CROSSINGAT 90°

NO REFRACTIONLAND

SEA

NDB

Such bearing errors may be minimised by any or all of the following:

Do not use beacons unless they are situated on islands or near to the coast. If using an inland NDB only use bearings at or near to 90º to the coast. Remember that coastal refraction is less as height is increased.

Multipath Signals When flying in mountainous regions, signals may be refracted (bent) around and/or reflected from mountains. The ADF may be affected by such multipath signals and the bearings will be unreliable.

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TRANSMITTER

Noise This is defined as any signal detected at the receiver other than the desired signal.

Man Made Noise Each published NDB has an associated published range. If use of that NDB is restricted to that range, the desired signal is protected from the harmful interference of ground waves from other known transmitters on the same or near frequencies. It should be remembered that, from sunset to sunrise, sky wave propagation of signals in the LF and MF bands is possible. This will cause the signal to noise ratio to be reduced and will result in errors as the null is displaced, usually randomly. Another localised source of man-made noise is overhead power cables. Many of these cables carry not only electrical power but also modulated signals used by the power companies for communication. These modulated signals radiate from the power cables and create mini NDBs. Such emissions are monitored but, in some states, monitoring may not be carried out. The rule is – if unsure, use with extreme caution.

Lightning There are an average of 44 000 thunderstorms over the earth’s surface in every period of 24 hours and more than half of these occur over or near land surfaces within 30º latitude of the Equator. Each thunderstorm generates electro-magnetic signals and these radiate in all directions from that storm. If you happen to be flying near one of these storms, your ADF will detect the signal and the bearing indication may well be deflected towards that storm. Such noise levels are normally quite low but they will increase

In temperate latitudes in the summer As you move towards the tropics At night as a result of sky wave propagation.

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Charged Water Droplets Water droplets held in a cloud have an electric charge. As an aircraft flies through the cloud the water droplets that contact the aircraft will discharge on the metal surface. The collective effort of the water droplet discharge can distort and blur the polar diagram such that the null position is displaced.

Noise effects can be indicated by:

Seeing the bearing indication randomly wandering. Using the audio output and noting audible signals such as voice/music/static.

If ‘noise effect’ is suspected, only use the published NDBs when well within the notified range. You could be at half the published range before a reliable signal is found. Synchronous Transmission Where two or more beacons are transmitting on the same frequency then the measured bearing becomes the resultant of the two received signals.

As long as the NDB is used within its promulgated range the effects of synchronous transmission should be a minimum. Promulgated Range Most NDBs are given a daytime only protection range where the unwanted signals are limited to ± 5°. Outside this range the error will increase. The propagation conditions at night also increase the bearing errors. Absence of Failure Warning There is no visible indication to the user that there is a system failure.

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Accuracy When used within the published range by day the ADF should give a bearing accuracy within ± 6.

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Chapter 4.

NDB Navigation

Introduction This chapter will take you through the uses of the NDB. Even though you are unlikely to use the instrument for plotting position lines the need to understand this procedure will be of help to your General Navigation course. The process of homing and understanding the Jeppesen plate is essential in the instrument flying for the Instrument Rating. ADF Bearing Procedure for obtaining an ADF bearing:

Determine the frequency, identification and modulation of the required beacon and ensure that your aircraft is within the published (promulgated) range.

Switch on the ADF and adjust volume. Tune the frequency and identify the station using ANT and BFO as necessary. Select ADF on the control panel and note the bearing on the indicator.

Line of Position (LOP) using the RBI With the help of the information we get from our instruments, we are now able to determine the line of position along which our aircraft is positioned. To draw this LOP on the chart we need the QDR or the QTE. Assume the aircraft is on a heading of 015ºM:

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The relative bearing from the indicator is 340° The QDM is the relative bearing plus the heading 340 + 015 = 355° The QDR is the reciprocal 175°

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Line of Position (LOP) using the RMI An RMI solves the bearing automatically. The RMI provides continuously QDMs and QDRs. Magnetic Bearings can only be used on charts that are oriented to Magnetic North. The beacons on most instrument charts have the direction of magnetic north attached with an arrow.

VOR

VOR

ADF ADF

33 N3

6E

30W

24

21S 15 12

Assuming that the single pointer is the ADF:

The QDM is 017 The QDR is 197

Homing The ADF needle always points towards the station, and the easiest way to reach the beacon is to constantly fly with the needle pointing to the top of the indicator. This procedure is known as homing. The easiest way to home to a station is to turn the aircraft in the direction of the needle until the needle points to the top of the indicator. This points the nose of the aircraft directly towards the station. Once aimed at the station, any crosswind component will displace the aircraft to either side of the straight track to the station and the ADF needle will swing away from the top of the indicator. The pilot will then have to make a correction of the heading towards the needle in order to continue heading to the station.

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This process will have to be repeated again and again since the crosswind pushes the aircraft away from the straight track. The resulting path to the station will thus be a curved one.

The crosswind component requires the aircraft to turn further and further into the wind in order to continue toward the station. The aircraft must turn until a point is eventually reached where the aircraft is headed directly into the wind. At that point, the aircraft will no longer drift off the direct track and is now heading straight to the station. The actual curved path that results will be different for each combination of crosswind and TAS. A strong crosswind component and low TAS will result in a large deviation. A weak crosswind component and a high TAS will result in a small deviation. Since the actual track over the ground will vary with every wind and airspeed combination, there is no way to ensure that any given aircraft will stay within the boundaries of an airway or approach path when homing. Homing is a very simple but extremely inefficient procedure, because of the uncertain demands on airspace, it is not commonly used. Intercepting a Course To navigate with the help of ADF and NDB:

Visualise your position Intercept the desired course, and Maintain the course to or from the station.

The first step is to visualise your position. Once you have visualised the aircraft’s position, you will be able to intercept the desired course, which in this case is 035° inbound. The second step is to make any turn necessary to the heading that gives you a suitable intercept. Observe the instrument readings during the turn.

Wind Direction

ATPL Radio Navigation 28 October 2003 4-4

Inbound to the Beacon

Now look at the corresponding plan view. The heading of 090° gives you an intercept angle of 55°. The desired QDM is 035° the aircraft will be on track when the RBI indicates a relative bearing of 305° as shown in the diagram below.

ATPL Radio Navigation ©Atlantic Flight Training 4-5

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When the needle is reaching the desired relative bearing, in this case 305° start your turn towards the station and your aircraft will be on the desired inbound track. Compare with the instrument indications. Outbound From The Beacon To intercept a track outbound, follow the same procedures. First of all visualise your position.

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The relative bearing of 100° combined with the magnetic heading of 125° indicates that you are North and East of the NDB.

ATPL Radio Navigation 28 October 2003 4-6

The desired track is 050° outbound. Our intercept angle is 40°. When the relative bearing is 140º we will have reached our outbound course. Observe the instruments’ indications. When the needle has reached 135° degrees, start turning to intercept the outbound course. Look at the instrument. Heading 050º (only in still air, otherwise drift will have to be applied) with relative bearing 180º, now you are on course. Example 1 In order to intercept a specific course:

First you have to know your position relative to the desired course Then you establish a suitable interception angle.

Consider the following situation – to help you to visualise the situation draw a plan and the instrument indications.

The aircraft is on a heading of 340º. The relative bearing to the NDB is 080º. The required course is 090º inbound.

By maintaining a heading of 340°, the aircraft will eventually intercept the 090 course. This would be a rather untidy intercept because a turn of 110º would be required when the aircraft is on track. A more efficient intercept can be achieved by turning onto an initial heading of 360º, for a 90° intercept. A heading of 030º will lead to a 60° intercept with the required inbound course.

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Since the aircraft is on QDR 240, a heading of 060º would turn the aircraft directly towards the station, and the QDM 090 will not be intercepted. Once you have turned to a correct intercept heading, the rule is a simple one. When the angle formed by the aircraft’s heading and the desired course is the same as the angle between the zero mark at the top of the indicator and the pointer, then the aircraft is on the desired course (QDR or QDM). If you are intercepting OUTBOUND, the aircraft is on the desired course when the intercept angle is the same as the angle between the zero mark at the top of the indicator and the TAIL of the needle. To intercept a specific course from an assigned heading with this technique, you have to know the interception angle. For instance, with a heading of 220º and a clearance to intercept QDM 180, the intercept angle is 40°. When the needle is 40° to the left of zero, the track has been intercepted. Example 2 The aircraft heading is 265º and the RBI indicates 005º. You are required to join QDM 240 at an intercept angle of 60º. The first step is always to visualise your position:

What is the QDR? You are east of the station, Which way do you have to turn to make the intercept, left or right?

The course is to the right of the aircraft, so a right turn has to be made for the interception. Which heading will you need in order to intercept the QDM 240 with an intercept angle of 60º? To intercept QDM 240 at 60°, the aircraft should be turned to a heading of 300º. Maintain a heading of 300º, and observe the RBI needle. Since this is a 60º intercept, wait until the pointer falls to 60º left of the zero indication on RBI. To mentally superimpose the RBI needle on the directional gyro is always a good crosscheck of your calculations. The aircraft should be turned a few degrees before the desired QDM is reached. Observe the instruments and initiate the turn a few degrees before reaching QDM 240.The RMI eliminates the need to do any mental calculation. It always displays the QDM under the pointer and the QDR under the tail. The procedure of intercepting QDRs and QDMs is made a lot easier if you maintain a mental picture of where the aircraft is and where you want it to be.

ATPL Radio Navigation 28 October 2003 4-8

Tracking With no crosswind, a direct inbound course is achieved by:

Heading the aircraft directly at the NDB, and Maintaining the ADF needle on the nose of the aircraft.

If there is no drift then the aircraft will home to the NDB. Any crosswind will cause the aircraft to be blown off track. In the cockpit, the ADF needle indicates this as it starts to move away from the top of the indicator. To fly a straight course to the station is called TRACKING. To track to the station, a wind correction angle (WCA) has to be established which compensates for the drift caused by the crosswind. If the exact W/V is not known, then use an estimated WCA obtained from the available information (forecasts, pilots’ reports, etc.). Remember that the higher the crosswind the greater the WCA and, for the same crosswind, slower aircraft will need to establish a greater WCA than faster aircraft. You are established on a course with a wind correction angle to compensate for drift. Observe the instruments and look at where the crosswind is coming from and what affect it is having on the aircraft. If the ADF needle indicates a constant relative bearing while you are maintaining a constant magnetic heading, your wind correction angle is correct and the aircraft is tracking directly to or from the station. A wind correction angle that does not compensate for the present wind will cause the aircraft to drift off course and the ADF needle to show a gradually changing relative bearing. If the head of the ADF needle moves to the right, it indicates that a turn to the right has to be made to maintain the course to the NDB and, conversely, if the head of the needle moves to the left, a left turn has to be made. How large each correcting turn should be depends upon the deviation from the course. A simple method is to double the angle of bearing change. Observe that if the aircraft has deviated 10° to the left, the needle will have moved 10° to the right. To double the angle of bearing change simply means that you alter your heading 20 degrees to the right. Having regained the course, turn left by only half the correcting turn of 20°. That is to say, turn left 10° to maintain the track. This WCA should provide reasonable tracking. In real life the perfect track is difficult to achieve and the pilot will make a number of minor corrections to the heading. This technique is known as bracketing the track.

ATPL Radio Navigation ©Atlantic Flight Training 4-9

The ADF needle will become more and more sensitive as the NDB station is approached. Minor displacements to the left or right of the track will cause larger and larger changes in the relative bearings and the QDM. When passing overhead the NDB, the ADF needle will oscillate then move toward the bottom of the dial and settle down. When close to the NDB do not change heading. Maintain the heading until accurate readings are obtained on the out bound flight. To facilitate the QDR calculations when tracking outbound, you should remember that the QDR is equal to the Magnetic Heading plus or minus the deflection of the tail of the needle. Suppose that the desired course outbound from an NDB is QDR 040 and the pilot estimates a WCA of 10° to the right to counteract the wind from the right. To fix the QDR 040 in a no wind condition is achieved by flying heading 040°. Since we have a right crosswind that requires a 10° WCA, the heading in this case is 050°.

ATPL Radio Navigation 28 October 2003 4-10

NDB Approach The approach chart (plate) shown below is for the Coventry NDB(L) Runway 23.

ATPL Radio Navigation ©Atlantic Flight Training 4-11

To understand the profile required the plate will be split into 3 parts in a similar method to the Cranfield VDF discussed in an earlier chapter. Part 1 - Administration

The top of the chart follows the same profile as the VDF chart. In the top right:

The name of the airfield Coventry The approach to be flown NDB(L) The L indicates that the beacon being used is a locater beacon. Normally, this

beacon is associated with the ILS. The frequency and identification CT 363.50 KHz

On the left:

The minimum safe altitude sectors. In the centre:

The four letter identifier for Coventry The categories of aircraft that can use the approach Airfield elevation, threshold elevation, transition altitude and variation Frequencies in use at Coventry

ATPL Radio Navigation 28 October 2003 4-12

Part 2 - The Plan View

The plan view shows information around the environs of Coventry:

Coventry airfield Coventry NDB (CT 363.5). This is also the Initial Approach Fix (IAF) The prominent obstacles Major airfields (Birmingham) Minor airfields (Baxterly, Bruntingthorpe) Disused airfields

The plan view shows a circle radius 10nm from Coventry. Superimposed on the chart are the tracks to fly:

From the VOR at Daventry (DTY) From the NDB at Lichfield (LIC) The tracks to fly for category A, B, C and D aircraft. Note that the two tracks are

different The missed approach Warnings

ATPL Radio Navigation ©Atlantic Flight Training 4-13

Part 3 - The Elevation View

The profile is complementary to the plan view. The altitude (height in brackets) and tracks are shown on the elevation view. The timing for the outbound (OUBD) legs are shown in a box to the right of the diagram. The missed approach is a written commentary from the Minimum Descent Altitude (MDA) Part 4 - Limits and Other Information

The bottom part of the chart is split into four columns:

The recommended profile The rate of descent and time to the MAPt from the Final Approach Fix (FAF) for

different groundspeeds The Obstacle Clearance Altitude (OCA) or Obstacle Clearance Height (OCH)

which is used in calculating the MDA. The Visual Manoeuvring (Circling) Altitude (VM(C) OCA)

ATPL Radio Navigation 28 October 2003 4-14

The bottom two rows show an alternate procedure and a note as to the lowest altitude the aircraft can start a procedure from.

ATPL Radio Navigation ©Atlantic Flight Training 5-1

Chapter 5.

VHF Omnidirectional Radio Range (VOR)

Introduction Recognised in 1949, by ICAO as the international standard for short-range navigation VOR is the most commonly used beacon in radio navigation. As opposed to the NDB, which transmits a non-directional signal, the signal transmitted by the VOR contains directional information. Two types of VOR will be discussed in this chapter, Conventional VOR (CVOR) and Doppler VOR (DVOR). For the user in the aircraft no difference will be seen in the indications. Principle of Operation Two independent modulations are placed on a VHF frequency, known as the reference and variable phase. The aircraft equipment measures the magnetic bearing of the station by phase comparison of these two waves. Frequency VHF – 108 to 117.95 MHz It is prudent to talk about the allocation of the VHF frequency band in this section. VORs are used for two separate purposes, as Terminal VORs (TVOR) and Airway VORs. These beacons are allocated different parts of the frequency band. To further complicate the allocation Instrument Landing System (ILS) is allocated frequencies in the above range as well.

TVOR Uses the first even decimal and first even decimal + 50 KHz up to 112 MHz

108.00 MHz, 108.05 MHz, 108.20 MHz, 108.25 MHz etc ILS Uses the first odd decimal and first odd decimal + 50 KHz up to 112

MHz 108.10 MHz, 108.15 MHz, 108.30 MHz, 108.35 MHz etc Airways VOR The remainder of the frequency band 112 MHz to 117.95 MHz at 50

KHz spacing Polarisation Horizontal.

ATPL Radio Navigation 28 October 2003 5-2

Emission Characteristics A9W Conventional VOR Reference Signal An omni-directional continuous wave transmission on the VOR frequency. The signal is frequency modulated (FM) at 30 Hz. The polar diagram of this omni-directional signal is circular; meaning that the phase detected by the aircraft’s receiver will be the same on all bearings. This reference signal effectively tells the VOR receiver where magnetic north is. Variable Signal The variable signal is transmitted from an aerial that is effectively a loop and is amplitude modulated (AM) at 30 Hz. As with the ADF, a figure of 8 polar diagram is produced. Unlike the ADF, the “loop” aerial is electronically rotated at 30 revolutions per second. By combining the reference and variable signal the resulting polar diagram is similar to a cardioid, however it does not have a “null” position. This polar diagram is called a “limacon” the French for edible snail. Aircraft Receiver The aircraft VOR receiver splits the received signal into three parts:

The first is connected to the aircraft communications system so that the beacon can be identified

The second and third parts are passed through a filter, this separates:

The FM reference signal The AM variable signal

The 30 Hz FM reference signal is then electronically changed so that a comparison can be made with the AM variable signal. Thus the principle of operation of VOR, bearing measurement by phase comparison.

Bearing Measurement The absence of a null position is compensated for by varying the power relationship between the reference and variable signals. This difference in field strength is graphically illustrated below.

ATPL Radio Navigation ©Atlantic Flight Training 5-3

ABOTH SIGNALSARE IN PHASEON A BEARINGOF MAGNETIC

NORTH

FM REFERENCE30 HZ

AM VARIABLESIGNAL 30 HZ

A

B

C

DTX

ROTATINGLIMACON REFERENCE

REFERENCE

REFERENCE

D90° PHASE DIFFERENCE

= 090 RADIAL

C

B

180° PHASE DIFFERENCE

270° PHASE DIFFERENCE

= 180 RADIAL

= 270 RADIAL

7

4

1

4

The resultant shape is that of an elongated cardioid, called a Limacon. In the diagram above the reference signal has been placed at the four cardinal headings. If the line of the variable phase plots the power curve, then it can be seen that at North it is 4 units of power, West 7 units, South 4 units and East 1 unit. The variable phase is rotated at the same rate as the reference signal is modulated so that at:

North The Limacon is set in the start position (remember that the aerial is rotating at 30 revolutions per second). By comparing the variable signal wave for magnetic north with the reference signal wave it is obvious that the wave diagrams are the same. By looking at the Limacon signal strength we would start North 4, West 7, South 4, East 1. Comparing this with the reference signal whose amplitude varies 4, 7, 4, 1 it can be seen that the two waves are in phase East On East the sequence starts East 1, North 4, West 7, South 4. When drawn as a sine wave and compared with the reference wave it shows that the variable signal lags the reference signal by 90°. Hence we are on a bearing of 090°M. South On South the sequence starts South 4, East 1, North 4, West 7. When drawn as a sine wave and compared with the reference wave it shows that the variable signal lags the reference signal by 180°. Hence we are on a bearing of 180°M. West On West the sequence starts West 7, South 4, East 1, North 4. When drawn as a sine wave and compared with the reference wave it shows that the variable signal lags the reference signal by 270°. Hence we are on a bearing of 270°M.

ATPL Radio Navigation 28 October 2003 5-4

Aircraft Equipment Aerial The aerial is a small, horizontal dipole, designed to receive the horizontally polarised signals transmitted from the ground station. Designed for the frequency band of 108 MHz – 118 MHz the aerial has to be mounted so that it offers 360° reception. It must be shielded from transmissions from the VHF communication radio aerial. Aerials are frequently mounted on the fin of an aircraft. Receiver The receiver compares the reference signal and the variable signal in order to detect the phase difference. The signal from the aerial is filtered through the high frequency part of the receiver and only the signals from the desired VOR station are passed through to the detectors and filters. Frequency Selector The frequency selector switch on the control panel is used to select the required station. The phase comparator compares the phase of the two signals and the difference is fed to the indicator. A special circuitry, within the receiver, detects the identification signal and amplifies it for a speaker or headphones. Some VORs can also transmit “voice”, either radio communication, identification, met-information or other voice transmissions. The receiver panel has a frequency selector knob, a dial indicating the selected frequency and a selector switch with a position for IDENT and Voice. The IDENT position is selected to hear the identification signal of the VOR. The identification is transmitted according to ICAO recommendations and consists of a two or three letter Morse code transmitted at a rate of:

Seven words a minute Is repeated at least once every 30 seconds, and Is modulated at 1020 Hz.

The VOICE position is selected to improve the reproduction of speech, and is selected when the transmission contains voice messages (for instance ATIS), or if the station serves as a regular voice transmitter. Indicators The indicator can be in many different forms, from the simplest to the most complex electronic flight information system. The basic indicator, its parts and functions are covered in the next chapter.

ATPL Radio Navigation ©Atlantic Flight Training 5-5

Monitoring All VOR stations are monitored by an automatic monitor located approximately 50 ft from the transmitter. The monitor performs functions when it detects any malfunction:

It warns the control point It removes the identification and navigation component of the beacon It switches the beacon off in extreme cases

The monitor limits are:

A change in bearing information of > 1° A reduction of more than 15% in the signal strength of either of the 30 Hz

modulations The monitor failing

If the beacon is switched off the standby system will come into operation. Time is needed for the standby beacon to become fully operational so no identification is transmitted until a full changeover is achieved. Terrain Where a VOR has uneven ground, high ground or man made obstacles nearby the VHF wave can be affected. If the terrain causes erroneous indications these are listed in the AIP under the Designated Operational Coverage Designated Operational Coverage (DOC) VOR operates in a range where the signals are line of sight. So the line of site formula can be used to calculate the maximum range a signal can be received. In The AIP the VORs are listed and with this are given a maximum range, altitude and bearings where reliable signals can be obtained. As with the promulgated range for an NDB the VOR should only be used with confidence within the DOC. VORs on the same frequency have to be spaced apart by at least 500 nm to ensure that they do not cause mutual interference. The DOC is applicable for both day and night operations as the VHF wave is not affected by returning sky waves like the NDB. Cone of Confusion Unlike the ADF, which has a cone of confusion of 40° from the vertical, the VOR cone of confusion, is 50° from the vertical. The area above the VOR will give no signal this causes problems with the indicators in the aircraft. It is possible that rapid bearing changes will occur at close range to the beacon making it impracticable to home or follow a radial. The easy option is to fly the required heading through the overhead until reliable indications are received.

ATPL Radio Navigation 28 October 2003 5-6

The cone of confusion is easily calculated by trigonometry.

Radius of the cone of silence = altitude (nm) x Tan 50°

Example An aircraft is flying at 30 000 ft overhead a VOR what is the diameter of the cone of confusion. 30 000 ft = 30 000 ÷ 6080 ft = 4.93 nm

Radius = 4.94 x tan 50° = 4.93 x 1.19 = 5.9 nm The diameter is twice the radius = 11.8 nm

Accuracy A number of sources account for the total accuracy of a VOR:

Site Error Caused by the nature of the terrain or obstacles in the vicinity of the transmitter, course displacement errors are limited to ± 1°. This site error is monitored as stated earlier. Propagation Error Caused by the travel of the signal over terrain or obstructions these errors are in the region of ± 1°. Airborne Equipment Error The tolerances of the equipment in the aircraft. Normally no more than ± 3°.

The normal accuracy of the VOR can be said to be ± 5°.

50° 50°

ATPL Radio Navigation ©Atlantic Flight Training 5-7

Airway Navigation It is possible that the student may be asked to calculate the maximum distance between two VORs. Again this is a matter of using simple trigonometry.

Distance = width of the airway ÷ tan 5° (accuracy of the VOR) x 2

Example What is the maximum distance two VORs on an airway which is

10 nm wide (5nm from the centreline to the edge of the airway) 5 / tan 5° x 2 = 5 / .087 x 2 = 115 nm Where the figures are easy try the 1 in 60 rule. We effectively have a track error of 5 nm caused by a 5° angle error We know that 1° in 60 nm will cause a 1 nm error 5° must cause 5nm, so the distance is twice this = 120 nm

Test VOR Certain airports have VOT transmitters installed these are VOR test transmitters and allow a pilot to check the airborne equipment on the ground. The test can be conducted at any position on the aerodrome:

Tune the VOT frequency Centre the needle on the Course Deviation Indicator (to be discussed in the next

chapter) The bearing indicates 180° with a TO flag 000° with a FROM flag If the indications are not within ± 4°, the aircraft installation should be repaired.

5°

Airway Centre Line

width

ATPL Radio Navigation 28 October 2003 5-8

Doppler VOR CVORs suffer from reflections from objects in the vicinity of the site. It was found that these errors could be reduced if the horizontal aerial dimensions were increased. This was found to be impracticable as the CVOR uses a mechanical rotating aerial and so a new system had to be devised.

The Doppler VOR is the second generation VOR, providing improved signal quality and accuracy. A fundamental change is that the reference signal of the DVOR is amplitude modulated, while the variable signal is frequency modulated. This means that the modulations are opposite as compared to the conventional VORs so the variable signal rotates anticlockwise so as to maintain the same phase relationship at the receiver. Because the frequency-modulated signal is less subject to interference than the amplitude modulated signal the received signals provide a more accurate bearing determination. The Doppler effect is created by “electronically rotating” the variable signal. Circular placed aerials (diameter 44 ft), rotate at a speed of 30 revolutions per second. The diameter of the circle is 13.4 metres, making the radial velocity of the variable signal 1264 m/s. This causes a Doppler shift, making the frequency increase as the signal is rotated towards the observer and decrease as it rotates away with 30 full cycles of frequency variation per second. This results in an effective FM of 30 Hz. A receiver situated at some distance in the radiation field continuously monitors the transmitter. When certain prescribed deviations are exceeded, either the identification is switched off, or the complete transmitter is taken off the air. This is similar to the CVOR. The VOR receiver does not know if it is receiving a signal from a CVOR or a DVOR and the pilot treats both types in the same way. Reference Signal Variable Signal

CVOR Frequency Modulated Amplitude Modulated DVOR Amplitude Modulated Frequency Modulated

ATPL Radio Navigation ©Atlantic Flight Training 6-1

Chapter 6.

VOR Navigation Introduction This chapter will take you through the uses of VOR. Three indicators of the VOR will be discussed:

Radio Magnetic Indicator (RMI) Omni-Bearing Selector (OBS) Horizontal Situation Indicator (HSI)

Radio Magnetic Indicator The RMI, combines the information from the radio navigation instruments with the directional information from the directional gyro. The RMI has two needles, which can indicate both ADF and VOR information. The two needles are usually marked with single and double lines to make it easier for the pilot to identify the stations.

VOR

VOR

ADF ADF

33 N3

6E

30W

24

21S 15 12

There are two small buttons on the bottom of the instrument. These enable the pilot to select either VOR or ADF to be displayed by the needles. The indicator needles indicate exactly the same information whether determined from the VOR or the ADF and they will constantly point towards the tuned station. The RMI card is slaved to the directional gyro, so that the heading of the aircraft can be read directly off the lubber line at the top. In this way, the needles will show the bearing to the ground stations continuously:

ATPL Radio Navigation 28 October 2003 6-2

The tip of the needles will indicate a magnetic bearing to the ground station QDM The tail indicates magnetic bearing from the ground station to the aircraft QDR

When tuned to a VOR, the tail of the needle indicates the VOR radial. In our example, the single needle points to a VOR station, indicating that the aircraft is on radial 195º. The needle marked with a double line indicates QDM 302º (QDR 122º). You should be aware that the bearing registered from the ADF is a magnetic bearing against the magnetic meridian passing through the aeroplane. If there is a significant variation change or meridian convergence between the station and the aeroplane the bearing indicated will not be the same as a QDM. If you fly at high latitudes, find a station with both a VOR and an NDB where there is a marked difference of variation and longitude between the station and your aeroplane. Select both transmitters and feed one to the single needle and the other to the twin needle of the RMI. Will they have the same bearing indications? Omni-Bearing Selector The indicator shown below has three components:

The Omni Bearing Selector A TO/FROM Indicator A LEFT/RIGHT Course Deviation Indicator

When using this instrument the pilot has certain selections that he can make:

OBS Selector The control knob selects the desired magnetic track that a pilot wishes to fly TO or FROM a VOR beacon. In the case above the pilot has selected a track of 100°M.

TO

FR

N S

6 E 12

153

21

24W30

33

OBS

E

E

W

N

ATPL Radio Navigation ©Atlantic Flight Training 6-3

TO/FROM Indicator When the required magnetic track has been selected the TO or FROM arrow will appear showing where the aircraft is relative to the beacon. In the case above the TO arrow is showing. The indication will change as the aircraft passes through the beacon. Course Deviation Indicator The indicator has 4 small dots and one large central dot. Each of these dots represent 2°, with a full-scale deflection of the needle being 10°. The vertical bar moves left or right according to the relative position of the aircraft to the magnetic track selected. With the vertical bar central the aircraft is on the magnetic track selected. In the instrument above the bar shows 3½ dots left. This means that the aircraft has a deviation of 7° from the selected course. To get back to track the aircraft should always be flown towards the needle, in this case left. Warning Flag A warning flag will appear when:

There is a failure of the aircraft’s receiving equipment There is a failure of the ground station There is a failure of the indicator Signals received are too weak or the aircraft is out of range of the beacon

The indications are totally independent of aircraft heading. The instrument shows the aircraft position in relation to the course selected.

TO Flag If the TO flag is showing the number shown by the arrow at the bottom of the instrument (if the vertical bar is central) is the radial of the aircraft FROM Flag If the FROM flag is showing the number selected at the top of the instrument (if the vertical bar is central) is the radial the of the aircraft

Using The OBS

The diagram below shows three aircraft at different positions, each is discussed in turn (Note that the angles shown are exaggerated):

ATPL Radio Navigation 28 October 2003 6-4

Aircraft 1

The aircraft in position 1 would have the indications shown above. The aircraft is flying TO the beacon and must fly left to regain the track. Note that when centralising the needle the aircraft will home to the beacon. When airways flying an aircraft must regain track as quickly as possible. With TO showing, if the aircraft is on track (vertical bar central) it will be on the 280° radial. The lubber line shows this at the bottom of the OBS indicator. We know that the vertical bar shows a fly left situation of 3½ dots, which is the deviation from track selected. 3½ dots are equal to 7°. If we take the radial minus the deviation because the aircraft is right of track we can work out the radial of the aircraft (280 –7) = 273°

TO

FR

N S

6 E 12

153

21

24W30

33

OBS

E

W

N

ATPL Radio Navigation ©Atlantic Flight Training 6-5

With TO in the window the deviation shown by the vertical bar should be:

ADDED to the OBS radial if the aircraft is left of track SUBTRACTED if the aircraft is right of track

Aircraft 2 Where an aircraft is within the area of ambiguity, 10° either side of the perpendicular cutting track, no positive indications will be given. Aircraft 3

The aircraft in position 3 would have the indications shown above. The aircraft is flying FROM the beacon and must fly left to regain the track With FROM showing, if the aircraft is on track (vertical bar central) it will be on the 100° radial. The top of the OBS indicator shows the radial. We know that the vertical bar shows a fly left situation of 3½ dots, which is the deviation from track selected. 3½ dots are equal to 7°. If we take the radial plus the deviation because the aircraft is right of track we can work out the radial of the aircraft (100 + 7) = 107° With FROM in the window the deviation shown by the vertical bar should be:

ADDED to the OBS radial if the aircraft is right of track SUBTRACTED if the aircraft is left of track

TO

FR

N S

6 E 12

153

21

24W30

33

OBS

N

W

E

ATPL Radio Navigation 28 October 2003 6-6

Horizontal Situation Indicator (HSI)

A more modern derivative of the CDI, this instrument is widely used and you will have to be familiar with its presentation and interpretation. As the name suggests, the HSI (Shown above) provides the pilot with a pictorial presentation of the aeroplane’s navigational situation in relation to a selected course as defined by a VOR radial (or ILS localiser beam). It also displays glide slope information, a heading reference and, on many units, a DME range indication. The instrument consists of a number of discrete elements:

1. HORIZONTAL SITUATION INDICATOR (HSI) - Provides a pictorial presentation of aircraft deviation relative to VOR radials or localizer beams.It also displays glide slope deviations and gives heading reference with respect to magnetic north.

2. NAV FLAG - Flag is in view when the NAV receiver signal is inadequate. When a NAV flag is present, the navigation indicator of the autopilot operation is not affected. The pilot must monitor the navigation indicators for NAV flags to ensure that the Autopilot and/or Flight Director are tracking valid navigation information.

3. LUBBER LINE - Indicates aircraft magnetic heading on compass card (10).

ATPL Radio Navigation ©Atlantic Flight Training 6-7

4. HEADING WARNING FLAG (HDG) - When flag is in view, the heading display is

invalid. If a HDG flag appears and a lateral mode (HDG, NAV, APR or APR BC) is selected, the Autopilot will be disengaged. The Autopilot may be re-engaged in the basic wings level mode along with any vertical mode.

5. COURSE BEARING POINTER - Indicates selected VOR course or localiser course

on compass card (10). The selected VOR radial or localiser heading remains set on the compass card when the compass card rotates.

6. TO/ FROM INDICATOR FLAG - Indicates direction of VOR station relative to

selected course.

7. DUAL GLIDE SLOPE POINTERS – Indicate, on glide slope scale (8), aircraft displacement from glide slope beam centre. Glide slope pointers in view indicate a usable glide slope signal is being received. The glide slope pointers will bias out of view if the glide slope signal is lost.

8. GLIDE SLOPE SCALES - Indicate displacement from glide slope beam centre. A

glide slope deviation bar displacement of 2 dots, represents full scale (0.7º) deviation above or below glide slope beam centre line.

9. HEADING SELECTOR KNOB - Positions heading bug (14) on compass card (10).

The Bug rotates with the compass card.

10. COMPASS CARD - Rotates to display heading of aeroplane with reference to lubber line (3) on HSI.

11. COURSE SELECTOR KNOB - Positions course bearing pointer (5) on the compass

card (10) by rotating the course selector knob.

12. COURSE DEVIATION BAR (D-BAR) - The centre portion of omni-bearing pointer moves laterally to pictorially indicate the relationship of aircraft to the selected course. It indicates degrees of angular displacement from VOR radials and localizer beams, or displacement in nautical miles from RNAV courses.

13. COURSE DEVIATION SCALE - A course deviation bar displacement of 5 dots

represents full scale (VOR = ± 10º, LOC =± 2.5º; RNAV = 5NM, RNAV APR = 1 NM) deviation from beam centre line.

14. HEADING BUG - Moved by knob (9) to select desired heading.

ATPL Radio Navigation 28 October 2003 6-8

VOR Navigation

The VOR is a very versatile navigational aid, which forms the basis of the Airway routes structure. It can be used to assist VFR pilots, as the main navigational aid for en-route navigation, a holding aid or also as an approach to landing aid. Let us have a look at the different ways of using the VOR, but first we point out a few important things that we have to do before using the information indicated by the instruments;

Always make sure you are within the coverage area of the VOR stations you plan to use. Checking the official AIP (Aeronautical information publication) or other published en-route manuals can do this. After having turned the receiver on, dial the frequency of the aid, then listen to the identification signal to make sure you are receiving the correct and desired station and that it is “on the air”. Make sure that the warning flag (NAV or OFF) is not visible, indicating that a satisfactory signal is being received and that the aircraft installation is working properly.

Establishing position Using the VOR to find our present position, we need either a VOR in combination with DME, or we can use two VOR stations. By turning the OBS to centre the needle with a FROM indication, we determine the radials on which the aircraft is located. This procedure gives us two crossing position lines, good enough to determine a fix position. Tracking a radial inbound from a present position If we have to fly towards a VOR station from our present position, all we have to do is to turn the OBS to centre the CDI needle with a TO indication and fly a heading equal to the indicated value in the selected course window. The inbound track will be the reciprocal of the radial on which you are positioned. In a no wind condition, a heading equal to the inbound track will take you to the VOR with the CDI needle centred. If there is any crosswind, heading corrections have to be made in order to keep the CDI needle centred. Initially make a small heading correction, if the needle drifts to one side, turn towards the needle, since the needle actually indicates the position of the desired track. Use only small changes in heading at any one time and wait for the needle to move back to centre position. This procedure of changing the heading to stabilise the needle in centre is called “bracketing”. Use small changes of heading and keep the new heading for a while to await needle movement. If we see that the needle remains still at a position off centre, we have found the correct WCA, but a correction will still be needed in order to regain track.

ATPL Radio Navigation ©Atlantic Flight Training 6-9

Intercepting a radial To plan an intercept and follow a specific radial, first determine your position in relation to the desired track. If you are tracking a radial outbound, set the CDI to the desired radial. CDI deflection will now tell you which way to turn in order to make an intercept. The intercept angle will depend on different factors. If ATC wants you to join the new track as soon as possible, you can make an initial intercept of up to 90° and, when the CDI starts to move, you start leading the turn to establish on the new radial. If you are close to the VOR station, the needle will move quite fast. Conversely, if you are far from the station the needle will move more slowly. Aircraft speed will also affect the needle movement. If there are no restrictions regarding the intercept, an intercept angle of 30° or 45° is normally a good alternative. This will be taught during your practical training. If we are to intercept a radial and track it inbound, the procedure will be as above, except that we set the reciprocal of the radial, which is the inbound course. The procedure is otherwise the same. When the selected course has been intercepted, the procedure is the same as described earlier. If you are tracking TO a VOR station and you are to continue on the same course after you have passed the station, when close to the station, needle-movement becomes very erratic. The TO/FROM flags will flicker during the passage of the station and the warning flag (NAV/ OFF) will appear momentarily. This is because of the cone of confusion that is directly overhead the VOR. When tracking along an airway between two VORs, the normal procedure is to switch from tracking FROM one VOR to tracking TO the next VOR when midway between the two facilities. Sometimes the changeover point is specified elsewhere on the route segment. This is because of signals being restricted by terrain or by frequency interference. When this is the case, it will be specified on the appropriate instrument chart. When tracking from one VOR to another, the published radial for the inbound track to one VOR should match the outbound track or radial from the other, but it is not always so. Radials are magnetic tracks from the VOR and the directions of the radials depend on the magnetic variation at the different VORs. The difference between the corresponding radials equals the difference in variation at the position of the two VORs. At high latitudes, convergence of the meridians will also contribute noticeable differences. VOR Approaches VOR approaches may be published for an approach to an aerodrome. The example shown below is for Cranfield.

ATPL Radio Navigation 28 October 2003 6-10

ATPL Radio Navigation ©Atlantic Flight Training 6-11

Part 1 – Administration

As with the NDB the top part of the chart is dedicated to giving the details of:

The Airfield and the type of approach the chart is for The Categories of aircraft that can fly the Approach The Minimum Safe Altitude

Part 2 – Plan View

ATPL Radio Navigation 28 October 2003 6-12

Part 2 shows a plan view of the airfield out to 10 nm. Restricted areas, danger areas and significant obstacles are marked. In the centre of the plan is the airfield with a plan view of the track to be flown. Part 3 – Elevation View

The elevation view is used in co-ordination with the plan view. Timings and altitudes to fly are given (as with all charts QFE heights are given in brackets). The MAPt is the IAF and a commentary of the missed approach is given in a box to the left of the diagram. Part 4 – Notes

The notes given include:

The rate of descent for a given groundspeed The OCA for the approach The OCA for visual Manoeuvring (Circling)

The holding procedure for the approach is given.

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The notes box at the bottom gives the restrictions applicable to the approach.

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Chapter 7.

Distance Measuring Equipment (DME) Introduction DME is a secondary radar providing the pilot with an accurate slant range from a ground transmitter. Normally paired with VOR the combination provides the standard for ICAO short-range navigation systems (know as rho-theta). More recent uses see the DME paired with ILS and MLS to give range from touchdown during a precision approach. Principle of Operation The system works on the principle of secondary radar:

The interrogator on board the aircraft transmitting an interrogation signal, and The ground based transponder (transponder meaning a transmitter that is

responding to an interrogation) The interrogation signal from the aircraft and the response are on different frequencies.

Frequency UHF – 960 to 1215 MHz Emission Characteristics P0N

Aircraft transmits the interrogation signal

DME Ground Aerial Replies

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Aircraft Equipment The airborne unit (interrogator) consists of:

An omni-directional blade aerial A transmitter A receiver A time measuring device, and A tracking unit

The interrogator transmits pulse pairs on the selected frequency. These pulse pairs are spaced by either 12µ seconds (X channel) or 36µ seconds (Y channel).

Note: For the X channel the transponder will reply with a pulse pair spacing of 12µ seconds. The Y channel reply is at 30µ seconds.

When the equipment is switched on, or when new DME channel is selected, the pulse pairs are transmitted at 150 pulse pairs per second (pps). This is the search mode. The equipment will stay in search mode till the equipment:

Locks on (normally 4 to 5 seconds), or 15 000 pulse pairs have been transmitted.

If lock on occurs then the transmitter reduces the PRF of pulse pairs to 24 to 30 pps, this is known as “tracking mode”. If the system transmits 15 000 pulse pairs the PRF will drop to 60 pps until the system locks on. Transponder The ground transponder consists of a receiver and a transmitter. When an interrogating signal is detected, the response is transmitted after a 50 µsecond delay. Response is at a different frequency to that received with the transponder being capable of generating up to 2700 PPS. When a signal is replied to, the ground transponder will reply at a rate of 24 – 30 pps. Frequency Allocation Interrogator and transponder operating frequencies are grouped into pairs, the two frequencies being 63 MHz apart. The airborne interrogator uses frequencies from 1025 MHz to 1150 MHz for transmissions, while the ground based transponder answers on frequencies in two groups, 962 MHz to 1024 MHz (low) and from 1051 MHz to 1213 MHz high). For each airborne interrogation frequency two reply frequencies are allocated, one at + 63 MHz and the other at – 63 MHz. These are the X and Y channels. An interrogation frequency of 1030 MHz will, therefore, have responding frequencies at 1093 MHz and 976 MHz. The responding frequencies at + 63 MHz are referred to as “X” channels, while those at – 63MHz

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are known as “Y” channels. This allows for 252 channels as opposed to the 126 previously available. ICAO recommends the pairing of DME channels with VOR/ILS or MLS. Thus a VOR on 112.30 MHz is always paired with the DME on Channel 70X (1094 MHz interrogation – 1157 MHz reply). A VOR on 112.35 MHz would pair with the DME on Channel 70Y (1094 MHz interrogation – 1031 MHz reply). Each DME channel is identified by a number and a letter (X or Y). The following table is an illustration of some of the available channels with their paired frequencies.

DME Channel VOR/ILS/MLS Paired frequency

20X 20Y 21X 21Y

- 70X 70Y

- 126X 126Y

108.3 108.35 108.4 108.45

- 112.3 112.35

- 117.9 117.95

The channel numbers and paired frequencies can be found in the relevant communications documents. As a pilot you will never select a DME frequency because of the pairing. Even though the system works in the UHF band you will select the paired VHF frequency. The major reason for this procedure is to reduce the workload on the flight deck. Jittered PRF If two aircraft transmit to a DME at the same time. The replies are on the same frequency. If both signals received by the aircraft are the same how can any differentiation of the correct reply be made. Which aircraft is being replied to?

ATPL Radio Navigation 28 October 2003 7-4

The equipment in the aircraft “jitters” the PRF before transmission. This random PRF is unique to the aircraft. When the ground station replies it manufactures exactly the same PRF reply for the aircraft. Any reply taken by the airborne equipment, which does not match the PRF of the initial transmission, is rejected. The responder will now respond to the new rate and since the interrogator PRF is randomly varied, only the responses to that interrogation will have the same random variation of PRF. Within the airborne receiver the ‘tracking unit’ looks for responses around the anticipated time interval that is compatible with the current range from the ground responder. Effectively a gate is created and only responses that arrive within that gate are considered. The receiver then determines a match between the PRF of the response and those that were transmitted. Once this match is achieved, the time difference is measured and, allowing for responder fixed delay, a range is derived. This is tracking mode. Reflected Transmissions The advantage of using secondary radar is that reflected transmissions from the ground or cloud will not be processed by the aircraft equipment, as the frequency of reply is incorrect. Memory If the responding signals are interrupted whilst the system is in tracking mode a memory circuit is activated. The system holds:

The last measured range value, and The receiver gate at the last measured time interval

?

Same frequency used for the reply

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Memory mode can be held for up to 8 to 10 seconds after which the system will return to the search mode. Beacon Saturation Since the ground based responder beacon is limited to a maximum PRF of 2700 pps and interrogations occur at 24 - 30 pps (27 pps average), it follows that up to 100 aircraft may be handled by one DME beacon.

The ground transponder has a set gain level that a signal must break to be replied to. This ensures that receiver noise or other returns that are weak are ignored. In the diagram above:

Signal A Is too weak to break the normal gain level so is not replied to Signal B & D Both signals have broken the normal gain level and so are replied to by the receiver as long as the beacon is not saturated Signal C If the beacon is saturated the normal gain level is raised to a saturation gain level and only the strongest 100 signals are replied to.

Co-location of Beacons As stated in the introduction the DME is usually paired with a VOR to provide the primary short range fixing required by ICAO. Where a VOR/DME transmit the same callsign in a synchronized manner the stations are called “associated”. This means:

The VOR and DME transmitter are co-located In a terminal area where the VOR/DME is used for approach purposes the aerials

are a maximum of 100 feet apart Where the VOR and DME are not used for approach purposes the aerials are at

a maximum of 2000 feet apart.

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Where co-location occurs the identification is synchronized and transmitted 7½ seconds apart. In a 30 second period:

The VOR will ident 3 times The DME will ident once

Where a VOR and DME serve the same area (within 7 nm) they may be frequency paired but the DME will generally use Z as the last letter of the ident.

VOR ident MAC DME ident MAZ

Where a VOR and TACAN are co-located the system is called VORTAC. The VOR uses the DME portion of the TACAN. Where DME is paired with an ILS or MLS the 50 µsecond time delay is gradually reduced to a minimum to allow the DME to read zero when the aircraft passes the runway threshold. Slant Range All aircraft displays will show the value of the measured slant range while some contain an arithmetic unit, which calculates the instant ground speed and time to the station. On most modern installations, you can select “GS” or “TIME” for this purpose. Some indicators show distance, ground speed and time simultaneously. It is important to note that the indications of ground speed and time will only be correct when flying directly towards the ground station. If you fly in any other direction, both the DME indicated ground speed and time to the station would be too low. In this case, only slant distance is correct. DME Navigation

All navigational aids provide the pilot with a position line, depending on the type of radio aid. The position line resulting from the DME, is a circle. When your DME indicator shows 55 nm, you know that you are at a slant range of 55 nm from the station, but you don’t know if you are south, east, north or west of the station. As a result of this, the position line from one DME station alone is only of little help. A radial from a VOR will provide a second position line that could intersect the DME circle at two places. This results in an ambiguity situation as can be seen from the diagram below.

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If the VOR and DME are associated you will have one clearly defined fix position. As an approach aid, the DME will provide, together with the tracking facility, positions like initial approach fix (IAF), final approach fix (FAF) and missed approach point (MAPt) etc. DME Procedures A DME procedure is one that is published for use with a particular ground facility. The most common type of DME procedure is known as “flying the arc”. This procedure requires the pilot to maintain a specific range from a DME, generally between two stated VOR radials.

Slant Range The DME indicates a slant range that is the straight line from the aircraft to the ground station, not the distance along the ground. The true range can be calculated by using Pythagoras’s theorem.

A2 + B2 = C2

Example

An aircraft is flying at 45, 000 ft with an indicated DME of 175 NM.

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The true range is?

45 000 ft = 7.4 nm

Range = 1752 - 7.42

Distance = 174.84 nm This results in a slant range error of 0.16 nm or 0.09%, which is negligible. Problems start when the aircraft is closer than 20 nm to the beacon.

Example

The aircraft is at 30 000 ft, indicated DME 20 nm The true range is?

30 000 ft = 4.9 nm Ground distance = 19.39 nm The error is 0.61 nm or 3%

The slant error is almost negligible at long distances, but increases both with altitude and with decreasing DME distance. If you are using a DME at less than 20 nm range you must apply a slant range correction. Flight Overhead the DME When an aircraft passes directly overhead a DME station, the DME will indicate the altitude of the aircraft in nautical miles. For instance, if the aircraft passes at an altitude of 40 000 ft, the indication will be about 6.6 nm. There is a cone of silence directly above the ground station. However, the arithmetic unit in the aircraft will remember the last computed data and continue to indicate the altitude for some time. Failure Indications If no signals of an acceptable strength are received the system will unlock and will only regain the tracking mode if the correct signal of an acceptable strength is received. The unlock condition is indicated by:

An off flag on rotary indicator A red bar across the face of the digital indicator

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Failure indications are also shown when the equipment is switched on and:

No signal is being received The received signal is below the minimum strength required The aircraft is out of range of the transponder

An off indication is also given when the equipment is switched off. Accuracy The DME is extremely accurate. ICAO prescribes a maximum system error of ± (0.25 nm + 1.25% of the slant range) on 95% of occasions.

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Chapter 8.

Instrument Landing System (ILS)

Introduction The instrument landing system is the primary precision approach facility for civil aviation. A precision approach is defined as an approach where both glideslope and track guidance are provided. ILS signals are transmitted continuously and provide pilot interpreted approach guidance. When flying the ILS approach, the pilot descends with approach guidance to the decision height (DH), at which point he takes the final decision to land or go around. All installations must conform to the standards laid down in ICAO Annex 10 and an appropriate performance category is allocated. Any exception to these standards are published in NOTAMs. Many ILS installations use an associated DME to provide a more accurate and continuous ranging facility than that provided by the markers. ILS installations may also be complemented with a low power NDB, known as a locator beacon, the function of which is to provide guidance, during intermediate approach, into the final approach path marked by the ILS. The ideal flight path on an ILS approach is where the localiser plane and the glide slope plane intersect. To fly this flight path, the pilot follows the ILS cockpit indications. Principle of Operation The ILS consists of the following components:

Localiser The localiser transmitter and aerial system provide the azimuth guidance along the along the extended runway centerline.

Glidepath The glidepath and its aerial system provide the approach

guidance in the vertical pane. Marker Beacons Separate beacons (up to three) along the approach path provide the aircraft with range check points on the approach (discussed in Chapter 9)

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.

5 MILES

3250 FT

MIDDLEMARKER

INNERMARKER

GLIDEPATH TRANSMITTER(MAY BE EITHER SIDE)

LOCALIZERTRANSMITTER

RUNWAY

Frequency The localiser and glidepath operate on separate frequencies:

Localiser VHF – 108 to 112 MHz using the odd first decimals and the odd first decimals plus 50 KHz. 108.10 MHz, 108.15 MHz, 108.30 MHz, 108.35 MHz etc Glidepath UHF – 329.15 MHz to 335 MHz at 150 KHz spacing

(The frequency band allocated is 328.6 MHz to 335.4MHz – These figures do not have to be remembered)

329.15 MHz, 329.3 MHz, 329.45 MHz etc The Localiser and glidepath frequencies are paired. The glidepath being automatically selected when the ILS VHF frequency (the localiser) is selected.

Marker Beacons VHF – 75 MHz

Emission Characteristics A8W The localiser and Marker beacons also radiate an A2A identifier Localiser The localiser transmitter aerial is located in line with the runway centre line, at a distance of approximately 300 metres from the “up-wind” end of the runway. The aerial, which is of frangible construction, may be 20 metres wide and 3 metres high, and consists of a number of dipole and reflector elements. The radio signal transmitted by the localiser aerial produces a composite field pattern consisting of two overlapping lobes. The two lobes are transmitted on

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a single ILS frequency and, in order to make the receiver distinguish between them, they are modulated differently.

COURSE SIDE LOBESPRODUCING SPURIOUS

EQUISIGNALS

1000 FT

90 Hz

RUNWAY

20°APPROX150 Hz

RUNWAY EXTENDED CENTRE LINE NIL DDM

MAIN LOBE

THE COURSE RADIATION PATTERN The lobe on the left-hand side is modulated by a 90 Hz tone. The lobe on the right hand side as seen by the pilot making an approach is modulated by a 150 Hz tone. A receiver located to the left of the centre line will detect more of the 90 Hz modulation tone and relatively less of the 150 Hz modulation. This difference is called DDM (Difference in Depth of Modulation) and it causes the vertical indicator needle of the ILS to indicate that a correction to the right is necessary. Conversely, a receiver right of the centre line receives more 150 Hz than 90 Hz modulation and therefore, the needle will indicate that a correction to the left is necessary. With the needle in the centre the difference in depth of modulation is zero. Because the beam of the ILS localiser is very directional unwanted side lobes are produced. To ensure that the aircraft does not pick up a false localiser signal the basic pattern shown above is covered with a clearance pattern. This changes the localiser signal to the one shown below.

CLEARANCESIDE LOBES

RUNWAYAPPROX 70°

NIL DDMAPPROX 20°

COMBINED COURSE AND CLEARANCE RADIATION PATTERNS

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Localiser Coverage The ILS localiser covers:

± 10° of the centreline to 25 nm range ± 35° of the centreline to 17 nm range

Localizers paired with steep angle Glideslope provide coverage in the following areas. From the centre of the Localizer to distances of:

± 10° of the centreline to 18 nm range ± 35° of the centreline to 10 nm range

The coverage of the localiser in elevation is determined as follows:

First calculate point “P”, which is the higher of a point 600 metres above the threshold and a point 300 metres above the highest point within the approach area.

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Connect this point to the threshold. Draw a line 7° above the horizontal. The resulting shaded area corresponds to the localiser vertical coverage within its

horizontal coverage. The vertical coverage extends from ¾° above the surface up to 7°. The maximum field strength is directed along the centreline out to a range of 10 nm. If an aircraft is outside the coverage of the course and clearance patterns, false localiser signals can be received. In some cases these signals caused by side lobes can give reverse indications. The localiser signals are protected out to a range of 25 nm and up to a height of 6250 ft. The localiser is checked for accuracy out to a range of 10 nm. The above criteria should enable the aircraft to undertake the manoeuvres that are necessary to capture the localiser course at the outer limit of the coverage pattern and to carry out the subsequent descent on the glide path. Glidepath The glidepath aerial is placed 300 metres upwind from the threshold and 150 metres from the centre line. It is placed at the optimum touch down point at which the extension of the glide path intersects the runway. This ensures adequate wheel clearance over the threshold and over any other object or terrain during landing approach. Glide path transmission is in the UHF band on 40 spot frequencies from 329.15 to 335 MHz. UHF is used to produce an accurate beam. The transmission is beamed in the vertical plane in two lobes similar to that of the localiser. The upper lobe has a 90 Hz modulation, while the lower lobe has a 150 Hz modulation. The DDM (Difference in Depth of Modulation) will energise the horizontal needle of the instrument, so as to indicate whether the aircraft is in the 90 Hz lobe or in the 150 Hz lobe. In this way, it gives the position of the centre line of the glide path. The line, along which the two modulations are equal in depth, defines the centre line of the glide path. It is generally 3° from the horizontal, but it could be adjusted to between 2° and 4° to suit the particular local conditions. A glide slope much in excess of 3° requires a high rate of descent and is not common in public transport operations. In the vicinity of the landing threshold, the glide path becomes curved and gradually flattens. This is of consequence when a fully automatic landing is considered. It is one of the reasons why a Category III landing requires the use of a radio altimeter. The siting of the glide path aerial and the choice of the glide path angle are dependent upon many interrelated factors:

Acceptable rates of descent and approach speeds for aircraft using the airfields. Position of obstacles and obstacle clearance limits resulting. Horizontal coverage

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Technical siting problems The desirability of attaining the ILS reference datum 50 feet above the threshold

on the centre line. Runway length.

Glidepath Coverage The coverage in azimuth extends 8 degrees on either side of the localiser centre line, to a distance of 10 nm

The coverage in the vertical plane extends from 0.45θ to 1.75 θ where θ is the nominal glidepath angle above the surface (1.35° to 5.25° for a 3° glidepath). Remember that correct signals are guaranteed only within the approved coverage zones and false indications can be received outside these zones.

Use of the glidepath below 0.45 θ, that is below 1500 ft QFE at 10 nm range (for a 3° glidepath) should only be attempted when the Promulgated Glide Path Intercept Procedures requires the aircraft to fly at this level. The aircraft should never fly below 0.3 θ (0.9° for a 3° glidepath) which is 1000 ft at 10 nm range. When the procedure is designed to join a glidepath from above the pilot must bear in mind that a false glidepath may exist at approximately 2 θ (discussed later in this chapter).

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Airborne Equipment The ILS airborne equipment consists of

A frequency control box A VHF localiser receiver A UHF glide path receiver A 75 MHz marker beacon receiver, and An ILS indicator.

Three separate aerials are included in the installation - one for each receiver. Since all the marker beacons transmit on the same frequency, there is no need for a marker beacon control box. Markers are automatically identified by an audio coded signal, by the related transmission audio tone and a coloured light. Frequency Pairing Both localiser and glidepath tuning are effected from a single control unit. This is possible because of international agreement under ICAO standards. Every localiser frequency has paired glidepath frequency. Since the frequencies are paired, only the correct localiser frequency need be selected, the glide path receiver will then be automatically tuned to the appropriate UHF channel. The VHF navigation receiver panel is used to tune the ILS frequency. Localiser and Glidepath Receivers Once the localiser frequency has been tuned, both the localiser and the glide path receiver are activated, and they send the received signal to the indicator. The two receivers are similar to each other in that they both detect the modulations on the carrier wave. The modulations (90 Hz and 150 Hz) are compared and the Difference in Depth of Modulation (DDM) is measured. This output, which is in the form of a DC electrical signal, is used to drive the pointers on the display. If the aeroplane is on the centre line, the 90 Hz and the 150 Hz signals will have the same amplitude and the indicator needle will be centred. If the aircraft is not on the centre line, one signal will be stronger than the other, dependant upon the position of the aircraft and the resultant DC output energises the needle displacement. To indicate whether the received signals are adequate or not, a warning system is incorporated into the receivers. A red warning flag appears on the ILS indicator if the signal is not reliable. There are separate flags for the localiser and for the glide path signals. ILS Indicator The indicator consists of a dial, similar to the VOR indicator, but with an additional needle. Localiser signals displace the vertical needle, while glide path signals displace the horizontal needle. The same indicator is normally used both for ILS and VOR guidance.

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When the localiser receiver detects that the 150 Hz signal is stronger, then a voltage is fed to the localiser needle that moves it to the left. This indicates that the localiser centre line is to the left of the aircraft on approach. If the 90 Hz signal then the voltage fed to the localiser needle moves it to the right, indicating that the pilot has to turn right to get back on centre line. Full-scale deflection will occur when the aircraft is displaced 2.5° or more from the centre line. In other words, when tuned to a localiser frequency, the indicator is four times more sensitive than it is when being tuned to a VOR (full scale deflection for a VOR corresponds to 10°). Unlike the VOR, which gives the pilot a choice of 360 radials using the omni-bearing selector (OBS), the localiser course is a single fixed beam. Once a localiser frequency is selected, all the needle indications will refer exclusively to the localiser centre line. Consequently, the fact that the instrument is fitted with the OBS has absolutely no significance and rotating it will have no effect on the ILS indications. However, you should always turn the OBS to the correct inbound course when flying the ILS. The localiser indicator does not give any heading information. It only gives information regarding the geographical position of the aircraft. It displays how many degrees the aircraft is displaced from the localiser centre line.

The diagram above shows only the vertical localiser needle, a full illustration with the glidepath needle is below. The important features of the localiser needle are:

Full scale deflection is 2.5° Each dot represents a deviation of 0.5° A warning flag will appear when the signal is unusable

The horizontal needle of the indicator indicates the position of the glide path, relative to the aircraft. The vertical glide path scale on the usual cockpit indicator consists of 5 dots above and below the central position.

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The full instrument has the horizontal glidepath needle, important features being:

Full scale deflection is only 0.7° Each dot represents a deviation of 0.14° As with the localiser a warning flag appears if the signal is unusable

If the 90 Hz signal is the stronger one, the aircraft is above the glide path and the indicator needle is deflected down. This indicates that the aircraft must fly down to recapture the glide path. Conversely, if the receiver detects a stronger 150 Hz signal, the needle will be made to move up. This is known as a fly up indication. The glide path has a total depth of 1½°, making the glide path indicator considerably more sensitive than the localiser indicator. This means that for a full-scale deflection of the needle the aircraft will be at least 0.7° above or below the glide path. A “half-scale” (2½ dots) fly-up indication should be considered to indicate the maximum safe deviation below the glide path. Deviation from the glide path is referred to in terms of dots instead of degrees, in that there are 5 dots above it and 5 dots below it on the instrument. Very accurate control is required when flying down a glide path. A more sophisticated instrument, used to fly an ILS approach, is the horizontal situation indicator. Horizontal Situation Indicator (HSI) With the HSI, the course arrow must be manually aligned with the localiser inbound course and the deviation bar is used for localiser guidance. A scale alongside the instrument provides the glide path position.

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ILS accuracy Up to now we have looked at the ILS as an instrument that provides assistance in approaches to landing. This means that the ILS provides guidance down to a specified height above the threshold. If the visibility at this point is good enough for landing, then the pilot may legally land the aircraft. It is clear that if the existing weather does not permit the pilot to see the visual references at the prescribed minima, the aircraft cannot land. Operators were not happy about the prospect of delaying a flight or wasting time and fuel while holding overhead an aerodrome and waiting for the weather to clear. Therefore, an improved ILS system was required. In order to obtain these improvements, certain limitations of the system have to be considered. The main problems come from bends and scallops in the beams.

Bending of the beam is a single angular displacement from the approach path Scalloping is where the guidance beam direction varies from side to side of the

intended approach path.

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These bends are produced by reflections from obstacles on and around the aerodrome such as airport structures, vehicles, aircraft flying overhead the localiser aerial, etc. The ability to use ILS installations for fully automatic landing has necessitated that ICAO lay down stringent requirements and that constant improvement is made to both ground and airborne equipment. These requirements concern the quality of the transmitted signal data, the suppression of bending of the radio beams by improvement in aerial design in order to reduce unwanted reflections. False Beams Even if all the ILS ground equipment is strictly monitored, there are unavoidable factors to consider. The first of these is the false signals. This problem is particularly associated with the glide path transmission and occurs because of the aerial’s propagation characteristics. The number of such false glidepaths produced at any ILS site depends on several factors, such as the design of the aerial, transmission power, obstacles and other such factors. These false glide paths occur at multiples of the nominal glide path and thus the first occurs at approximately 6° above the horizontal for a glide path of 3° (2 ). There will never be a false glide path below the true one. Therefore, it is the recommended practice that when carrying out an ILS approach, to lock onto the localiser first and then intercept the glide path from below.

Outside the localiser ‘protected area’, it is possible to encounter false localiser beams. The angle from the actual centre line to the false beams will vary with the number of aerial elements. Six elements produce a false beam at approximately 40° and 12 elements at 50° to 60°.

SIDE-LOBES PRODUCINGFALSE GLIDE PATHS

GP NOT CORRECTLYDELINEATED INFINAL STAGES

ILS REFERENCE

RUNWAY

1000 FT

GP AERIAL OFFSETA SAFE DISTANCE

FROM RUNWAY

50 FT APPROX. ABOVE THRESHOLD

VERTICAL COVERAGE

TO 1.75 ABOVE HORIZONTALηAT LEAST FROM 0.45 BELOWη

η BETWEEN 2° AND 4°(NORMALLY 3°)

90 Hz PREDOMINATES

G.L. GLIDE PATH - NIL DDM

150 Hz PREDOMINATES

SIMPLIFIED DIAGRAM OF GLIDE PATH RADIATION PATTERN

ATPL Radio Navigation 28 October 2003 8-12

Localiser Back Beam Some localisers transmit in the opposite direction of the ILS inbound course and the signal can be received when flying behind the aerial. This signal is called the back beam and should normally not be used. Some transmitters, however, are designed to radiate a back beam. This beam can provide a back course approach to the reciprocal runway. It must be noted that when using a back course you do not have the benefit of a glide path. Usually, back beams are less accurate than front beams. They are not checked for accuracy unless they are a part of published procedure. Do not use a back beam unless it has a published procedure.

Note that, when flying the localiser back beam approach, you must be very careful when using the course selector. If you are using a conventional ILS indicator, the localiser needle will give a “fly left” indication when you are left of the centre line and vice versa. In other words, you will experience a reverse sensing. Such reverse sensing will occur regardless of course selector setting. Conversely, if you are flying an HSI equipped aircraft, you will get normal indication (i.e. “fly left” when being to the right of the centre line) if the course selector is set to inbound track on the localiser front beam. If the course selector is set to the back beam course, you will get reverse sensing.

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Above is shown a plate for a back beam approach at Lista in Norway. These forms of procedure are not common in Europe. ILS Performance Categories A system of facilities performance categories has been established to define the capability of a particular ILS system. These categories state that the ILS must be capable of providing guidance from the coverage limit and as follows, for:

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Category I to a height of 60 metres above the horizontal plane containing the threshold Category II to a height of 15 metres above the horizontal plane containing the

threshold Category III with the aid of ancillary equipment when necessary, down to and along the runway.

ILS Operational Performance Categories A similar categorisation exists for operational purposes and it is to these limits that the pilot flies. These categories establish practical weather minima for an approach. As a pilot you must be familiar with the following ILS operational minima.

ILS Cat I DH down to 200 feet, RVR 550 metres ILS Cat II DH down to 100 ft, RVR 350m ILS Cat IIIA DH 0 to 100 ft, RVR 200m ILS Cat IIIB DH 0 to 50 ft, RVR 50- 200m ILS Cat IIIC No external visual reference

Where a category I approach is flown reference is to the barometric altitude, where a category II or III approach is flown reference is to the radio altimeter. In a category I ILS approach, also called CAT I, the pilot may manually follow the ILS indications down to the decision height (DH), which is not less than 200 feet. At that point, if visual contact has been established, the landing can be made. If not, a go-around has to be initiated. Note that the ILS coverage, which is described earlier in this chapter, refers to ILS category I. Category II and III requirements are more stringent. ILS CAT I, although still widely used, is gradually being replaced by CAT II & CAT III facilities. On a CAT II approach, the aircraft must be flown by the autopilot down to the DH. From there, if visual contact has been made, the pilot can make the landing. Otherwise, a go-around must be initiated. A CAT II approach can only be made at an airport that is category II certified. The localiser and glide path transmissions must meet stricter standards than for a CAT I system. The transmissions must be monitored and failure indications must be available in the control tower. In addition, two RVR (Runway Visual Range) transmissometers must be operating on the runway, and extensive lighting requirements must be met. Finally, a CAT II approach requires an aeroplane whose CAT II equipment has been certified by the regulatory authority. Additionally, special training programmes must be certified and conducted for the flight crews. For category III approaches the same criteria as those for CAT II are followed but with additional, more stringent, requirements. This is because the aircraft must be provided with guidance all the way down to the runway. The CAT III approach must be performed by the autopilot.

ATPL Radio Navigation ©Atlantic Flight Training 8-15

The improvement in the ground equipment must be matched by improved performance of the airborne equipment. To this end, operational performance categories have been established. They correspond to the facility performance categories. In accordance with its airborne equipment, an aircraft is certified in one of the listed categories. Naturally, aircraft and airport performance categories are needed to conduct any of the approaches. Remember that, in order to perform a CAT II or CAT III approach, not only must the airport and the aircraft be properly equipped and checked, but so also must the flight crew. Unqualified and untrained pilots are not permitted to carry out a precision approach. Protection Range and Monitoring National and regional frequency plans have been established by the ICAO and are adhered to by contracting states. These plans take many factors into account, such as the sensitivity and selectivity of receivers, the channel spacing and the geographical proximity of transmitters. In this way, interference between facilities is reduced to negligible proportions. Within Europe, the congested radio frequencies have resulted in FM transmissions from aerials that are close enough to allow side band interference to spill over into the ILS frequencies. These can cause random displacement of the localiser so be aware. Monitoring equipment automatically and continuously checks both localiser and glide path transmitters. Whenever a shift or change in the basic transmission is sensed the monitors will take action. If the ILS is category II or III the transmissions must be stopped within 2 seconds. If category I the transmissions will be stopped within 6 seconds. The localiser and glidepath monitors operate when:

The mean course line shifts by:

Category I ± 35 feet Category II ± 25 feet Category III ± 10 feet

The glidepath angle changes by > 0.075θ

3° Glidepath 0.225°

A reduction in power of 50% or more in any transmission If a monitor operates then the standby unit will be used, before this happens:

All radiation will stop The identification will stop For a Category II or III operation the system may allow degradation to a lower

category operation.

ATPL Radio Navigation 28 October 2003 8-16

Use of ILS ILS Identification Because the localiser and glide path frequencies are paired, selecting a localiser frequency automatically activates the glide path receiver so that the corresponding glide path signal is automatically received. The ILS must be identified before use. The identification is transmitted on the localiser frequency and is amplitude modulated by a 1020 Hz (A2A transmission) tone. A two or three letter Morse code transmitted at a rate of seven words per minute. The letter “I” may precede the identification. When an ILS is undergoing maintenance or is being used for test purposes:

The identification will be completely removed, or The coding is replaced by a continuous tone.

In both cases the ILS must not be used. Flying the Localiser When initiating the approach, the localiser indicator shows the position of the aircraft in relation to the centre line and that no heading information is provided. Thus the term “follow the needle” is only valid when flying inbound within the coverage area. For an aircraft on approach, the localiser needle indicates which way the aeroplane should move to regain the centre line. If the localiser needle is to the right, then the aircraft should be flown to the right. To regain the centre line, fly towards the needle. The aim is to fly a heading that will maintain the aircraft on the centre line. If a crosswind exists, a wind correction angle (WCA) will be required and the aircraft heading will differ slightly from the published inbound course. The localiser beam narrows as the runway is approached. Therefore, corrections should become smaller and smaller.

Flying the Glidepath The horizontal glide path needle should be flown in the same way as the localiser needle. To regain the glide path fly towards the needle. The needle is your glide path. If the glide path

1000 FT NOMINAL

LOCALIZERTRANSMITTER

(100 WATTS)

4° ON LONG RUNWAY6° ON SHORT RUNWAY

TOTAL COURSE WIDTHIS 700 FT WIDE AT THRESHOLD

5 DOTS OR FULL SCALEDEFLECTION

ATPL Radio Navigation ©Atlantic Flight Training 8-17

needle is below centre, you are too high and a steeper descent must be initiated. Remember that the closer the aircraft is to the threshold the more dangerous a high descent rate. If the aircraft is not properly established on the glide path then the approach should be stopped. Do not continue to hunt the glide, if in doubt carry out a missed approach. With an angular depth of only 1.4° (0.7° above and below), the glide path needle is three times more sensitive than the localiser is and 15 times more sensitive than the VOR. When following a glide path, the rate of descent is your reference, so the vertical speed indicator becomes important. The vertical speed should be determined before starting the descent on the glide path. Instrument approach plates usually give the rate of descent related to the ground speed of the aircraft during the approach. As a ‘rule of thumb’ or guide, the rate of descent (in feet per minute) may be calculated as half the ground speed, in knots, with a zero added. This is valid for a glide path of 3°. If your aircraft has a ground speed of 120 kts during final approach, you would have to fly the glide path using 600 feet per minute descent rate. As a general rule, use the pitch attitude to control the glide path and the throttle to control the airspeed. When flying the ILS procedure the pilot has to continuously monitor all the instruments, and follow both the ILS needles at the same time. This phase of flight requires a great deal of accuracy and attention. Look at the indications of the needles relative to the aircraft’s position. The same rules apply when using the horizontal situation indicator, but naturally the localiser tracking is simplified. Normally the HSI has a scale along either side of the instrument, on which glide path information is presented. As you have seen, a lot of instruments are involved in an ILS approach: The localiser and glide path indicators, the directional gyro, the airspeed indicator, the vertical speed indicator, and the altimeter. Lately, there has been a tendency to group all these instruments into a single display. ILS Without Glidepath If glide path information is not available, either because of equipment failure on the ground or glide path receiver failure in the aircraft, the ILS automatically becomes a localiser approach. In some cases, ILS installations may be purposely commissioned without glide slope. Since no vertical guidance is provided, localiser approaches are non-precision approaches. They are similar to VOR approaches except for the fact that a localiser course is four times more sensitive than a VOR course. In addition, localiser approaches normally include marker range indicators. The minimum descent height (MDH) for a localiser approach will never be lower than 250 feet, whereas the DH for ILS with glide path can be 200 feet (ILS category I). The next two pages show the approaches to Coventry on Runway 23. The first plate is for a normal ILS approach. The second plate is for the localizer only approach.

ATPL Radio Navigation 28 October 2003 8-18

Note that for a Category A aircraft the OCA 431 feet.

ATPL Radio Navigation ©Atlantic Flight Training 8-19

The OCA on the localiser only approach for a Category A aircraft is 635 feet. The Category A aircraft OCA for the ILS on the previous page is 431 feet. The difference occurs because a

ATPL Radio Navigation 28 October 2003 8-20

precision approach such as an ILS allows the aircraft to a lower DH than the localiser only non-precision approach where a MDA is used. Distance Measuring Equipment It is common to find a DME paired with an ILS frequency to:

Supplement the information given by the marker beacons, or Replace the marker beacons

All information is zero referenced to the threshold. The is protected from other DME services only within the localiser service area described earlier up to a height of 25 000 ft. Rate of Descent (ROD) Unfortunately the rule of thumb is only valid for a 3° glidepath. To calculate the ROD for any other glidepath the 1 in 60 rule can be used:

ROD = θ x groundspeed (miles per minute) x 101.3

Example For a 4° glidepath at a groundspeed of 300 knots, the rate of descent should be: 4 x 5 x 101.3 = 2026 feet per minute

Height Passing on the Approach Using a similar method the height an aircraft should be passing at a range from either the threshold or touchdown can calculated.

Height Passing = θ x Range from touchdown (nm) x 101.3

Example For a 4° glidepath at an aircraft is 4.5 nm from touchdown, at what height should it be passing: 4 x 4.5 x 101.3 = 1824 feet

If range from the threshold is given add 50 feet to your answer as the aircraft is assumed to be passing 50 feet as it passes over the threshold.

ATPL Radio Navigation ©Atlantic Flight Training 9-1

Chapter 9.

Marker Beacons Introduction Marker beacons are radio beacons transmitting their power vertically towards the sky. All marker beacons transmit on the same frequency. The marker beacon can only be received when the aircraft is directly overhead. The marker beacon cannot be used as a tracking aid for navigation purposes. Principle of Operation The transmitted beam forms a polar diagram in the shape of a vertical fan or funnel-shaped lobe. There are four types of markers:

Airway marker (fan marker) Outer marker Middle marker Inner marker

The markers are indicated on the control panel by small lights marked:

A Airways Marker O Outer Marker M Middle Marker

No inner marker indicator is shown. The inner marker is now found on military installations only. The beacon and its limitations will be discussed as military airfield systems can be used for both training and commercial practices. Frequency VHF - 75 MHz Emission Characteristics Amplitude modulated, A2A emission, with different audio frequencies in order to distinguish between the different types of markers.

ATPL Radio Navigation 28 October 2003 9-2

Airborne Equipment All marker beacons transmit on the same frequency and there is no need for a frequency selector in the receiver. Besides the receiver unit, the airborne equipment consists of three coloured lamps and a sensitivity switch. Audio-output is channeled through the audio panel of the nav/com installation of the aircraft. Signals received from the marker beacons are first passed through a 75 MHz filter, then to the RF amplifier and detector. The sensitivity switch (HI/LO) acts as a gain control on the RF-amplifier input. Detector output is activated by whichever audio tone that has modulated the carrier. Three audio filters discriminate audio tones. The passage of the different markers is indicated as shown in the diagram below.

The outer, middle and inner markers are parts of the ILS installation and are used when conducting an ILS approach to an airfield. Remember that the inner marker will only be found at military installations.

Inner Marker The frequency of the modulating signal is 3000 Hz with an audio tone of 6 dots per second. The white indicator lamp will flash on the indicator panel. Middle Marker The frequency of the modulating signal is 1300 Hz with an audio tone of alternate dots and dashes at a rate of 2 dashes per second. The amber indicator lamp will flash on the indicator panel. Outer Marker The frequency of the modulating signal is 400 Hz with an audio tone of continuous dashes at a rate of 2 dashes per second. The blue indicator lamp will flash on the indicator panel

ATPL Radio Navigation ©Atlantic Flight Training 9-3

Position Marker Distance From Threshold

Deviation Allowed

Light Signal Modulation

Outer 3.5 to 5 nm - Blue Continuous dashes

400 Hz

Middle 3500 feet ± 500 feet Amber Alternate dots and dashes

1300 Hz

Inner 250 feet ± 25 feet White Continuous dots

3000 Hz

Since the transmission pattern is fan-shaped, the horizontal area of reception will depend on aircraft altitude. Where the aircraft is on the glidepath the distance of travel where the beacon will be received is:

Outer Marker 2000 feet ± 650 feet Middle Marker 1000 feet ± 325 feet Inner Marker 500 feet ± 160 feet

With airways markers, in order to get a more precise indication and to avoid receiving ILS markers at high altitude, the ILS Marker sensitivity switch on the panel is set to low. Airway Marker The airway marker is, as the name indicates, used while route flying along airways. It is used:

To identify certain fixes along routes where there are no other means of

establishing the fix. It can be used over mountainous areas where it is difficult or impossible to

receive other navigation aids than the one being tracked To supplement an NDB providing vertical cover above the cone of silence.

Some markers, usually modulated with 3000 Hz, are used to mark important points such as significant positions in a noise abatement procedure. Airway markers are becoming rare, as are the inner markers of ILS. Ground Installation Airway markers are gradually being phased out but can be found along airways in order to establish accurate reporting points. In areas with poor radio-coverage, such as mountainous areas, airway markers can provide a point source fix. With the sensitivity switch set to high, these markers can be received at 50 000 ft. The outer, middle and inner markers are parts of

ATPL Radio Navigation 28 October 2003 9-4

the ILS Installation and are all installed along the extended centre line in the approach end of the runway. The outer and middle marker are installed no more than 75 metres from the extended centreline. The inner marker is installed no more than 30 metres from the extended centreline. Glide-path crossing heights at the different markers are published on the appropriate approach plate for the actual ILS approaches. When on the glide slope, outer marker crossing height will be approximately 1500 to 2000 ft dependent on the glideslope angle. The purpose of the outer marker is to provide height, distance and ‘equipment functioning’ checks to aircraft on intermediate or final approach. The middle marker will be crossed at approximately 200 ft aal, which is close to decision height for a Category I ILS approach. The purpose of the middle marker is to indicate the imminence, in low visibility conditions, of visual approach guidance. The purpose of the inner marker is to indicate to the pilot that the threshold is about to be passed, and the height will be the lowest decision height applicable in Category II operations.

ATPL Radio Navigation ©Atlantic Flight Training 10-1

Chapter 10.

Microwave Landing System Introduction ILS has served as the primary precision approach and landing aid since the last war. Since the mid 1980’s, the limitations of the system prompted ICAO to develop a replacement system to fulfil the needs for future aviation. The limitations of the ILS are:

Procedurally, ILS limits aircraft to long straight-in final approaches of at least 7 miles. This creates potential airspace conflicts in multi-airport environments and constrains the number of approach paths that can be provided. Each ILS provides only one approach path for one aircraft at a time.

Part of the ILS guidance signal is formed by a direct and ground reflected signal requiring a significant level of site preparation. ILS can only be installed at locations where site preparation is practical.

ILS is limited to 40 frequency channels constraining the number of sites that can be allocated a frequency in a given geographical area.

The ILS frequency band suffers from interference from high power FM transmitters operating in adjacent bands. Aircraft receivers are equipped with FM filters, which narrow the band of reception and reduce this ‘noise’ interference but it is still a limitation. At some airports interference can cause the ILS to receive local radio broadcasts.

ILS is sensitive to signal diffraction and blockages caused by ground traffic, necessitating the use of large protected areas on the airport surface (critical areas). Within these areas the ground movement of vehicles and aeroplanes must be prohibited. This reduces the effective capacity of the airfield when low visibility operations are being conducted.

These limitations led to the development of a microwave landing system. Parallel to the development of MLS, the civilian use of satellite based Global Positioning System (GPS) was also under development, both as an en-route navigation aid and, with augmentation systems, as an approach aid. The development of GPS is now so advanced that, in some countries, further development and installation of MLS has been abandoned. In practice, this chapter describes a system that you may not encounter in your career as a pilot. Principle of Operation The MLS system is a precision approach system that provides the pilot with highly accurate azimuth, elevation information. It also utilises a precision DME (DME/P) which provides highly accurate ranging information. The system is also capable of transmitting other types of information to the aircraft such as station identification, system status, runway information and weather.

ATPL Radio Navigation 28 October 2003 10-2

Frequency SHF – 5031 to 5090.7 MHz. Spacing 300 KHz giving 200 channels Polarisation Vertical Ground Installation A completely digital system that is not influenced by weather or other common sources of disturbances. The system allows for several approach paths, both in azimuth and elevation at the same time. As with visual approaches, MLS lets the air traffic controller clear the aircraft for curved approach paths, with a straight-in final segment being as short as 1.5 N.M. This leads to a significant reduction in air traffic delays. The ground installation consists of the following three main elements:

Azimuth (AZ) Elevation (EL), and Precision DME (DME/P). Some installations can also have Back Azimuth (BAZ)

Azimuth Coverage The azimuth (AZ) part of the installation can be compared with the localizer of the ILS but it provides a much wider area of information; up to 40º on each side of the extended centre line.

ATPL Radio Navigation ©Atlantic Flight Training 10-3

The AZ is provided out to 20 nm while the BAZ is provided to 5 nm (ICAO minimum). The azimuth coverage is:

± 40° of the runway centreline out to 20 nm The vertical coverage of the beam is 0.9° to 15° The beam is no more than 4° wide

Elevation Coverage The elevation (EL) part can be compared with the glide path of the ILS, the main difference is that the pilot can choose the desired glide path angle (up to 15º).

APPROACHELEVATIONANTENNA

APPROACHAZIMUTHANTENNA

45 m (150 ft)

45 m (150 ft)MLS DATUM

POINT

THRESHOLD

40°

40°

37 km (20 NM) CL

APPROACHDIRECTION

LATERAL COVERAGE ADDITIONALCOVERAGERECOMMENDED

6000 m (20 000 ft)

600 m (2 000 ft)

30°20°

15°

2.5 m (8 ft)

APPROACHAZIMUTHANTENNA

0.9°

HORIZONTAL

37 km (20 NM)

VERTICAL COVERAGE

COVERAGE OF LOCALIZER EQUIVALENT

ATPL Radio Navigation 28 October 2003 10-4

The elevation coverage is:

± 40° of the centreline to 20 nm The aerial scans vertically from 0.9° to 7.5° above the horizontal (most systems

can scan to 15°) The beam is no more than 2.5° wide

DME/P The DME/P is an integral part of the MLS system. The DME/P signal defines two operating modes:

Initial Approach (IA) Final Approach (FA)

.

APPROACHELEVATIONANTENNA

APPROACHELEVATIONANTENNA

MLSDATUMPOINT

75 m (250 ft)

37 km (20 NM)

EQUAL TOAPPROACH AZIMUTHPROPORTIONALGUIDANCE SECTOR

C/L

APPROACHDIRECTION

LATERAL COVERAGE

THRESHOLD

6000 m (20 000ft)

DATUMPOINT

2.5 m (8 ft)

75 m (250 ft)

7.5°

0.9°

HORIZONTAL

37 km (20 NM)

ADDITIONALCOVERAGERECOMMENDED

VERTICAL COVERAGE

COVERAGE OF GLIDE PATH EQUIVALENT

ATPL Radio Navigation ©Atlantic Flight Training 10-5

The IA mode is designed to give improved accuracy for the initial stages of approach and landing. The FA mode provides substantially improved accuracy in the final approach area. The DME/P coverage is at least 22 nm from the ground transponder. The interrogator does not operate in the FA mode at ranges greater than 7 nm from the transponder site, although a transition from the IA mode may begin at 8 nm from the transponder. These ranges assume that the transponder is situated beyond the stop-end of the runway at a distance of 2 nm from the threshold. Back Azimuth The back azimuth (BAZ) is installed to provide navigational guidance for precision departures and for missed approach procedures. In practical installations, the coverage in the horizontal plane can vary according to local conditions and needs and it does not have to be symmetrical on each side of the centre line. Signal Transmission Format The AZ and EL elements transmit on the same frequency while the DME uses a paired channel in the UHF band. The format of the digital signal is very flexible and the information from the different elements can be sent in any desired order. Each group is started by a preamble, which tells the processor in the receiver which functions are being sent. As soon as one group has been decoded, the processor is ready and waiting for the next element. There are two types of signals sent; basic data and auxiliary data:

Basic Data are associated directly to the operation of the landing guidance system. Station identification is a part of the basic data. Auxiliary Data is other data used for siting information and other information not directly related to the guidance system.

Measuring the time difference between successive passes of the highly directional fan shaped beams derives the angular measurements required by the aircraft. Distance is given by the DME/P. By using a system called time division multiplexing all information required by the aircraft is accommodated on the same channel. This means that an aircraft can decode the incoming signal in a sequential manner. Each function is a separate entity within the format and is identified by a preamble. This preamble sets up the receiver processing circuits which then decode the remainder of the function transmission. Once the decode is complete, the receiver waits for the next function preamble and the process is repeated. In the diagram below the approach azimuth signal is explained. As well as information regarding the position of the aircraft, extra information can be carried by this signal.

ATPL Radio Navigation 28 October 2003 10-6

Each angle function transmission consists of four elements:

The preamble consisting of a synchronising code plus a function identity code. A series of pulses for azimuth guidance The “TO” and “FRO” angle scan Two pulses which give a system check

The process then starts for the flare information if the aircraft is close to the ground. Time Reference Scanning Beam Angular Measurement in Azimuth and Elevation The aerial transmitting the AZ beam, forms a vertical narrow fan shaped beam, which is scanned from one side to the other and back at a constant angular velocity.

ATPL Radio Navigation ©Atlantic Flight Training 10-7

The total scan of the beam lasts 9000 microseconds:

4000 microseconds for the “TO” scan 1000 microseconds resting time 4000 microseconds for the “FRO” scan

In the diagram above the aircraft is 15° left of the centreline. The “TO” scan is received after x microseconds microseconds. The “FRO” scan will occur after y microseconds. The measured intervals of time give the azimuth the aircraft is on. Vertical position (EL) is calculated exactly in the same way as the horizontal except for the scanning beam moving in the vertical plane, up then down. Normally the horizontal AZ- scan is repeated 13 times per second, while the vertical EL - scan is repeated 39 times per second. Airborne Equipment The aircraft receiver measures the time between the passing of the “TO” and “FRO” scans of both the AZ and the EL elements. From these times both azimuth and elevation angles can be determined and, when coupled with a range measurement from the DME/P, a three-dimensional aircraft position can be determined.

ATPL Radio Navigation 28 October 2003 10-8

In its simplest form this position can be compared with a planned approach path and, if not on that path, can be used to create an error signal, which can be used to drive the conventional ILS indicator to show displacement from the selected azimuth and glide path approach. The conventional ILS indicator is used since it is also required for conventional ILS approaches. The indicator is, therefore, multi-mode. More sophisticated, computerised systems would allow the full potential of MLS to be realised, making it possible to follow curved and segmented approaches. If the DME/P is not available, the system still provides an ILS look-alike approach. Accuracy When used with a Category III system:

In azimuth ± 20 ft In elevation ± 2 ft

ATPL Radio Navigation ©Atlantic Flight Training 11-1

Chapter 11.

Radar Principles and the Cathode Ray Tube Pulse Techniques and Associated Terms In the following the uses of radar are discussed. For you to be able to understand how these various types of radar operate you will need to refresh and expand some fundamentals and expressions related to radar. Chapter 1 gives some of the basic principles of radar. This chapter is designed to expand on those principles. Firstly a review of some of the used:

Frequency f Hertz Hz Wavelength λ metres m Speed of light C metres per second m/s Time intervals – microseconds ms. microseconds ms Pulse Width PW microseconds ms Pulse repetition interval PRI microseconds ms Pulse repetition frequency PRF pulse per second pps Duty cycle DC no units Radar mile microseconds

The Components of a Radar Unit A radar unit will consist of:

A transmitter An aerial A receiver (and possibly a receiver aerial) A timebase A display

ATPL Radio Navigation 28 October 2003 11-2

We will take a look at each of those elements in turn – in simple terms.

Within the transmitter, the timer unit triggers a supply of RF signal to the modulator. The modulator forms the pulses and passes these to the magnetron where they are amplified to a very high energy level. This high energy pulse is sent through a waveguide via the T/R switch to the aerial where it is then radiated outwards. The receiver is isolated from the aerial to ensure that there is no damage from this high energy pulse. After the pulse has passed the T/R switch, the switch recovers and the receiver, using the same aerial, waits for an echo of the energy returning, after it has bounced off a target. The Timebase The timebase is connected to the transmitter and receiver and knows when a pulse has been sent and when an echo has been received. The time between these two events is measured and, using the speed of an electromagnetic wave a range is determined. The Display In simple radar systems, the timer and display are a single unit known as a ‘plan position indicator’ or PPI. In many modern applications, however, the information from this timer is processed and sent, with other information, to a suitable display. This is particularly evident in modern Air Traffic Control systems. The Transmitter Like other transmitters, the radar transmitter consists of a RF generator, a modulator and an amplifier. The RF generator creates the transmission frequency and the modulator creates the pulses of energy.

Transmitter T/R Switch Receiver

Modulator Master Timer Indicator

ATPL Radio Navigation ©Atlantic Flight Training 11-3

Choice of Frequency The choice of frequency (wavelength) is governed by a number of factors as follows:

Attenuation Attenuation due to intervening weather can be high so returning signals are very weak. Target Size The relative size of desired target must be considered. Smaller targets will require shorter wavelengths in order to create reflection of energy. Aerial Size If space is limited and aerial size is restricted, shorter wavelengths give narrower beams. Pulse Length The energy content of a pulse is increased if the number of cycles transmitted during the pulse is increased.

The following frequencies are commonly used:

1000 MHz 33 cm Long Range Surveillance 3000 MHz 10 cm Surveillance Radar & Approach Radar 10 000 MHz 3 cm Approach Radar

The modulator creates pulses of the desired length and at the desired rate. Factors affecting the choice of pulse length include:

Minimum Detection Range Since the aerial is common to both transmitter and receiver, it is important to protect the sensitive receiver from the high power pulse. The receiver is therefore ‘disconnected’ from the aerial during the transmission of the pulse (and for a short interval after). Range Discrimination The ability to detect separate targets, which are on the same bearing (azimuth) and are close together, is dependent on pulse length. For example, if a pulse length is 4 microseconds (4ms.) its physical length will be 1200 metres. If two targets lie on the same bearing and are within 600 metres of each other, they will both be illuminated at the same time and their echoes would be merged at the receiver. If the two targets are so close that the beamwidth covers both aircraft then only one target will be seen. Range The longer the pulse the greater the energy content and therefore a greater range is possible.

ATPL Radio Navigation 28 October 2003 11-4

Choice of PRF is also affected by a number of factors including:

Design Maximum Range The transmitter must remain ‘silent’ while the receiver is ‘listening’ for echoes. If the design maximum range is 200 nm the receiver must be allowed to ‘listen’ for the period of time from when a pulse has been transmitted until it can go 200 nm and then return. That is a round trip of 400 nm which would require a silent period of 2473.3 ms. This would be the minimum PRI (pulse repetition interval) but in practice the minimum PRI would be increased to allow for receiver recovery time. The PRF is the inverse of the PRI so that, if the PRI was 2500 ms the PRF will be 400 PPS. Data Acquisition If the pulses are of short duration (1m or less) it takes up to six echoes to cause the radar display to show the target echo. The PRF must be sufficiently high to allow for this to be achieved in the period of time that the effective part of the aerial is pointing towards the target.

The Aerial A highly directive aerial system is necessary in order to concentrate the transmitter power and increase the effective pulse power and provide azimuth information. The systems most commonly used consist either of a wave-guide horn and parabolic dish or a flat plate “phased array” system. Either system is designed to focus the radiated energy into a narrow beam.

The radar pulses are sent through the wave-guide horn and reflect from the parabolic dish. This acts very much as the reflector in a car headlamp. Beamwidth The polar diagram from such an aerial assembly will appear with a main lobe and a number of smaller side-lobes. The side-lobes are not desirable and may, on some systems, be suppressed by modification of the reflector or by use of suppression systems.

ATPL Radio Navigation ©Atlantic Flight Training 11-5

The beam width is the angle contained between the ½ power points on the polar diagram. This determines the radar’s ability to discriminate between targets that are close together and at the same range. If, for example, two targets are at a range of 60 nm and are separated by 1 nm, a beam of more than 1° will allow both targets to be ‘illuminated’ at the same time (1 in 60 rule). The reflections from these targets will merge at the receiver and they will appear as one echo on the display. As long as a target is within the beam, it will be illuminated and will continue to paint an echo. All targets will therefore show as having an azimuth dimension equal to the beam width irrespective of the physical size of the target. For a parabolic reflector, the beam width can be calculated from the following relationship:

Beam width° = 70λ ÷ d

Where λ is wavelength of the transmission and d is the dish diameter. Remember that both λ and d must have the same units.

Example To get a 1° beamwidth at a 25cm wavelength requires a dish diameter of? 1 = (70 x 25) ÷ d d = 70 x 25 d = 1750 cm = 17.5 m

On 10 cm radar the same beam width is possible at with a 7 metre dish. 1 = (70 x 10) ÷ d

d = 70 x 10 d = 700 cm = 7 m

In order to provide coverage, the aerial is rotated in azimuth so that the beam is also rotated. The direction in which the aerial is pointed when a target is ‘illuminated’ provides the azimuth (or bearing) information. This direction is relayed to the display by an electronic link, which may be either analogue or digital. The rate of rotation of the aerial must be matched to the criteria that affected the selection of the PRF. It must be sufficiently high to allow for target information to be renewed at short enough intervals of time while, at the same time, it must be

ATPL Radio Navigation 28 October 2003 11-6

slow enough to allow sufficient echoes to be compared in order to allow the display to show the target. The Receiver This unit is designed to detect the extremely low energy signals reflected from a target. The receiver is therefore extremely sensitive and has a very powerful amplifier circuit. It must be protected from the very high energy of the transmissions from the same aerial and is therefore electronically ‘switched off’ during transmission. This means that the receiver is dead during transmission and for a short period afterwards (known as receiver recovery time). The minimum range at which a target can be detected is governed by the duration of time that the receiver is inactive. Echoes from the target, after being detected and amplified, are sent to the display. In the case of ATC radar, the echoes will include returns from fixed objects on the ground. These may well hide the returns from aeroplane targets. In order to remove these targets, a circuit known as ‘Moving Target Indicator’ (MTI) is introduced. If the target is moving radially towards or away from the aerial, the echo pulses will be different in that the radio frequencies within the pulses will be increased if the target is moving towards the aerial and decreased if moving away. This is known as ‘Doppler’ effect. Stationary targets will not be affected by this phenomenon and the receiver circuit can be designed to reject all signals that do not exhibit a change in frequency. Unfortunately, if a target is moving at a tangent to the radar, there will be no radial motion and no Doppler effect. The MTI circuit could reject such targets unless measures are taken to counteract this problem. In modern units this is achieved by varying the transmissions. Timer – Cathode Ray Tube In its simplest form, the timer is combined with the display and consists of an electron gun and screen assembled into a unit known as a Cathode Ray Tube (CRT). This is illustrated below. The elements of this unit have the following functions:

ATPL Radio Navigation ©Atlantic Flight Training 11-7

FIRSTANODE

THIRDANODE

HEATER/CATHODE/GRID ASSEMBLY

SECONDANODE

X PLATES

Y PLATES

AQUADAG-WALL ANODE(INTERNALLY CONNECTED

TO THIRD ANODE)

FLUORESCENTSCREEN

Cathode When heated the cathode emits electrons. Due to the great heat required to expel the electrons from the parent metal tungsten is generally used.

Grid The annular grid restricts the flow of electrons. By applying a negative voltage the electron flow can be:

Cut off if the grid is at a maximum negative voltage A maximum if the grid has no voltage applied

The grid controls the brilliance of the CRT. Focussing Assembly The focussing assembly consists of 3 anodes at different potentials. These anodes both focus and accelerate the electrons. The electrons are attracted at high speed to this group of annular anodes which are all at a high positive potential with respect to the cathode. The higher the potential used the higher the electron velocity. Due to this high velocity large numbers pass through the apertures which means the emerging beam has enough energy to strike the screen at the end of the tube. The potential in anodes 1 and 3 are constant with the potential in anode 2 being variable. By using a variable potential in anode 2 the focal length of the beam can be adjusted.

ATPL Radio Navigation 28 October 2003 11-8

Electron Beam Deflection To write usable information on the screen the beam must be deflected in a controlled manner. Two plates are set in the CRT, the X for horizontal scanning and the Y plates for vertical scanning. Screen The screen is coated with a chemical salt that gives a fluorescent glow when the electron beam strikes it. When the electrons strike the screen, the heat produced allows electrons from the chemical screen to be freed (called secondary emission). This forms a cloud of electrons which can inhibit the energy of the electron beam and cause a loss of brilliance. To remove the secondary emission the screen wall is covered with a carbon coating – Aquadag. The aquadag is charged to a high positive potential which attracts the secondary emissions. The screen wall is also known as the final wall anode. Time Base In the CRT, the stream of electrons, striking the interior surface of the screen, causes the screen to fluoresce at the point of impact. If the electron stream is deflected, the spot will leave a trace image. Assume that you are looking at a radar screen below.

If a high negative electrical potential is applied to plate X1, the electron beam is held to the left at plate X2. This equates to zero range. If this negative potential at X1 is now changed at a constant rate from negative to positive the electron will move towards X1. The spot will move from the left to the right at a constant speed leaving a straight line traced behind. Knowing the speed of travel allows us to calibrate across the scope for range. This can also be done for the Y plates allowing us the possibility of moving the beam around any part of the screen.

ATPL Radio Navigation ©Atlantic Flight Training 11-9

If the frequency at which the potential applied to the plate is made to match the PRF then the time taken for a single movement of the spot from X1 will match the interval between pulses and is therefore compatible with the maximum range. The trace left by the spot is called a time-base. Saw Tooth Voltage Once the electron beam is fully displaced the right hand plate X1 is made rapidly negative. This causes the electron beam to “fly back” to X2. If nothing was done in this fly back period, a visible trace would be left on the screen. During the fly back a high negative voltage is applied to the grid which stops any electron flow. This ensures that there is no visible trace left on the screen. This is known as a “saw tooth voltage” and is illustrated below.

+

0

-

0

-VE

SAW-TOOTHVOLTAGE

GRIDBIAS

VOLTS

TIME

CUT-OFF BIAS

ELECTRONS DO NOT FLOWDURING FLY-BACK PERIODS

VARIABLE BIAS(BRILLIANCE)

ELECTRONS FLOW

Most radar timer/display CRTs use a rotating time base. In this the time base is rotated by varying the potentials, in sequence, to the deflector arrangement. Targets are now shown by causing the spot to become brighter. This is achieved by connecting the receiver output to the grid and causing the flow of electrons to be increased momentarily. If the time base is made to rotate in sympathy with the aerial then bearing information can be derived. Radar Performance Radar uses frequencies that are normally in the higher bands of the electromagnetic spectrum. Normally propagation follows a direct wave path. In general, the range of radar is

ATPL Radio Navigation 28 October 2003 11-10

therefore ‘line of sight.’ Radar is subject to certain factors, other than those of design, which affect its performance as follows:

Atmospheric Conditions At the very high frequencies used, super refraction caused by inversions of temperature and/or humidity (Anaprop), may cause the direct wave range to be considerably increased. It is possible for echoes to return from a range greater than the design maximum range and to appear on the screen as false targets at any range. Sub refraction will cause poor radar performance at the upper range limits. Weather Rain and snow will attenuate the radar signal and will cause at least reduced performance and possibly even blind sectors. Target Size, Shape and Aspect As previously mentioned, the size and shape of the target will have a tremendous effect on its ability to reflect radar signals. How it appears to the radar is also important. A B747 head on is much harder to see than one that is side on and radar has the same difficulty with targets of differing size.

By the same reasoning, a flat surface at right angles (to the direction to the radar aerial) will produce a much stronger return than a similar sized curved surface.

Secondary Radar The principles of operation of primary radar and some of the factors which affect a radar’s performance have been illustrated. Some of the effects can be minimised by using Secondary Radar techniques. The principle of measuring range from a time delay is still applicable, but the target plays an active role. The interrogating radar unit sends out a pulse (interrogation pulse). When this pulse is detected at the target, it triggers a transmitter to respond, sending a signal back to the interrogator. This signal will be stronger than an echo, will not be dependent on how well the target has reflected the energy and could be coded with additional information.

ATPL Radio Navigation ©Atlantic Flight Training 12-1

Chapter 12.

Ground Radar Introduction In today’s Air Traffic Control systems, the role of radar is crucial in allowing for the safe and efficient controlling of an ever-intensifying air traffic density. To provide for the needs of this task the differing Air Traffic Control environments demand different performance parameters from radar. The main types of ground radar can be summarised as follows:

Surveillance Radar

Long range surveillance radar TMA surveillance radar Aerodrome surveillance radar

Secondary surveillance radar (SSR) – discussed in Chapter 13 Precision approach radar Surface movement radar Weather radar – a meteorological service

Long Range Surveillance Radar The radar has the following properties:

A range capability of 200 NM to 300 NM An ability to penetrate intervening weather The ability to detect small targets out to maximum range Moderate target discrimination capability in range and bearing.

Using two radar systems generally fulfils these needs:

Primary Radar Wavelength of 50 cm Pulse length 4 ms PRF 270 PPS Horizontal beamwidth 1.7° Aerial rotation 5 RPM.

Secondary Radar This is provided as a complement to the primary radar,

improving the possibility of detecting targets at long ranges and allowing for the identification of co-operating targets.

ATPL Radio Navigation 28 October 2003 12-2

Terminal Surveillance Radar This radar provides separation between aircraft within the terminal area during transit, approach and departure. It may be used to provide a radar approach. The service is provided by primary radar with the following characteristics:

Ranges up to about 60 or 80 NM Ability to refresh the target information at short intervals Ability to penetrate intervening weather Good target discrimination properties Good accuracy.

The radar has the following properties:

Wavelength of 25 cm Pulse length 3.9 ms PRF 350 PPS Beamwidth 1.2° Aerial rotation rate 8 RPM.

An SSR element is also normally used in the terminal surveillance radar environment. Surveillance radar displays for long range and terminal radars are normally processed and combined with the information from the primary radar. The superimposing of the SSR information on the primary display shows the controller a complete situational picture of the relevant airspace on an easily viewed screen. Aerodrome Surveillance (Approach) Radar Where provided, the aerodrome surveillance radar is normally a short-range (25 nm) primary radar which is capable of being used to provide guidance during the initial, intermediate and even final approach phases of the flight. As such it requires the following:

Very accurate range and bearing Excellent target discrimination Rapid refreshing of the information.

The properties of the radar are:

Wavelength 10-cm which allows a very short pulse length to be produced (pulse length1 ms)

PRF of 700 PPS

ATPL Radio Navigation ©Atlantic Flight Training 12-3

Beamwidth 1º Aerial rotation rate increased to 15 RPM

The shorter pulse length and narrower beam width will improve both accuracy and target discrimination. The increased speed of aerial rotation will allow for an increase in the rate of target information renewal. This type of radar may be used to provide a “Surveillance Radar Approach”. Range, Accuracy and Limitations of Surveillance Radar Surveillance Radar is capable of providing coverage to ranges greater than 200 nm. However, both Primary and Secondary elements are strictly “line of sight” so if an aircraft is below the radar horizon it will not be detected. If the aircraft is above the horizon other considerations have to be taken into consideration:

The size of the aircraft The aircraft heading straight towards or away from the radar aerial If the aeroplane is made of glass reinforced plastic

Any of the above make the aircraft a poor target for primary radar. SSR provides information which is not size or aspect related, but this depends on:

The ground unit being equipped with an interrogator Your aeroplane being fitted with a transponder You using the transponder correctly.

The accuracy of Surveillance Radar is dependant on the type of unit used but will be sufficiently effective to allow for a traffic separation of 5 miles and this may be reduced to 3 NM within a range of 40 NM of the radar aerial. Surveillance Radar Procedures En-Route Procedures are limited to those required for identification. This may involve:

Carrying out turns as directed by the controller, or At the request of the controller identifying your position as a radial and range from

a VOR/DME beacon, or At the request of the controller identifying your position as a geographical point

ATPL Radio Navigation 28 October 2003 12-4

In European regions, identification is more frequently carried out by the SSR element. In the event of a failure of primary radar, the controller will introduce “non-radar” separation standards using SSR to assist. Approach Surveillance radar may be used to provide the pilot with approach guidance including azimuth information and altitude advisories. The success of such an approach is dependent upon:

The skill of the controller The ability/willingness of the pilot to carry out the controller’s instructions.

You must remember that this type of radar has no height finding capability so that all height information is advisory and is the height you should be at the range and bearing that the controller observes. You must also remember that if you do not have the runway in sight when you reach Minimum Descent Height (MDH) you must not descend below this height. When the aircraft reaches the Missed Approach Point (MAPt) a go-around must be completed. Precision Approach Radar (PAR) Many military airfields have PAR installations. These are primary radar units that are designed to provide guidance during final approach to landing. The PAR consists of two elements:

One providing azimuth and range information, and The other providing elevation/range information.

ATPL Radio Navigation ©Atlantic Flight Training 12-5

Each element utilises 3-cm wavelength (10 GHz) radar with high PRF and short pulse length (less than 1ms). Both elements must be capable of providing detection to:

Range of 9 NM Elevation of 7°, and Azimuth sector of 20° (10° each side of the extended runway centre line).

Within this volume of airspace, the PAR must be capable of detecting a target with a radar cross section of 15 m2 or greater. The maximum allowable error is ± 30 feet on azimuth and ± 20 feet in elevation. The two elements are sited at the approach end of the runway to the side of the landing threshold.

Azimuth The azimuth element scans a very narrow beam (0.6°) backwards and forwards over a sector which covers the required minimum azimuth sector.

ATPL Radio Navigation 28 October 2003 12-6

Elevation The elevation element has a narrow vertical beam width (0.6º) but a broader azimuth beam width (up to 30º). It sector scans vertically from an elevation of about 0.5º up to 8º.

Both scans are at a rapid rate in order to ensure that the target information is refreshed quickly. Target information is presented to the controller on two screens mounted one above the other. The upper screen shows the range to the target and its position relative to the nominal glide path. The lower screen shows the range and the position relative to the extended runway centre line. PAR Procedure A typical procedure is detailed below:

Prior to commencing the approach, the controller will advise the pilot of the Aerodrome QFE. All heights will be referred to this datum. Instructions are designed to help you on the glide path and centre line and, in addition, the following calls will be made: You must remember that it is the pilot’s responsibility to ensure that the runway is in sight before DH. If the runway is not in sight at DH a missed approach procedure must be initiated.

ATPL Radio Navigation ©Atlantic Flight Training 12-7

Airfield Surface Movement Indicator (ASMI) This is a highly specialised primary radar unit that is designed to assist controllers in maintaining safe separation between aircraft and vehicles on the ground and to monitor all ground movements. The maximum range is approximately 2.5 nm. It requires only short range but must be capable of:

Very low minimum range 360° cover Very high level of accuracy Excellent target discrimination.

For some time it was considered necessary to use a radar operating in the “Q” band (35000 MHz) as this provided:

A very narrow beam width with a small aerial Excellent bearing discrimination

Modern signal processing techniques have led to equivalent definition being achieved with a cheaper “X” band (10 000 MHz) radar and this is now the favoured option. The following are the typical specifications for an ASMI:

Frequency 10 000 MHz PRF 15 000 PPS Pulse length 0.05 µs Beam width 0.4° Aerial rotation 60 to 75 RPM

A typical ASMI picture is shown below.

ATPL Radio Navigation 28 October 2003 12-8

Weather Radar The weather radar found at an airfield is not an ATC radar but one for the use of the meteorological services used to supplement their forecast information.

ATPL Radio Navigation ©Atlantic Flight Training 13-1

Chapter 13.

Secondary Surveillance Radar (SSR) Introduction The primary radar element of the ATC Surveillance Radar System provides detection of suitable targets with good accuracy in bearing and range measurement but with the following limitations: Targets that are too small, built of a poor radar reflector material or have a poor aspect may not be detected.

Targets cannot be identified directly Radar energy suffers attenuation (losses) both on the path out to the target and

on the return path of the reflections. To overcome these problems, a Surveillance Radar installation will often consist of both a primary radar and a secondary radar, the latter being known as a Secondary Surveillance Radar (SSR). The role of the SSR is to complement the primary radar element. Principles of Operation SSR operates on secondary radar principles. An SSR “link” uses one ground-based transmitter and receiver, called the interrogator and one airborne transmitter and receiver, referred to as the ATC transponder, or simply ‘transponder’. The interrogator transmits pulses. A receiver within the interrogator’s beam receives these pulses and decodes them. The transponder then responds by transmitting a pulse train (many pulses in a stream) back to the interrogator. The pulse train contains information according to what the interrogator requested. All interrogations are transmitted at a frequency of 1030 MHz and all transponder responses are transmitted at a frequency of 1090 MHz.

ATPL Radio Navigation 28 October 2003 13-2

The SSR aerial consists of a radiator and reflector similar to that used in the primary radar. Because the return is much stronger than that of a primary radar reflection, it is much smaller. Because of this small size and the frequencies used, the beam width tends to be large. This results in a considerable number of side lobes being transmitted. Pulse Spacing There are four modes and their applications and pulse intervals are as follows:

Mode Use Pulse spacing A ATC 8 µs B ATC 17 µs C Auto P. Alt. report 21 µs D Not Assigned 25 µs

ATPL Radio Navigation ©Atlantic Flight Training 13-3

Mode A is known to the military as Mode 3. Modes B and D are not currently in use and the conventional aeroplane transponder is designed to use only modes A and C. Side Lobe Suppression The large beam width reduces the bearing accuracy and increases the chances of false interrogations being generated by reflections from buildings and other obstacles. The side lobes can cause aircraft responses from any direction. This problem has been minimised by aerial design.

Side lobes still create a problem since aeroplane receivers, especially at close range, will detect them and this will trigger a false response outside the main beam. This can result in the effect of side lobe clutter which is shown below. To counteract this, a process known as side lobe suppression (SLS) is introduced. This is achieved by modifying the method of interrogation so as to electronically cancel the effect of side lobe radiation.

ATPL Radio Navigation 28 October 2003 13-4

SIDE LOBECLUTTER

AIRCRAFT ATLONG RANGE

RING-A-ROUND

MEDIUMRANGE

SHORTRANGE

Interrogations are sent in the form of a group of three pulses that we will identify as P1, P2

and

P3. P2 is a control pattern that is placed over the main beam.

The spacing between P1 and P2 is constant at 2.0 ms. Pulse P2 is used in the electronic side lobe suppression. The spacing between P1 and P3 is set at a value dependant upon the mode required from the aeroplane transponder as shown earlier.

Position 1 The control pattern is omni-directional except in the direction of the main beam. This means that an aircraft painted by the main beam will have a P2 pulse which is lower in amplitude than the P1 and P3 pulse as in the pulse diagram shown above.

ATPL Radio Navigation ©Atlantic Flight Training 13-5

Position 2 If an aircraft is painted by one of the side lobes the P2 pulse will be of a greater amplitude than the P1 and P3 pulse. The side lobes only have 50% of the power of the main beam and thus P2 is shown as the pulse with the greater amplitude (see diagram below).

It can be seen that whenever the transponder receives P1 and P3 at a greater amplitude than P2 then a reply will be transmitted. Should P2 be greater than P1 and P3, it indicates that the aircraft is being painted by sidelobes and will suppress any reply transmission. Operation The pilot sets the transponder to the mode and code as instructed by ATC.

OFFL R

ALT RPTGOFF ON

ATC

IDENT

FAULT

0767

TRANSPONDER SELECTOR- SELECTS DESIRED TRANSPONDER

ATC CODE INDICATOR- DISPLAYS CODE SET BY ATC CODE SELECTOR

ALTITUDE REPORTING SELECTORON - ENABLE ALTITUDE REPORTING

ATC FAULT LIGHT (AMBER)- ILLUMINATES WHEN A TRANSPONDER

MALFUNCTION HAS BEEN DETECTED

ATC CODE SELECTORSROTATE - SETS CODE IN ATC CODE INDICATOR

AND BOTH TRANSPONDERS

ATC IDENT SWITCHPUSH - TRANSMITS IDENT SIGNAL

TYPICAL TRANSPONDER CONTROL If the transponder is set to the “ON” position, the unit will respond to Mode A interrogations. If set to ALT, the transponder will respond to Mode A and C interrogations, sending identification and automatic altitude information. The transponder’s response will be in the

ATPL Radio Navigation 28 October 2003 13-6

form of a pulse train. This consists of two framing pulses separated by 20.3 µs. Between the framing pulses is the facility for up to 13 coding pulses to be transmitted. Pulse ‘X’ is not used at this time, so only 12 pulses are used. The pulses are used as follows:

A pulses form the first digit of the four-figure code B the second C the third, and D the fourth

The diagram below shows the arrangement of A, B, C, and D pulses for sending the digits. You will note that, for each digit, there are 8 possibilities ranging from 0 to 7. This leads to a total of 4096 selectable codes (8 x 8 x 8x 8).

After the main framing pulses a special position indicator (SPI) pulse is transmitted 4.35 ms after the last pulse when the ident button is used on the main control panel. This is discussed in the SPI code paragraph. The system uses a form of binary numbering to identify the code. The altitude information is relative to the 1013.2 hPA level no matter what pressure setting is on the altimeter. SPI Code A special “IDENT” feature is utilised in order to allow ATC to confirm an aeroplane’s identity. The pilot activates this when instructed by ATC. When the IDENT button is pushed, an additional pulse is transmitted 4.35 µs after the second framing pulse. At the controller’s display, the ident pulse will cause the particular aeroplane’s echo to brighten or flash. This lasts for approximately 15 – 30 seconds. Use of Transponder Pre-departure the transponder is set to stand by which allows the equipment to warm up but not transmit. The test function is then activated in order to establish the operational status of the equipment. When instructed the pilot sets the mode and code given by ATC, and when told to “Squawk” sets the controller to “ON” or “ALT” as appropriate. Normally ALT is selected, as the altitude encoding has to be selected in all cases unless ATC instructs otherwise. In order to avoid causing interference, do not change the mode or code without first selecting “STBY” on the controller. When in an abnormal situation, there are three codes to alert the controller. These codes have a predefined meaning and, with one of these selected, a signal

ATPL Radio Navigation ©Atlantic Flight Training 13-7

indicating a “special condition” will be triggered on the controller’s screen. The aircraft symbol may change colour to attract his attention. On some radar systems, an aural alarm will be triggered together with the target on the screen either flashing or brightening.

Code 7500 Hijack Code 7600 Radio failure Code 7700 Emergency

From time to time the ATC controller may ask you to “SQUAWK IDENT”. By pushing the “IDENT” button, the transponder is activated to transmit the additional pulse. This is shown at the radar display as a flashing target. This function, when first enabled, will continue for approximately 15 - 30 seconds. The “IDENT” button should not be pressed unless instructed by air traffic. Presentation and Interpretation SSR information is presented together with the primary radar information. The difference between the two is that the primary information is very accurate in bearing and range, but does not consist of any extra information. The secondary radar information is inaccurate in bearing and range, although it can be used as a back up, but provides reliable information that can identify every aircraft and provide altitude information. The primary radar element provides the necessary bearing and range and the use of computer generated displays allows calculated information, such as course and ground speed, to be shown. A common style of displaying combined (primary and secondary surveillance radar) information on the air traffic controller’s radar screen is shown below.

Limitations SSR compliments the primary radar system and, although effective, is likely to be replaced by Mode S in the near future. We have already noted that the bearing and range information is not as good as that of the primary radar. In addition there are two other major problems.

ATPL Radio Navigation 28 October 2003 13-8

Fruiting

Although ground based interrogators have a nominal range of approximately 200 nm, the propagation is “line of sight” and it is not unusual for aeroplanes, especially at cruising altitudes over well developed ATC regions, to be within range of two or even more ATC interrogators. Since all SSR units operate at the same frequency, this can result in an aeroplane’s response to one interrogator being detected by other ground units. Such responses will be out of synchronisation and will cause random responses to appear. This is called Fruiting. Electronic circuits are employed (de-fruiters) to remove this effect but they do not remove all random responses and the situation becomes worse as traffic density increases.

Garbling Another problem is known as garbling. This occurs when targets are close to one another e.g. in a holding pattern or progressing along an airway one above the other. If both aircraft are in the interrogation beam at the same time and are close enough to each other, the ground interrogator will be unable to differentiate between them and will record only one confused return. Although fruiting and garbling effects can be controlled at this time, future traffic growth will place more and more stress on the system and the controllers. Mode S This is a development of the basic SSR, which is being introduced. The Mode S ground interrogators and airborne transponders are fully compatible with the conventional Mode A and C units and use the same basic frequencies of 1030 MHz and 1090 MHz. However, Mode S units working together have much greater capabilities. Operation of Mode S The Mode S interrogator and receiver operate on the same frequency as standard SSR. The initial part of the interrogation signal is such that the standard SSR modes will be recognised by the normal airborne transponder unit. The second part of the Mode S interrogator consists of a message of up to 112 data bits within which 24 bits are allocated to aircraft address. This permits the controller to interrogate a specific aircraft. The 24 bits allocated are sufficient to provide for over 16 million individual addresses, which is thought to be sufficient for the registration of all aircraft in the world.

ATPL Radio Navigation ©Atlantic Flight Training 13-9

In order to detect further Mode S transponders, a special feature known as SSR/Mode S “ALL CALL” is broadcast at intervals. Normal SSR transponders respond to this in Mode A or C (dependent on the P1/P3 relationship). Mode S transponders will recognise the special character of the “ALL CALL” interrogation as a roll call request and will transmit a response which will include the aircraft’s identity/address along with details of the capability of the relevant on board equipment. This is a 56-bit message. The other interrogations and responses are summarised as follows:

INTERROGATION RESPONSE Type Content Type Content Surveillance

16 control bits 16 altitude echo 24 address

Surveillance

13 SSR identify & altitude 19 control purposes 24 address

Comm A As for Surveillance + 56 bits ground to air data

Comm B As above + 56 bits air to ground data

Comm C 112 bits for data transmission of long messages

Comm D 112 bits data transmission of long air to ground messages

The altitude echo function in the Surveillance interrogation is intended to indicate (to the pilot) the flight level that ATC are being given by the aeroplane’s transponder. Comm A and Comm C interrogations can be used to send longer messages by breaking the messages up into suitable sized blocks and transmitting on successive cycles. Comm D, from the airborne transponder, has a similar capability. Comm D cannot be used for position update, as the messages contain no altitude information. The expanding use of Mode S will have the following benefits over standard SSR:

Elimination of synchronous garbling Elimination of ‘fruiting’ Increased traffic capacity Improved accuracy.

In addition, the ability to send messages will allow for a reduction of congestion on the current R/T communication frequencies. Transmitted data will be presented to the pilot on a CDU either integral with the Mode S transponder or on the FMS screen. Mode S information, transponder to transponder, can also be integrated with the airborne Collision Avoidance system allowing the systems of conflicting aircraft to communicate and resolve convergent situations. ATC Services Mode S data link can serve as a back up to many ATC services that are provided today by VHF ‘voice communications’. This data link back up will improve system safety by reducing communications-related errors within the ATC system. Many types of messages are potential candidates for data link back up and other ATC services. These include:

ATPL Radio Navigation 28 October 2003 13-10

Flight identification altitude clearance confirmation

Take-off clearance confirmation New communication frequency for sector hand-off Pilot acknowledgement of ATC clearance Transmission to the ground of aircraft flight parameters Minimum safe altitude warning.

ATPL Radio Navigation ©Atlantic Flight Training 14-1

Chapter 14.

Airborne Weather Radar (AWR) Introduction All transport aircraft, and some small general aviation aircraft, are equipped with weather radar. The weather radar has four main functions:

Locating clouds ahead of the aircraft Assisting the pilot to avoid turbulent clouds Determining the location and height of cloud tops. Mapping of the terrain ahead

The weather radar is an airborne pulse radar designed to locate turbulent clouds ahead of an aircraft. Some weather radars have a secondary ground mapping capability. Principle of Operation Cumuliform clouds are associated with both rising and descending currents of air, which lead to turbulence. In the case of towering cumulus and cumulo-nimbus cloud this turbulence can be severe. The turbulence within the cloud retains the water droplets within the cloud until they are of a size to fall as precipitation. It is the precipitation, and in particular the large water droplets, that reflect the radar energy. The reflection depends upon the form and size of the droplets within the cloud:

Hail Hail is covered by a film of water and gives the strongest return Light Rain and Snow These give the weakest returns

Non-turbulent cloud is not detected as the water droplet size is too small. Frequency 9 to 12 GHz

ATPL Radio Navigation 28 October 2003 14-2

Frequency Range The frequency range above gives good returns from droplets of water, rain or hail in clouds.

If too high a frequency is used the smaller wavelength will start to reflect off the smaller water droplets of non-turbulent clouds

If too low a frequency is used the radar penetrates the cloud and inadequate returns are given from turbulent clouds

AWR Aerial The aerial consists of either a parabolic dish or, in modern units, a flat plate phased array. It is mounted in the nose of the aeroplane and scans ahead of the aircraft from side to side with a narrow beam. The sweep can be up to 90° to each side of the aircraft nose. The beam can be tilted manually ± 15° in the vertical plane. A gyro stabilises the scan horizontally in order to prevent loss of target if the aircraft rolls or pitches. This stabilisation is effective up to ± 20° of combined roll, pitch and tilt. The radar can be selected to either “weather” or “mapping” beam. The weather beam is used for detecting clouds and is a conical pencil beam with a width of 5º. Because of the directional properties of the radar, side lobes are produced. One side lobe goes vertically down to the ground and is received back by the radar receiver. This received signal produces a height ring on the display. The ring indicates that the radar is working and appears at the approximate height of the aircraft above the ground. An aircraft flying at 12 000 feet will have a permanent return at approximately 2 nm. Control Panel

OFF

OFF

POWERONSTAB

STANDBY

20

50

150

CONTRAST

DOWN

MARKBRILL

UP TILT15

15

10

10

5

5

0

CONTWEA

MAN

MAP

POWER SWITCH MARKER BRILLIANCE

TIMEBASERANGESWITCH

CONTRASTCONTROL

TILTCONTROL

MANUALGAIN

CONTROL

FUNCTIONSWITCH

ATPL Radio Navigation ©Atlantic Flight Training 14-3

Power Switch and Timebase Range Switch These two switches are used together. On the ground the timebase range switch is set to standby. The power switch is then set to either STAB ON or STAB OFF (the significance of these positions will be explained later in this chapter). The radar will warm up but will not transmit. When the aircraft is airborne the appropriate range selection is made, in this case 20 nm, 50 nm or 150 nm.

The angle markings are etched on the radar screen every 15° and are permanent. The angles represent the relative bearing of a return left and right of the nose. The range markers are electronically produced dependent on the range selected. The brightness of these settings is controlled by the marker brilliance control, which has no other function.

30 3045 45

60 60

0 5 10 15

30 30

45 45

60 60

0 10 20 30 40

30 30

45 45

60 60

0 50 100

20 nm range scale

50 nm range scale

150 nm range scale

ATPL Radio Navigation 28 October 2003 14-4

Function Switch The function switch selects the mode of the radar:

WEA/CON Weather and Contour, both of these selections are used in cloud detection. Automatic gain control in these selections. MAN/MAP Manual and Mapping, both selections are used for ground mapping purposes. Manual gain control in these selections.

Weather Function (WEA) When WEA is selected a conical beam is produced by the radar. A form of “swept gain” is used. Swept gain is an automatic gain setting that adjusts the returns within a radius of 25 nm. A short-range non-hazardous return at 5 nm may show up on the radar while a hazardous return at 25 nm may only just be painted. The swept gain adjusts these returns to remove the non-hazardous cloud off the screen but enhances and amplifies the hazardous cloud return. Contour Function (CON) With a monochrome screen it is difficult to distinguish between the severity of turbulent clouds. When CON is selected an ISO-ECHO circuit is activated. An ISO-ECHO level supplements the Automatic Gain Visual Threshold set in WEA mode. The shape of the return on the radar depends upon the gain level of the cloud:

Automatic Gain Visual Threshold If a cloud return breaks this threshold then it will still appear on the radar scope but only as a normal cloud return Iso-Echo Level If this gain level is broken then the effect is to create a black hole in the cloud by reversing and amplifying the signal. This reversal brings the signal below the Automatic Gain Visual Threshold and so no return is seen in the middle of the cloud.

ATPL Radio Navigation ©Atlantic Flight Training 14-5

Severity of turbulence can be assessed from the picture:

The hole shows an area of high turbulence The outer ring shows the severity of the turbulence gradient within the cloud:

The narrower the retaining ring the steeper the gradient The wider the outer ring the slacker the gradient

Mapping Function (MAP) The conical beam used in the WEA and CON modes is modified into a fan shaped beam – a cosecant2 beam. To spread the beam a system of spoilers are placed on the aerial. When MAP is selected these spoilers and the aerial position change the conical beam into the one shown in the diagram below. In MAP the power is spread across the beam so that the maximum range usage is approximately 60 nm.

IN P U T S IG N A L - IN V E R TE DA B O V E TH E IS O -E C H O L E V E L

IS O -E C H O LE V E L

A U TO M A TIC G A INV IS U A L TH R E S H O LD

3 0 30

4 54 5

6 06 0

R E S U L T IN G P A IN T

T U R B U L E N T C L O U D P A IN T - C O N T O U R M O D E

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Manual Function (MAN) The conical beam is used in this mode. Because of the concentration of energy within the beam ranges of up to 150 nm can be seen on the radar. Contrast This rotary switch determines the degree of amplification to the video pulse. It influences the brightness of the display. Manual Gain Signal strength can vary with altitude and the type of terrain the aircraft is flying over. This control varies the amount by which the returning echoes are amplified by the video processing unit.

ATPL Radio Navigation ©Atlantic Flight Training 14-7

Tilt Control The dish aerial sweeps in azimuth and the variation in elevation is determined by the setting of the tilt control, normally 15° up or down. In association with the tilt control the aerial settings STAB ON and STAB OFF are used:

STAB OFF The beam is fixed to the aircraft nose.

STAB ON The beam is gyroscopically fixed to the earth horizontal. The beam will always look at the tilt angle selected no matter what the pitch angle of the aircraft.

Colour Displays The airborne weather radar will display echoes from clouds, which have sufficient concentration of liquid water. However, it will not be able to discriminate between clouds that are likely to give turbulence and those which are not likely to do so. To overcome this an “iso-echo contour” circuit is introduced. This is selectable by the pilot. Any clouds having heavy concentrations of large water drops will be associated with turbulence and will also be good reflectors of radar, especially at 3 to 5 cm. When echoes return, the strength of which are above a pre-set level (if the contour circuit has been selected), these echoes will be processed to show as black holes on a monochrome (single colour) screen. Most modern weather radars use colour and the processing of the returns produces graded colour paints as follows:

UP

TILT

DOWN

15

15 10

10

5

5

0

ZERO TILT SELECTED

STAB OFFPENCIL BEAM LOCKED IN

AIRCRAFT PITCHING PLANE

EARTH HORIZONTAL

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Green Light concentration, slight turbulence Yellow Moderate concentration, light turbulence Red Heavy concentration, moderate/severe turbulence Magenta Heavy concentration of large drops, severe turbulence

Cloud Height Determination On detecting an active cloud, it is possible to determine the height of the top of the cloud by using the tilt control. Tilt the beam upwards until the cloud echo disappears. Note the tilt angle and range. Then, using the 1/60 rule, determine the height above the aeroplane level. Remember to allow for beam width.

ATPL Radio Navigation ©Atlantic Flight Training 14-9

Echo range 60 NM Tilt angle at disappearance 4° Beam width 4° = 4NM @ 60NM Beam bottom 2° above horizontal. Range 60 NM Beam height 2 NM = 12 000 feet above aeroplane The simple formula to use is

Cloud height = (tilt angle - ½beamwidth) x 100 x range Cloud height = (4 – 2) x 100 x 60 Cloud height = 12 000 ft above the aircraft A negative figure will mean that the cloud is below the aircraft.

At lower altitudes, tilting the aerial down may increase ground returns and you will find it impossible to differentiate between ground and cloud returns. In this case an upward tilt of the aerial may be required. Shadow Water surfaces at any reasonable range will reflect the radar energy away from the aeroplane and will give little or no return, causing the screen to remain quite clear. However, beware, shadow areas will appear behind hills and could easily be mistaken for water.

4°5°3°

BEAMWIDTH

EARTH HORIZONTAL(STAB ON)

RANGE 30 NM

AIRCRAFTALTITUDE

5000 FT

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Adjusting the gain control can often make a substantial improvement to the radar picture. In order to interpret the bearing and range information, you must interpret between the bearing marks to obtain the relative bearing and between the range markers for the range (which will be a slant range for ground targets). Degradation of the performance of the radar will be experienced if the radome is contaminated with ice, snow or rain. Test Most modern radars have a Built in test equipment (BITE) selection. This enables checks to be carried out on the display colour patterns and the internal workings of the radar. Hold The hold selection is a method of assessing cloud movement. Once selected a cloud position is frozen on the radar screen. Deselecting hold releases the cloud paint to its current position. Thus allowing the pilot to assess the mean track of the cloud relative to the aircraft. Target Alert When selected a yellow T in a red square is set at the top of the screen. If a turbulent cloud is detected beyond the range selected the T is replaced by TGT and the symbol flashes. Use of the Radar on the Ground The following is a short checklist for using an AWR on the ground:

Prior to start - AWR off, this avoids any surge currents passing through the system

Post Start – Range to SBY and power to STAB OFF. STAB OFF is selected because this fixes the radar aerial to the aircraft axis and stops any damage to the gyroscopic system used in the gimbals of the radar. The radar warms up.

Tilt fully up When clear of all personnel, buildings etc the timebase can be checked for radar

returns on all ranges Return to standby Before take-off on the runway – Power STAB ON, WEA selected and tilt as

required. The radar can be switched on before take-off if the aircraft is lined up on the

runway.

ATPL Radio Navigation ©Atlantic Flight Training 15-1

Chapter 15.

Doppler Introduction Airborne Doppler Radar systems provide a pilot with ground speed and drift angle information continuously and automatically. Doppler, unlike most airborne navigation equipment, does not need any active system outside the aircraft. Although the use of Doppler has been superseded by more modern systems it may still be encountered on older transport aircraft and helicopters. In 1842 the Austrian scientist, Christian Doppler, noted the changing pitch of the sound generated by moving vehicles as they approached and then passed beyond a stationary point. If a wave propagation source is moved towards a person, the frequency of the sound observed by that person is higher than the actual frequency at the source. The shift of frequency (increasing when source moves towards the object, decreasing when it is moving away) is proportional to the velocity of the source. This phenomenon is called Doppler shift. Other names are Doppler effect or Doppler frequency. Applying the Doppler principles to an aircraft in flight, with no relative motion between the radiating source and the receiver (both transmitter and receiver are on board the aircraft), a Doppler shift will still occur if the transmitted energy is returned to the receiver from the earth. In this case the surface of the Earth acts as a reflector. Frequency 8.8 GHz or 13.5 GHz Doppler Effect

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As the Transmitter moves to the left the waves are compressed towards the blueshift. The intervals between the waves diminish and this apparent shortening in wavelength causes an increase in frequency or pitch. The frequency appears to increase. As the transmitter passes the waves are now stretched. The interval between each wave increases and causes a decrease in frequency and pitch at the redshift side of the diagram. The frequency has appeared to decrease.

The diagram above shows a stationary transmitter. Assume that the frequency of transmission is f Hz. The receiver will receive the successive wave fronts at f Hz.

Now the transmitter is moving towards the receiver. Assume the velocity is V m/s. The first wave front is centred on 1, the second on 2 etc. As shown in the first diagram. The effect is to decrease the wave front spacing as the transmitter travels towards the receiver. This will appear to the receiver as an increase in the received frequency. Behind the transmitter the wave front is increased and so a receiver placed opposite Rx would experience a reduced frequency. This change in frequency is the Doppler shift.

ATPL Radio Navigation ©Atlantic Flight Training 15-3

The change in frequency, the Doppler shift, is represented by fd and is proportional to the transmitter’s velocity:

fd = Vf = V c λ

c is the speed of propagation f is the transmitted frequency V is the velocity of the transmitter

Example Assume a transmitter (9 GHz) is moving towards a stationary receiver at 300 kilometres per hour. What frequency will be recorded at the receiver?

First convert 300 kmh to m/s 300 x 1000 ÷ 3600 = 83 m/s The wavelength for a frequency of 9 GHz is = 0.03 m

fd = V λ

fd = 83.3 ÷ 0.03 = 2777 Hz

This is the Doppler shift which must be added to the transmitted frequency to get the received frequency (if the transmitter is moving away from the receiver then the Doppler shift would have to be subtracted from the transmitted frequency)

Received frequency = 9 000 000 000 + 2777 = 9 000 002 777 Hz It is not likely that you will have to make any Doppler calculations in the JAR exam. This example above is probably the style of question you would be asked. Doppler Measurement of Groundspeed Since the aircraft cannot fly directly towards a reflecting surface, the radar beam must be tilted downwards. The measured Doppler shift has to be mathematically corrected for a tilt angle (θ) in order to resolve the aircraft’s forward horizontal velocity. The diagram below shows an aircraft transmitting a narrow radar beam towards the ground at an angle θ (this is called the depression angle) The beam will be received at a different frequency than that transmitted.

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The basic Doppler shift now becomes:

fd = 2VF cos.θ C

The frequency received (Fr) on a forward facing beam will be:

Frequency transmitted (Ft) + 2VF cosθ C

where θ is the angle of depression of the beam (the angle between the horizontal and the direction of the beam). The equation shows that the Doppler shift is at its highest when the angle θ is shallow or when the beam is directed forward in the flight direction. This is impossible as there is no reflecting surface in front of the aircraft. If we make the beam depression too large the Doppler shift will be very small, becoming zero when the beam is directed vertically downwards. The choice of beam depression angle is a compromise between these two extremes, trying to obtain reliable energy returns with sufficient Doppler shift to be accurately measured. This is normally achieved at an angle of approximately 67°. The system we have looked at is a single beam system that has certain disadvantages:

Pitch Error If the aircraft changes pitch then θ is changed. If the aeroplane pitches nose down, the reflected beam in front would be depressed greater than θ and the Doppler frequency shift would reduce, even if aircraft speed remains constant. For the system to work correctly, the aircraft would have to fly straight and level at all times or the aerial has to be stabilised in the pitching plane. Vertical Motion Any vertical motion of the aircraft generates a change in the Doppler shift not associated with the groundspeed. Drift If the aerial is fixed the system will be measuring the speed along the heading. Groundspeed is calculated along track. A system has to be designed so that the aerial can move to obtain maximum Doppler shift. This would then give us a drift angle as well as a groundspeed. In practice this is not done as the system is inaccurate.

ATPL Radio Navigation ©Atlantic Flight Training 15-5

Two Beam Janus Array A common way of solving this problem is to transmit a second beam backward from the aircraft, at the same depression angle as the front beam. The system then algebraically adds the two sensed Doppler shifts and calculates the ground speed based on this information. This principle is called a Janus system after the Roman god, Janus, who could look forward as well as aft at the same time.

The frequency that is received from the forward beam (fr) will be higher than the frequency transmitted.

fr = f + fd

The frequency that is received from the rear beam (fr) will be lower than the frequency transmitted.

fr = f - fd

With a two-beam system we summate the two received frequencies to give us a total received Doppler shift ft. This gives us:

ft = (f + fd) - (f - fd) = 2 fd or

4VF cosθ C

The total frequency received is double that received from a single beam system.

Pitch Error Aircraft pitch will cause an increase in depression angle for one beam and a decrease in depression angle in the other beam; thus effectively cancelling each other out. Vertical Speed The change in vertical speed is sensed by both front and back beams and in the summation process cancel each other out.

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Further Janus Arrays Four different types of array are shown below.

By using any of the systems above, the drift angle will not be sensed since drift occurs across the intended track. In order to ensure that drift angle is calculated we must use three or four beams, each pointing in different directions. The configurations are given names that reflect their appearance. For example the three-beam lambda configuration is similar to the Greek letter λ. In the four-beam system the aerials transmit front left/rear right and then front right/rear left. The Doppler shift measured by the two sets of aerials, if the aircraft has no drift, will be the same in each sequence of transmissions. Once the aircraft is experiencing drift, one set of aerials will receive a larger Doppler shift than the other set. This difference is electronically calculated as an error signal that rotates the aerial to ensure that the two signals received are the same. The angle of movement is the drift. Doppler Aerial The Doppler aerial system is made up of three or four slotted arrays giving shaped beams each with a width that varies between 5.5° to 11°. The beams are depressed to an angle of 67° to provide a measurable Doppler shift. Aerial systems may be hard strapped to the airframe and, if so, will cause some small errors in measured groundspeed during prolonged climbs. Alternatively they may be gyro stabilised in pitch in order to reduce these errors.

ATPL Radio Navigation ©Atlantic Flight Training 15-7

Signal characteristics

The sensitivity and thereby the accuracy of a Doppler radar increases with frequency. However, the higher frequency the more the signal is affected by rain reflections, scattering effects and absorption. A compromise is made and as a result Doppler airborne radar equipment operates on two frequency bands: 8.8 GHz and 13.5 GHz. The actual frequency used is selected, taking into account:

The operational height required The power output available, and The speed of the aircraft.

A helicopter Doppler will normally use the higher frequency in order to increase the Doppler shift at the low speeds. Output and Presentation Doppler equipment measures drift and groundspeed and if combined with a heading input, will give track and ground speed. This can be fed to a specific display or, more commonly, integrated into a navigation computer, which can add the measured values to a start position to produce outputs such as:

Position Distance to go Bearing and distance to a waypoint Current W/V

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On a Doppler control unit shown in the diagram below the functions important to the pilot are:

A Display Switch B Latitude/Longitude selection switch C Slew switches D Light bars E/W or N/S as selected E Track error indicator F Warning bars G Power supply test bars H Numeric displays J Function switch K Display dimmer switch L Waypoint selection For the syllabus certain functions need to be understood:

STBY Equipment warm-up SLEW Allows the pilot to ‘set’ a drift and ground speed if the equipment is

operating in ‘Memory’ mode

ATPL Radio Navigation ©Atlantic Flight Training 15-9

LAND/SEA Creates a mathematical solution to compensate for partial loss of the beam when flying over the sea or flat surfaces the switch should be set to ‘sea’. (See sea bias).

Accuracy and limitations Modern Doppler radar systems are quite accurate. Errors in the order of 0.1 % on ground speed and 0.15% on drift angle can be expected. Sea Bias When flying over the sea, the leading edge of the forward beam (trailing edge of the rear beam) will be lost due to increased reflection away from the aircraft. This will cause the measured values of ground speed to be lower than the true ground speed. Selecting ‘Sea’ position on the land/sea switch provides a bias, which offers some compensation for this effect. The Land/Sea switch discriminates between the Doppler frequency over water (fdw) and land (fdl) when switched to sea. The calibration of the tracker unit is altered to increase the groundspeed by a nominal 1 – 2%.

Memory Mode If the sea becomes too smooth (surface wind less than 5 Kt) nearly all the energy will scatter away from the aeroplane and no measurements will be possible. On such occasions a ‘Memory Mode’ is activated and the drift and ground speed are frozen at the last measured values. These may not be correct values but will be used by any associated equipment. Examples of other situations that will cause the system to go into Memory are:

Flight over hot desert regions where the attenuation is high Flight in adverse weather such as severe thunderstorms. The water content of

such storms cause excessive scattering of the energy.

ATPL Radio Navigation 28 October 2003 15-10

Pitch and Roll Error Errors due to pitch or roll are cancelled by the Janus array. When both pitch and roll occur simultaneously, an error is likely to occur. Limitations for pitch and roll are typically ± 20° and ± 30°, respectively. Beyond these, the system will lose data and enter the Memory Mode. Height Hole Error In a pulse modulated system, when the Doppler radar is transmitting the receiver is shut off. Low flying may introduce an error to the system because the transmitted signals are reflected back to the aircraft before the receiver is functioning again. Likewise, when flying at high altitudes, the time delay between transmission and reception may cause problems, reception may occur at exactly the same time that a new transmission is taking place. In such cases the receiver will be short-circuited. Sea Movement Error Doppler measures the drift and groundspeed relative to the terrain beneath the aircraft. If the surface is moving, as is the case with the sea, errors can be induced by:

Tidal Stream Tidal streams normally affect narrow waterways. Since the time the aircraft will be over this type of feature is small the effect is minimal

Ocean Currents Ocean current speeds are slow so will have little effect Water Transport The surface wind causes movement on the surface of large

tracts of water. This error is complex to understand but the resultant error can affect both drift and groundspeed

Computational Errors Processed Doppler information may be subject to heading input errors that will probably be more significant than those of the measured ground speed. The Doppler is based on an assumption that 1 nm is equivalent to 1’ of latitude. This is only correct at 47° 42’ N/S. This is on the surface of the earth. As soon as an aircraft is at height then again the assumption is not true. Both errors are small and are not corrected for. Heading Error This is the greatest error in Doppler. The system relies on the accuracy of the heading input information which if wrong can cause considerable accumulative errors.

ATPL Radio Navigation ©Atlantic Flight Training 15-11

Summary of Errors

Error Measurement of Error Distance Track

Aerial Misalignment No Yes Pitch Error Yes No

Sea Movement A vector error dependent on the time spent over the water, the direction and movement of the sea and the wind velocity

Sea Bias Yes No Heading Error No Yes

Altitude and Latitude Yes No Advantages As mentioned above, the Doppler navigation system is quite accurate. It does not require any external equipment for basic operation (although updating would require external means). The system has good, long term accuracy as irrespective of flight time, the measured ground speed and drift will retain the same accuracy potential. If combined with a short-term accuracy system, the overall accuracy is excellent. Disadvantages Doppler can only give an instantaneous value of drift and ground speed and must be linked to other equipment. This makes the derived position information dependent upon the accuracy of such inputs as heading and TAS.

The equipment is very prone to ‘loss of signal’ and entry into Memory Mode. The equipment is costly to maintain.

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Intentionally Left Blank

ATPL Radio Navigation ©Atlantic Flight Training 16-1

Chapter 16.

Hyperbolic Navigation

Introduction Hyperbolic navigation systems determine present position from the intersection of lines of position. While radials and bearings from VOR and NDBs are straight lines, in hyperbolic navigation a line of position is based on a curved line - specifically, a hyperbolic curve and, more precisely, a hyperbolic surface.

Hyperbola A line joining all points where the difference of distance from two fixed points is the same.

Hyperbolic Family

Draw a straight line of 4 cm, label the ends M – S Draw a line perpendicular to M – S, 2 cm from M Draw circles starting with a radius of 1 cm from M and S Do this for 2 cm, 3 cm, 4 cm etc Eventually you will arrive at a diagram like the one below. Ignore the curved lines

for the moment.

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Look at the line DC. Where DC cuts the line MS is the meeting point of the 2 cm radius circle drawn from M and the 2 cm circle drawn from S. The difference between the two circles is (2 – 2) zero. The 3 cm circles meet at two points above and below the line MS along the line DC. The same for the 4 cm line etc. The difference between the circles is zero (3 – 3, 4 – 4 etc). The line DC is a hyperbola. Now plot the following points on the diagram that you have drawn.

Distance from S – 5 cm, Distance from M – 3 cm Distance from S – 4 cm, Distance from M – 2 cm Distance from S – 3 cm, Distance from M – 1 cm

Each of the circles cut at two points. The diagram you now have should look like the one below with the line PQ drawn.

The difference between the circles is 2 (5 - 3, 4 – 2, 3 – 1 etc) If we now plot all the variables we can establish a family of hyperbola. Which are shown by the curved lines. Now assume that these are range lines and if we can establish the difference in range between that from our own position to the M and S points respectively, we could use these hyperbolas as LOPs.

ATPL Radio Navigation ©Atlantic Flight Training 16-3

Hyperbolic radio navigation aids operate on the principle that the difference in time of arrival of signals from two stations is a measure of the difference in distance from the point of observation to each of the stations. Since radio signals can be considered to travel at a constant speed equal to the speed of light, these measurements can be used to translate the differences in time directly into differences in distance. Note that each hyperbola cuts the base line at half the difference in range from the central 0 hyperbola.

The 1 cm line cuts the base line at ½ The 2 cm line cuts the base line at 1

The line joining M - S, in a hyperbolic navigation system, is called the ‘Base Line’. The lines behind M and S are referred to as ‘base line extensions’ while the straight line at right angles to the centre of the base line is called the base line bisector.

The base line extensions and the base line bisector are straight hyperbolas. The spacing of the position lines vary. They are constant along the base line but diverge as we move away from the base line, the rate of divergence being least near the base line

ATPL Radio Navigation 28 October 2003 16-4

bisector and increasing towards the base line extensions. The accuracy is therefore reduced more rapidly in the vicinity of the base line extensions. Lines of Position (LOP) Due to the curvature of the hyperbola, you can find the same LOP on both sides of the base line extensions. This gives the risk of an ambiguous situation that is not acceptable. In the application of this principle one transmitter (M) is used to stabilise or control the transmissions of the second transmitter (S). The control transmitter is known as the MASTER and the other is called the SLAVE (or Secondary). Since the signals from the Master are used for control and synchronisation of the master/slave pair it follows that there must be reliable communication between the two beacons. This communication is normally done by radio so the two transmitters must be within radio range (of each other) which ensures predictable and reliable signal to noise ratios. It requires one Master/Slave combination to produce a single LOP. (Line of position). To determine a fix will require a minimum of two LOPs and therefore the use of two Master/Slave combinations. Like all fixes, however, the angle of intersection of the LOPs will have an effect on the fix accuracy. Each LOP will have a ‘band of error’. If the LOPs intersect at 90° the respective bands of error form a small rectangle which encloses the position of the receiver. If the LOPs intersect at 30° the area becomes an elongated diamond which considerably increases the area of possible positions. In a hyperbolic system, base line bisectors intersecting at 90° give the best fixes.

Errors of Hyperbolic Navigation Propagation Errors Since the determination of the LOP is dependent upon the comparison of time of arrival of the two signals, anything which affects the accuracy of that timing will cause errors. The timing assumes that the speed of propagation is constant and the same for both signals. If the Master signal travels over the sea to reach the receiver and the Slave signal propagates over

ATPL Radio Navigation ©Atlantic Flight Training 16-5

the land, the propagation speeds will be different and the accuracy of the timing will be disrupted. Where these propagation errors are known to exist they are generally accounted for either by distorting the hyperbola or by publishing corrections. Height Error We should also note at this time that the hyperbolas are indeed three-dimensional. A constant range differential can be observed not only along the surface of the Earth but also on a curved ‘hyperbolic’ surface. The hyperbolic surface is a vertical plane over the base line bisector but, in the vicinity of the transmitters, it becomes noticeably more curved. When charts are drawn for a hyperbolic navigation system it is normal for the hyperbola to be drawn for Sea Level. Although, at altitude, we may be on the same curved hyperbolic surface, when we plot our LOP it will assume Sea Level and this will cause our plotted LOP to be in error. This causes Height Error.

Simple Hyperbolic Calculation

Example A hyperbola cuts the base line 100 Km from the Master end and 150 Km from the Slave end. When on the same hyperbola at a range of 120 Km from the Master, the range from the slave will be: Draw the situation

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Hyperbola is a line joining all points where the difference of distance from two fixed points is the same. The difference between the M – S base line cut is 50 km Therefore the difference in distance must be the same at 120 km Therefore the distance from the slave is 120 + 50 km = 170 km if the aircraft is on the same hyperbola.

Example An aircraft is 120 km from the master and 170 km from the slave. How far from the slave will the hyperbola cut the base line. Master – Slave distance is 250 Km:

Draw the situation. Look at the range difference between the master and the slave = 50 km Halve this difference = 25 km This is the distance the hyperbola will cross the base line from the central perpendicular Therefore the hyperbola will cut 125 + 25 km = 150 km from the slave

ATPL Radio Navigation ©Atlantic Flight Training 17-1

Chapter 17.

LORAN C

Introduction LORAN is an acronym for LOng RAnge Navigation. Developed in the second world war with the intention of providing aircraft with a hyperbolic navigation system that did not suffer from sky wave contamination. Principle of Operation LORAN C is a hyperbolic navigation system. The system is scheduled to be discontinued in 2000 but some users are making strong moves to retain its operational status. LORAN C stations are grouped in a network or chain, with one station as a master and the other stations arranged around it as secondary (slave) stations. Secondary stations are identified by the letters W, X, Y or Z. The chains covering the North Atlantic are shown below. The distance between master and slave is between 600 to 1000 nm.

Coverage extends from Asia, over the USA, North Atlantic Region and Europe. LORAN C uses the principle of differential range by pulse technique. Frequency 90 to 110 kHz

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Typical LORAN C Chain In the diagram above three chains cover the North Atlantic, looking at the individual composition of the Norwegian Sea Chain is as follows.

Station Position Designation Location Eidhi Faroes Master 7970 Faroe Islands

Sylt Slave 7970W Germany Bo Slave 7970X Norway

Sandur Slave 7970Y Iceland Jan Mayen Slave 7970Z Arctic

A station’s location with respect to other stations in the chain is determined by the coverage requirements. A monitor station is also part of a LORAN C chain. The monitor station is usually located at one of the transmitting stations. The distance between master and secondary stations (base line) varies between 600 and 1000 nm. LORAN C signals are transmitted at low frequencies around 100 kHz. Low frequency signals travel as surface waves over the earth and this gives the LORAN C a theoretical range of up to 1500 nm over water. The master station in a chain transmits first and its signal is used as a reference. Each secondary station has a unique emission delay that allows the aircraft to receive the signal from the master before it transmits. LORAN signals are transmitted in pulses in the shape of an elongated pear. Signals from the master stations are sent out in groups of nine pulses with a pause after the eighth. Signals from the secondary stations are sent out in groups of eight pulses. LORAN C Transmission All the master and slaves transmit on the same frequency of 100 kHz. To ensure that there are no chain identification problems a specific PRI is allocated to each chain. The time that elapses from the beginning of one master pulse group to the beginning of the next is different for each LORAN chain. This time interval is called the group repetition interval (GRI). For each chain a minimum GRI is selected. It must be of sufficient length to allow time for the transmission of the pulse group from each station, plus the time between each pulse group. The GRI is used as a way to label LORAN chains. For example, a GRI of 79700 microseconds becomes chain 7970 by dropping the last significant zero. Chain 7970 corresponds to the Norwegian Sea Chain. Permissible GRIs are multiples of 10 microseconds from 40000 through 99990 microseconds. Operation Using the principle of differential range by pulse technique it is necessary to look at the way an operator deciphers the system. Weather or static build up can affect transmissions. The multi-pulse system minimises the problems of low frequency long-range transmissions. The master and slave pulses are transmitted in groups of eight, each pulse separated by 1000 microseconds. The master pulse is identified by an extra pulse which is transmitted 2000 microseconds after the main group. The pulses are then summated to give one strong pulse. Each pulse is between 180 to 270 microseconds long.

ATPL Radio Navigation ©Atlantic Flight Training 17-3

The operator will receive a train of pulses as shown above. The master transmits first and then the slaves follow in the order W, X, Y and Z. The operator will look at the chain and choose the two slaves which will give the most accurate fix. At the same time the master is gated to align the timing of the system.

Once the system has been gated the matching of the received signals is carried out to match the cycles. The measurement of time difference between the master and relevant slave is obtained from the pulse. The third cycle is always used as this is never contaminated by returning skywaves once it has been identified. For a 100 kHz radio frequency the period (1/T) is 10 microseconds which is also the accuracy we can obtain.

The LORAN C receiver uses the pulse envelope to find the timing point on a particular cycle. If the pulse is distorted, for instance due to a poor signal, the cycle tracking point may jump to

ATPL Radio Navigation 28 October 2003 17-4

the next pulse causing an error. This is how the 10 microsecond accuracy is determined. An error of 10 microseconds in the time measurements equals a typical 1 nm position error in the area around the base line. The position error increases away from the base line.

To minimise this error, LORAN C determines the time intervals by calculating the time between pulses and ‘cycle matching’ within the pulses by comparing the cycles of radio frequency within the master signal with those within the slave signal. This makes it possible to determine the time interval to an accuracy of 10 microseconds. The measured difference will be dependent on your position in relation to the Master Slave pair. On the slave base line extension, the total time difference will be base line propagation time plus slave delay and will be the smallest value. On the master base line extension, the delay will be twice base line propagation time plus slave delay and will be of the largest value. The area in the vicinity of the base line extensions is not a good LOP area due to spread of the hyperbola and high risk of ambiguity. Each pulse within a group may have its radio frequency cycles organised in a pattern of regular or inverted pulses. This phase coding enables the receivers to distinguish the master signal from the secondary stations and also helps to reduce sky wave and noise interference. In summary the propagation delay between the pulses from the Master and slave at every chain will be:

Baseline Extension From the Slave The time delay at the slave

ATPL Radio Navigation ©Atlantic Flight Training 17-5

Baseline Extension From the Master Twice the time the pulse takes to travel from the master to the slave plus the time delay at the slave The Baseline Bisector The time of travel of the pulse to the slave plus the time delay at the slave

All other propagation delays will be between the above figures. Coverage, Limitations and Accuracy LORAN C Coverage Even though most of the major avionics manufacturers now produce LORAN C systems, LORAN C’s nautical origin can still be seen in several ways. One of the most obvious is LORAN C’s continuing orientation toward over-water areas. Most of the existing chains are located in coastal areas or on islands, providing fairly complete North Atlantic and Pacific coverage. The western part of northern Europe and the Mediterranean Sea have almost complete LORAN C coverage. No coverage currently exists in the Southern Hemisphere. This limits LORAN C capability as an international long-range navigational system. Sky Waves LORAN C is a low frequency system, which means that the sky wave will sometimes be received by the LORAN C unit in addition to the normal and preferred ground wave. Since the sky wave travels a greater distance than the ground wave, it takes a longer period of time to reach the receiver. At certain distances from the transmitter this may adversely affect the accuracy of the plotted LOP. Up to distances of 1000 nm from the transmitter the ground wave dominates and the receiver can fairly easily distinguish ground waves from sky waves. At greater distances however, usually over 1500 nm, the ground wave becomes so weak that the sky wave begins to dominate. The ground wave is more stable and reliable than the sky wave and is preferred for navigation purposes. Modern LORAN C receivers have circuits that enable them to differentiate between the stronger ground wave and the weaker sky wave. Most LORAN C receivers are able to make use of the sky wave signals at this range but accuracy is reduced. The problems arise at the intermediate distances, from approximately 1000 to 1500 NM. At these distances the sky wave and the ground wave are about equal in strength and this may confuse the receiver and distort the results, particularly at night. The phase coding of the transmitted pulses helps the receiver in solving the problems of interfering sky waves. Static Disturbances The modern LORAN C receiver constantly monitors the signal to noise ratio (SNR). The SNR is a ratio given by the signal received divided by noise due to static or other disturbances. Precipitation static causes electronic “noise” and this noise causes the SNR to drop. LORAN C receivers will normally issue a warning when the ratio drops below a certain minimum level. The static charge picked up when flying through precipitation can seldom be totally eliminated

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but it can be reduced by airframe grounding and by installing static discharge wicks. Other electrical disturbances such as those produced by thunderstorms may also have an influence on LORAN C accuracy. Radio Propagation Speed The accuracy of LORAN C is related to the propagation speed of the radio waves, which is assumed to be constant. This is not the case, the speed of radio waves is influenced by factors such as the time of day, the season and, especially, by the different nature of the terrain. LORAN C was designed as an over-water system and all of the assumptions as to the signal speed and transmission are based on signals propagated over seawater. Calculations based on signals propagated over land will be slightly in error. Modern LORAN C receivers have compensating circuits, called additional secondary factors (ASF) circuits, to correct for this condition. These propagation errors are usually not significant. They only affect the accuracy slightly and within predictable limits. The pilot should be aware, however, that peak accuracy can only be obtained with LORAN C when operating over oceanic areas using signals from transmitters situated on islands or coastal regions. Geometry of Crossing Angles A factor that affects accuracy is the angle at which the hyperbolic LOPs intersect each other. LOPs that cross at 90º angles produce small, square areas of position between hyperbolae. LOPs that cross at oblique angles produce large, diamond shaped areas between hyperbolae. Furthermore, the distance from the LORAN C station affects accuracy by increasing the distance between LOPs. A 10-microsecond change will represent approximately one nautical mile close to the base line, compared to 8 nautical miles near the base line extension. These distances will both increase since the hyperbolae spread with increase of distance from the base line. Due to ambiguity, the area around the base line extension is not usable for LORAN C navigation even though you may be close to the station. Interference from navigation and communication stations within the same band as LORAN C may influence LORAN C accuracy. To cope with interference from other transmitters within the same band, the manufacturers install so called notch filters at the receiver input. These are designed to reduce interference from transmissions on adjacent frequencies. Use of LORAN C The use of LORAN was more complex before the introduction of new electronic processors. Signals from the master and secondary stations were presented on the horizontal trace of a cathode ray tube and appeared initially as vertical blips on the linear time base (as shown on page 17.2). The cathode ray tube displays the master and secondary pulses. The operators were able to move the two pulses and compare them and the time difference between the master and secondary pulses could be read as the distance X. This distance in turn corresponded to the time difference relative to the hyperbolic LOP. Once the operator had determined the hyperbolic line of position, he plotted it on a LORAN chart that had all the hyperbolic lines drawn on it. Then he had to start the operation all over again in order to find another line of position by using another master/secondary pair in the chain. A new generation of LORAN C receivers has been developed with the introduction of electronic processors. These very sophisticated receivers make all the calculations the navigator used to make but in a very short time and provide present position instantaneously. The new LORAN receivers not only provide present position instantaneously but can also

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provide a lot of additional information such as bearing, track, ground speed and drift angle without the need of any manual operation. There are many different types of control display units (CDU) for LORAN C operations.

There are also more expensive and sophisticated CDUs, which are intended for larger aircraft and are generally designed to fit in the control stand on the pilot’s side with the bulk of the electronics located in a remote unit. Although they will still be found in many aircraft, it is unlikely that they will be fitted in newer aircraft since the future of the system is very much in doubt. There are currently 15 chains worldwide. Most LORAN C units require that the pilot manually select the specific LORAN C chain to be used. The GRI is used by the pilot to tune the desired chain. In more sophisticated units an internal memory will remember the last position when the set was shut off. This information will then be used to find the applicable chain when power is switched on again. For instance, the GRI for the Norwegian Sea is 79700 microseconds. The identification for this station is 7970. Once a GRI has been selected, the unit then goes into search, identify and track cycle. During the cycle the unit attempts to receive the master station for the GRI selected and then looks for the two best secondary signals in that chain. LORAN C Navigation LORAN C can provide point-to-point navigational guidance with great reliability and accuracy and is easy to use. Specific procedures vary from unit to unit, but basically once the point of origin is established, the pilot can determine a route between waypoints and follow it, using all the additional information provided by the unit. The point of origin is entered into the unit by several methods. For instance, latitude and longitude co-ordinates can be entered manually. Selecting a previously stored airport identifier or waypoint can also enter Latitude and Longitude. Finally, the point of origin can be entered automatically by selecting position mode and allowing the LORAN C to determine the present position by itself. This last procedure is usually accomplished in flight. Once present position has been entered, the destination waypoint is then entered either by LAT/LONG co-ordinates, or by waypoint identifier. At this point the unit can be switched to navigation mode and track guidance will then be reflected on a dedicated instrument or on a CDI or HSI. The most direct method of tracking using LORAN C is by using the cross track distance (XTD). It is the distance between the desired track and the actual track. XTD is the actual distance the aircraft is off track and is displayed directly on the receiver’s monitor. If, for instance, the LORAN C unit indicates a cross track distance 1.2 N.M. right of track, the pilot can simply correct to the left and monitor the cross track distance decreasing as the aircraft is approaching the desired track. Many modern sophisticated Navigation Processing units will accept LORAN C position information to be used alongside that from other sources.

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The development of GNSS would appear to mean that the use of LORAN will become rapidly less important in aviation. Transmitter Fault Indication The ninth pulse in the master transmission can be coded to indicate a variety of problems, normally the blink is the Morse code letter R (. _ .). A slave station can also blink the first two pulses of transmission to indicate a fault. A blink code is used to indicate:

Station not transmitting Incorrect phase coding Incorrect number of pulses Incorrect pulse spacing Incorrect pulse shape Time difference outside specified limits.

Both master and slave stations blink if either station is operating incorrectly. Range and Accuracy Both range and accuracy can be affected by the following:

The power and accuracy of the ground transmission The path of the surface wave The propagation conditions for sky waves The receiver and aerial employed Noise in the reception area The aircraft’s position in the chain

The approximate daytime ranges of LORAN are:

Ground Wave 1200 nm over water 900 nm over land

Sky Wave Up to 2500 nm by day or night

Accuracy Limits

Ground Wave ± 0.5 nm on 95% of occasions up to 1000 nm over the sea Sky Wave Sky wave should only be used when out of ground wave range. 10 – 15 nm at 1500 nm. Up to 20 nm at 2500 nm

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Chapter 18.

Global Navigation Satellite Systems (GNSS) Introduction Satellite navigation was initially developed for military operations. The system is now used extensively in the civil aviation world. Available 24 hours a day the system provides accuracy and reliability never seen before in aviation systems.

Two systems have been developed:

GPS Navstar Global Positioning Service developed by the USA GLONASS The Russian equivalent

This chapter refers to GPS, GLONASS is discussed at the end of this text. System Capability For civil aviation, GPS is capable of providing users with the following information:

Position, in three dimensions Velocity determination Time

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Frequency Fundamental Frequency f0 10.23 MHz Carrier Frequency L1 1575.42 MHz Carrier Frequency L2 1227.60 MHz

The use of the frequencies will be discussed later in this chapter. Basic Principle of Operation

GPS uses a similar principle of operation to radar:

The satellite transmits a signal The receiver measures the time the satellite signal is received Knowing the time of transmission the time difference is measured and the range

is calculated Example

Assume that 2 people on the surface of the Earth are positioned a set distance away from a satellite (also on the Earth’s surface).

The satellite transmits a signal which is received by both people 10 seconds after transmission.

Assume that the speed of sound is 340 metres/second. Each person is 3400 m from the satellite.

10 seconds

10 seconds

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The range depends upon:

The satellite transmitting at the correct time The speed of sound being exactly 340 metres per second The clocks of the receivers being correct and synchronised to the satellite clock

In addition to the above only one range is available. Each person is somewhere on a circle of position at a range of 3400 metres. Assume that a pilot of an aircraft receives the same satellite signal 10 seconds after transmission. The aircraft could be anywhere on a spherical surface radius of 3400 metres from the bell.

For a person on the ground one satellite only gives one position line. Three satellites will give a three position line fix. As soon as a fix is required in 3 dimensions then a fourth satellite is required.

10 seconds

10 seconds

10 seconds

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If the time piece in the receiver is in error then the range from each satellite will have the same range error. This would present you with three possible positions. By biasing each of these ranges equally you could deduce the accurate position and, at the same time, determine the magnitude of the clock error (known as clock bias).

The GPS System

The GPS system consist of three segments:

The Space Segment The Control Segment The User Segment

The Space Segment This is made up of a group of satellites known as a ‘constellation’, which provides the navigation signals. To date GPS consists of 24 satellites forming the GPS constellation, of which 21 are operational and 3 are held in reserve as back-up against other satellite failures. The Constellation is arranged in:

Six orbital paths with 4 satellites in each Orbits which are inclined at 55º to the equator Orbits which are separated from each other by 60º of latitude as they cross the

Equator. A satellite is said to be “masked” when it is less than 5° above the horizon and will not be used in the navigation solution.

Clock error is the distance between the solid and the dotted line for each satellite Denoted by the arrows

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GPS Satellite Constellation

Orbit Height Approximately 20 200 kilometres Orbit Time Approximately 12 hours.

There is a time difference of 3 minutes 57 seconds between two orbits of the satellite and one rotation of the earth around its axis. This is due to the length of the sidereal day being different from that of the solar day. The system is designed so that an observer can always detect signals from a minimum of five satellites. The satellite receiver includes:

Wide coverage aerials normally on the top of the aircraft fuselage Operating transmitters, receivers and Quartz clocks

The Atomic Clock uses a combination of caesium and rubidium atomic frequency standards. This gives a clock accuracy of 10-13 x 3, about 0.003 seconds every 1000 years. The receiver system usually has a back-up capability for failures of any part of the system.

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GPS Timing GPS timing is measured in seconds from a start date of 00:00:00 UTC, 06 January 1980. The system then runs for 1024 weeks after which the time clock restarts at zero. Because the start date was UTC the GPS has to run referenced to UTC. The time difference between the satellite clock and UTC at present is approximately 13.5 seconds. The accurate time difference is transmitted in the satellite broadcast. Frequency and Coding GPS satellites transmit on two frequencies:

L1 1575.42 MHz L1 transmits the coarse/acquisition (C/A), Precision (P) codes and system data message

The C/A code is repeated every millisecond on a frequency of 1.023 MHz The P code is repeated every 7 days on 10.23 MHz The navigation and system data message on 50 Hz L2 1227.60 MHz Transmits the P code only This frequency determines ionospheric delay

New satellites will also have a capability for the inclusion of a new frequency (L5) upon which the calculations for automatic correction of ionospheric time delays will be based, although as yet the final frequency for L5 has yet to be decided. It must be greater than 200 MHz difference from L1 in order to optimise the ionospheric corrections. Sufficient separation must also be allowed for other Radio Aids in the UHF band such as DME.

C/A Code 1.023 MHz

Nav/System Data 50 Hz

P Code 10.23 MHz

L1 Carrier 1575.42 MHz

L2 Carrier 1227.60 MHz

L1 Signal

L2 Signal

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These signals use Pseudo Random Noise (PRN) to carry messages. PRN uses binary mathematics generated in a predictable manner by the on board clock. This is achieved by using Binary Phase Shift Keying (BPSK) in which the phase of the carrier wave is reversed when the PRN code changes. Since the PRN sequence is generated by the on board clock, the start time of each sequence is precisely known and can be carried with the sequence. The PRN sequences also make up two codes known as:

Coarse/Acquisition code C/A Precision code P

Remember: L1 carries both C/A and P

L2 carries only P. The P code is only available to authorised users. As a security measure, the military introduce a security feature which changes the coding of the P code. It is then known as the Y code. This does not affect the C/A code which civilian aviation uses. Navigation Message Superimposed on both C/A and P codes is a navigation message. (NAV-msg). This message contains five discrete sub-frames containing:

Clock data for the satellite being tracked Ephemeris (the satellite orbit) for the satellite being tracked Message – data on obtaining UTC and, for C/A users, ionospheric delay

corrections Almanac data – information about all the satellites in the constellation.

TLM HOW SV Clock Correction Data

TLM HOW SV Ephemeris Data (1)

TLM HOW SV Ephemeris Data (2)

TLM HOW Other Data

TLM HOW Almanac Data for all SVs

25 Pages of Subframes 4 & 5 take 12.5 minutes to download

1 2 3 4 5

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TLM Telemetry HOW Handover word SV Space Vehicle (satellite)

Each sub-frame takes 6 seconds to transmit, the total frame time taking 30 seconds. The 4th subframe contains the ionospheric propagation data. Frame 5 transmits the current SV constellation data. Frames 4 and 5 are changed in every message; a series of 25 frames is required to download the whole almanac. The almanac information taking 12.5 minutes to download. The almanac is usually downloaded every hour and is valid for 4 hours to several months The Control Segment This provides the control and support system for GPS. It is made up of:

Master Control Station (MCS) Colorado Springs Monitoring Stations (MS) Ascension

Hawaii Kwajalein Diego Garcia Back up MCS Onizuka

The Master station tracks, monitors and manages the satellite constellation. It also provides an updating service for the Navigation Message

Hawaii

Colorado Springs

Ascension

Onizuka

Diego Garcia

Kwajalein

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The monitor stations are precisely surveyed and consist of very accurate receivers which receive from each GPS satellites in view:

Ranging data Navigation message

The satellite internally computed position and clock time are checked at least once every 12 hours. This data is transmitted to the master control station. The MCS establishes the satellite’s exact orbit and location, the satellite ephemeris along with the actual and predicted clock parameters. The MCS then transmits this information to each satellite the updated ephemeris and clock data to be included in the Navigation Message. The User Segment The GPS receiver. Each receiver decodes the space segment to determine position.

Multi-Channel Receiver Preferred for civil transport aeroplanes. This system monitors all the satellites in view and selects the best four satellites in determining the position. Sequential Receiver This type of receiver scans the satellites in a sequential manner in order to determine the pseudo range. This means that fixing can be quite slow. Multiplex Receiver Quicker than the sequential receiver the system is still only single or twin channel.

Advantage can be gained by using more than four channels in an “all in view” system. If five channels are used, it is possible to track all five satellites “in view”. If a satellite is temporarily obscured from the aerial, there are still four satellites “in view” and providing the full position, velocity and time data. GPS Operating Principles Current satellite signals are transmitted on two frequencies. These are identified as L1 (1575.42 MHz) and L2 (1227.60 MHz). Each RF signal is modulated by “Binary Phase Shift Keying” (BPSK).

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This modulation is used to provide Pseudo Random Noise (PRN) sequences that carry messages and make up two codes. These two codes are known as:

Coarse/Acquisition code (C/A) The Standard Position Service (SPS) is provided on this code. This is available to all users. Precision code (P) A Precision Position Service (PPS) is provided. The availability of the “P” code is limited to users authorised by the US Department of Defence.

The PRN code is generated and transmitted from the satellite, however the code is not truly random but follows a strict mathematical process, so it is predictable and can be reproduced within the receiver and referenced to GPS time. It is important to remember that the signal reproduced with the receiver is not transmitted. At the GPS user unit the received satellite signal at the aerial and is fed to the RF amplifier and then to the Phase Modulated receiver where it is compared and matched with the appropriate PRN code held in memory which determines:

The identity of the satellite The time at which the signal was transmitted from the satellite

During the time that the receiver is identifying the PRN code it is also downloading the navigation message which is modulated onto the L1 carrier signal. This information provides the basis upon which the initial range can be refined into an accurate value to provide the required accuracy for which GPS is certified.

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By matching up the received signal and the reproduced signal, a “time of arrival” can be determined. This TOA is the measured value of elapsed time between transmission and reception of the satellite signal and a range can be derived. This derived range takes no account of the fact that the receiver clock may be in error and is referred to as a Pseudo Range (PR). Pseudo Range A Pseudo Range measurement is equal to the TOA value determined by the receiver. It contains:

Signal travel time errors and The GPS receiver’s clock bias (error).

Pseudo Ranges (PR) are fed from the receiver to the data processor. Within the data processor (sometimes called the navigation processor), each PR is corrected for:

Satellite clock errors (which are the difference between the satellite clock time and the GPS system time)

Atmospheric distortion of the radio signals Effects of relativity Receiver noise

The data for these corrections is determined from information that the processor collects from the satellite’s navigation message. This message is superimposed on both C/A and P code signals. The navigation message consists of 25 data frames. Each individual data frame contains 1500 bits of information and is divided into five x 300 bit sub-frames. Each frame takes 30 seconds to transmit so that the entire 25 data frames are repeated every 12.5 minutes. The navigation message contains

GPS system time of transmission A hand-over signal for processors transferring from C/A to P code use Orbital position data for the satellite (its ephemeris) Clock data for the particular satellite being tracked Almanac data giving information about the operational status of all the satellites in

the constellation. Coefficients for the calculation of UTC Coefficients for the determination of ionospheric propagation delay.

The navigation message data is normally valid for up to four hours. It is monitored and maintained as part of the master control station’s task.

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Once the GPS receiver/processor has applied the corrections derived from the navigation message, it can now set out to resolve its own “clock bias”.

In the example above, three ranges have been determined from a TOA system in which all variable errors have been reduced to zero. If there were no ‘clock bias’ the three ranges would intersect at a single point – the dotted lines. Clock bias will affect all measured ranges equally and therefore the actual position will lie in the geometric centre of the shaded area. By adjusting the measured TOA values by equal amounts until the three ranges intersect at a point, the clock bias can be determined. This is what the receiver processor does in a mathematical solution but, instead of solving for a two-dimensional fix, it solves for four unknown quantities, namely:

Users x co-ordinate Users y co-ordinate Users z co-ordinate Time (t)

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The same x, y, z co-ordinates are used in defining the positions of the satellites and they are referred to an origin located at the centre of the earth as shown below. Using the origin, the processor then converts the x, y, z co-ordinates into a fix referenced to the WGS – 84 ellipsoid. This conversion allows for the shape of the ellipsoid and results in a position in terms of latitude, longitude and height.

The GPS receiver/processor uses the information in the navigation message to compute the exact location of the satellite, the relative velocity of the satellite and the aeroplane does not affect the fix accuracy in any way. The time that it takes the GPS receiver to position the aircraft is dependent on:

The almanac currency. The position and time in the receiver.

If these three parameters are correct then the fix will be obtained within 30 seconds. If the almanac needs updating or the position and time are wrong then a fix will be obtained only when all the subframes have been transmitted and the receiver updated . In this case the time to fix will be over 12.5 minutes. Velocity Measurement The processor generates a carrier signal at the same frequency as L1which is compared to the frequency received from the satellite L1 signal. A difference will exist due to Doppler shift.

Prime Meridian

Equator

Z

Y

X

Position X1 Y1 Z1

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This relative motion between the receiver and the satellites is used to derive the aircraft velocity. The receiver can also derive an accurate UTC from the navigation message and display this to the user. The GPS receiver solves for the three elements:

Position Velocity Time

GPS Receiver A schematic diagram of a typical GPS avionics receiver is shown below. The receiver is fed from an aerial, generally mounted on the top of the fuselage.

The aerial is designed to provide uniform sensitivity for all signals from satellites above a specified angle of elevation (normally 5°). It is shielded from low elevation signals that are prone to multi-path signals especially signals arriving from a low elevation satellite. Multi-path signals may occur from:

Reflections and refraction from the earth and Its environment

If received multi path signals would cause significant errors. The RF amplifier sets the receiver’s noise level and rejects interference from other RF sources that are not the satellite frequencies. The Reference Oscillator provides the receiver’s time and frequency reference. The output from the RO is fed to the Frequency Synthesiser, which converts the output to local oscillators. The frequency synthesiser also provides the essential clock information to the Signal Processor. Here in the signal processor the following critical functions are performed:

Reference Oscillator

Frequency Synthesiser

Preamplifier Down Converter

F Signal Processing

Navigation Processing

Position Time Velocity

Antenna

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Splits the signal into multi channels so that the separate satellites ‘in view’ of the aerial can be processed in parallel

Generates the reference PRN codes Acquires the signals from the individual satellites Tracks the code and carrier of the individual satellites De-modulates and ‘reads’ the nav message Extracts the pseudo range rate and pseudo range from the satellite signals Maintains the relationship between the receiver and GPS time.

This information is fed to the Navigation (data) Processor, which performs some or all of the following tasks:

Selects satellites to be acquired and tracked by the signal processor and computes signal acquisition and tracking aiding information for that function.

Collects navigation data messages and measurements from the signal processor and maintains a database.

Computes the positions, velocities and time corrections for each satellite and corrects the measurements.

Uses external navigation data to assist with navigation processing. Determines position, velocity and time.

When switching any GPS receiver on, or if the GPS signal has been interrupted, the signal processor/ navigation processor must go through a search procedure in order to:

Detect and identify the visible satellites Carry out the tracking and information gathering functions Determine the position, velocity and time.

These processes, if not assisted, will occupy a noticeable period of time, which is known as “Time to First Fix” and could take up to 15 minutes on older systems. Pre-setting the receiver processor to the current position can reduce the time delay. System Limitations

There are two levels of positioning service:

PPS Precision Positioning Service this is not available to civil. SPS Standard Positioning Service is carried on the C/A code, is available to all and is the one that concerns us.

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Number of Users Since the pseudo ranges are determined from signals, which are broadcast without specific address, the number of users that can be served at any instant is virtually limitless. Coverage Of the 24 satellites in orbit, only 21 need to be fully operational. The remaining three can be considered as reserve satellites that can be rapidly re-deployed to replace other failed units. Current Minimum Operational Standard calls for a minimum of five satellites to be above a mask angle of 7.5º. Reliability/Integrity Monitoring of the satellite is carried out by

Internal systems within the satellite By the five monitor stations and The master control station of the ground segment

These ensure that signal performance degradation is detected at an early stage and give GPS a similar reliability to most well used navigation aids. It is possible that the satellite health message, a component of the satellite’s navigation message, may only be changed after a cycle of 25 frames transmission. So it is possible that a period of at least 12.5 minutes may elapse before a receiver/processor corrects the source message. ICAO does not accept this delay and GPS is not acceptable in precision approach landings. Receiver Autonomous Integrity Monitoring (RAIM) RAIM is system integrity monitoring within the GPS unit. The receiver/processor evaluates the information from the five satellites visible. The position is determined. From the positions determined the receiver looks to see if one of the satellites is giving an incorrect range and if so removes it from the position calculation. For RAIM activity there must be:

Five satellites visible and The receiver/processor must be capable of handling the extra data.

The GPS receiver/processor can be integrated into on-board navigation systems, taking inputs from other navigational sources. This is known as receiver augmentation. If the navigation computer detects a sudden marked deviation of the GPS position, a satellite failure can be suspected.

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GPS Integrity Broadcast (GIB) A ground-based satellite monitoring system which is sited and surveyed accurately. These sites measure satellite range on a continuous basis and from the measured values range errors are computed and broadcast, via satellite, to all users. Modern equipment uses a combination of RAIM and GIB. Coverage Problems GPS coverage can be interrupted or degraded by transient “holes” in the coverage. These holes can appear in certain areas on a regular basis lasting from a few minutes to a few days. Normally the GPS signal is weakened causing a degradation of accuracy. In some cases a total loss of the GPS service can occur. The cause of these problems can be the result of any of the following:

Other transmissions on the same frequency Multiples (harmonics) of other transmissions Inter-modulation which causes a similar effect to above Suppressing or blocking interference where the GPS receiver is swamped by

strong RF signals and is de-sensitised. Deliberate jamming.

Accuracy and Error Sources Accuracy for Civil Use The accuracy of the SPS on 95% of occasions is expected to be

35 metres horizontally 75 metres vertically

In practice however the accuracy achieved is much greater. The reason for the discrepancy between the horizontal and vertical figures is to do with geometry. The surface of the sphere around each satellite is predominantly in the vertical plane when perceived on or near the Earths’ surface, which give rise to a stretching of the area of uncertainty in the vertical plane.

The derived velocity will be accurate to within ± 0.2 metres/second The time will be accurate to:

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For GPS time 52 nanoseconds For UTC 340 nanoseconds For interest the PPS accuracies are listed as:

5 metres horizontally 27.7 metres vertically

User Equivalent Range Errors User Equivalent Range Error (UERE) is the result of a number of sub elements as follows:

Errors in the content of the satellite navigation message. Predictability of the satellites’ orbit – this may be disturbed by perturbations which

result from:

Asymmetry of the earth’s gravitational field Lunar and/or solar gravity Atmospheric drag Electro-magnetic forces Solar wind

Stability of the satellite’s clock.

The satellite clock is monitored and its general error trend is established and extrapolated to form part of the navigation message. If the trend changes this will establish an error source:

Precision of the PRN tracking in the GPS receiver. The heart of this is a stable clock. If the receiver clock drifts, the tracking sequence will be in error.

Errors in the processor’s ionospheric model. If the ionosphere/troposphere create different refractive indices from those used in the model, errors will be caused.

The UERE is minimised by:

Good satellite signal quality Good receiver/processor design.

Dilution of Precision (DOP) Like all position determining systems that use lines of position, the geometry of the intersecting lines greatly affects the potential accuracy of any resultant fix. If the satellites in use, as viewed by the GPS receiver aerial, are close together the surfaces of position will

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have a poor angle of intersection and, as a consequence, there will be a significant loss of accuracy. This is referred to as a high dilution of precision. A high DOP would cause any UERE to have a greater effect.

DOP can be referred to as:

GDOP geometric DOP as just described VDOP DOP in the vertical (altitude) HDOP DOP in the horizontal PDOP DOP of position, which is a combination of VDOP and HDOP TDOP DOP in time.

Error Predictions Up to a point, both UERE and DOP error sources can be predicted for a specific receiver at a specific time in a specific place. Not all error sources are totally predictable such as ionospheric refraction and solar wind. There is always the possibility of uncompensated errors existing. Differential GPS (DGPS) GPS provides a world wide navigation capability with a high level of accuracy. At present the system cannot be used for precision approaches unless a differential system is used (ICAO requires that a 2 second warning of failure is given for a precision approach and 8 seconds for a non-precision approach) Differential GPS involves the cooperation of two receivers:

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A stationary receiver that is accurately surveyed A moving receiver that requires accurate position information

DGPS Principle of Operation GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals has some error or delay. If each timing signal has an error, the calculation of position is going to be a compounding of the timing errors. The satellites are so far out in space that the short distances we travel on earth are made to appear insignificant. If two GPS receivers are close to each other the signals that reach them will have travelled through the same portion of the atmosphere. This means that both will have been affected by the same errors. If these variable errors can be established then they can be corrected. If a reference receiver is used to measure these errors it can be used as a referencing system. This reference station receives the same GPS signals as the aircraft. Knowing its own position accurately it calculates the time that it should take for a satellite signal to reach the earth. Comparing the calculated to the actual time gives the error correction required for an accurate position. The receiver transmits this error correction to the aircraft which then corrects its own messages. The reference receiver catalogues all visible satellites as it does not know which of the many available satellites the aircraft might be using. The aircraft receives the complete list of errors from the reference receiver and applies the corrections for the particular satellites it is using. The use of DGPS significantly improves the accuracy of GPS as illustrated: Summary of GPS Error Sources

Typical Error in Metres Standard GPS Differential GPS Satellite Clocks 1.5 0 Ephemeris Errors 2.5 0 Ionosphere 5.0 0.4 Troposphere 0.5 0.2 Receiver Noise 0.3 0.3 Multipath Reception 0.6 0.6

Typical Position Accuracy

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Horizontal 29 1.3 Vertical 45 2.0 Pseudolite/DGPS By using a pseudolite (a pseudo-satellite), it is possible to reduce the VDOP. Trials have revealed that VDOP can be reduced from 2.3m for DGPS to less than 1m for a pseudolite augmented DGPS. The reference station is exactly the same as in DGPS. The pseudolite is set at a fixed location and accurately surveyed. The pseudolite generates a GPS style of signal, which is capable of being coded and used as a ranging signal. The reference station monitors the transmission from the satellites in view and that from the pseudolite. From monitoring these signals, differential corrections are derived and sent to the pseudolite. The pseudolite broadcasts these corrections so that user equipment can use the broadcast to:

Determine the differential corrections Establish an additional ranging input.

Pseudolite augmented DGPS provides a significant advance, especially in applications for precision approach landing. In addition to the enhanced accuracy, a pseudolite provides the capability of overcoming multi-path effects and their signals are unaffected by ionospheric and tropospheric delays. Because the range to the pseudolite changes rapidly, the receiver is subjected to a much greater variation in signal strength. If the pseudolite is set up on the approach path its transmitter power may be set to provide a coverage of 20 nm. Approaching the pseudolite, the intensity of the received signal will increase inversely with the square of the range from the pseudolite. At 0.1 miles the signal is 40 000 times stronger, this could cause avionic saturation and swamp the satellite signal. This problem is resolved by modifying the broadcast signal structure.

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Aerial screening can also be a problem. The pseudolite is on the ground and the satellite navigation system receiver aerial is located on top of the aeroplane. The fuselage can screen the aerial and prevent reception of the pseudolite signal. This problem could be solved by using separate aerials; satellite aerial on top of the fuselage and pseudolite aerial below. This is not the ideal solution. Normally, the pseudolite is positioned offset from the approach path and, if possible, on an elevated site so that fuselage screening is overcome. Satellite Based Augmentation Systems (SBAS) The pseudolite is capable of providing high accuracy in small areas. The accuracy deteriorates rapidly away from the pseudolite until 100 kilometres there is little gain from its use. SBAS is being implemented to allow high accuracy at greater ranges. Three systems under development, all of which work on the same principle. These systems are:

Wide Area Augmentation System (WAAS) – developed in the US European Geostationary Navigation Overlay System (EGNOS) – Europe Metsat Satellite Based Augmentation System (MSBAS) – Japan

All operate on the same basic principle and aim to provide accuracy sufficient to enable Category 1 precision approaches.

←Pseudolite Signal Differential Correction→

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SBAS uses:

A ground segment A space segment and A user segment – just like GPS.

The ground segment consists of a number of precisely surveyed:

Wide Area Reference Stations (WARS) Wide Area Master Station (WAMS) and A Ground Earth Station (GES).

Ground Earth Station (GES) Reference Station (WARS) Master Station (WAMS)

GPS Satellite

Geostationary Satellite

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The WARS and WAMS stations are connected by communication data links. The network of reference stations (WARS) track the satellites and data link information to the WAMS. The WAMS:

Collates all the data, Determines the differential corrections for each satellite being tracked Organises the data and Then formats a data broadcast.

This is sent to the ground earth station (GES) and uplinked to geostationary satellite. In the American WAAS and the European EGNOS systems, the geostationary satellites used are from the INMARSAT 3 series of marine communications satellites. Up to 4 of these will be equipped with navigation packages. The Japanese system envisages the use of their meteorological satellites MTSAT 1 and MTSAT 2. The geostationary satellite receives the data and transmits it to all users as a broadcast. The broadcast uses the GPS L1 frequency, modulated with a C/A code that is of the same category as the GPS C/A codes and uses the same time basis. The signal also includes a ranging message so that the geostationary satellite can also be used as an extra positioning satellite. The message, broadcast by the geostationary satellite consists of:

An integrity message indicating the status of all GPS satellites in a use/don’t use format

Wide Area DGPS error corrections Ionospheric delay model Ephemeris and clock data for the geostationary satellite.

At the aircraft, the broadcast from the geostationary satellite is decoded. Using a wide area ionospheric delay almanac, the aircraft receiver determines the ionospheric delay for its position and applies the necessary corrections. No tropospheric delay correction data is included so any correction must be calculated from the stored standard model. RAIM in the Wide Area Augmentation System The Required Navigation Performance for a sole means of navigation makes it necessary for the role of Receiver Autonomous Integrity Monitoring to be extended. In a single navigation solution RAIM has to:

Detect Isolate and Exclude the failed source.

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The RAIM specification for RNP must include:

Alarm Limit Alarm Response Time Limitations On Nuisance Alarms High detection probability

GLONASS

The Russian developed counterpart to GPS. Basic concepts of the GLONASS system The Russian Global Navigation Satellite System (GLONASS) is based on a constellation of active satellites in a similar manner to GPS Navstar. The planned space segment is:

A constellation of 24 satellites Arranged with 8 satellites in 3 orbital plane Satellites are identified by a slot number, which defines the orbital plane (1-8, 9-

16,17-24) and the location within the plane The three orbital planes are separated by 120 ° of longitude at the equator and

the satellites within the same orbit plane by 45° of arc. GLONASS orbits are roughly circular with an inclination of about 64.8° The orbit is at 19 000 Km with a period of 11hrs.15m.44s.

The ground control segment of GLONASS is entirely located in the Soviet Union. The co-ordinate system of the GLONASS satellite orbits is defined according to the PZ-90 system, formerly the Soviet Geodetic System 1985l1990. The time scale is defined as Russian UTC. One difference from GPS, the GLONASS time system also includes leap seconds. Satellites transmit simultaneously in two frequency bands this allows the aircraft receiver to correct for ionospheric delays. Each satellite is allocated a particular frequency within the band, determined by the frequency channel number of the satellite. These different frequencies allow the aircraft receivers to identify the satellite.

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Superimposed on to the carrier frequency, the GLONASS satellites modulate the navigation message. Two modulations can be used for ranging purposes, the Coarse Acquisition code, with a modulation length of 586.7 metres and the Precision code, of 58.67 metres. The satellites also transmit:

Orbital information An almanac of the entire constellation and Correction parameters to the time scale.

The orbital values are predicted from the ground control centre for a 24 hour period and the satellite transmits a new set of orbital data every 30 minutes. The almanac is updated once per day. Integrated Navigation Systems The integration of the Navstar GPS and the GLONASS systems is a method of providing a truly Global Navigation Satellite System (GNSS). The system could provide for a high level of redundancy that is essential if the Required Navigation Performance (RNP) levels are to be achieved. Receivers are available that have been designed to decode both GPS and GLONASS signals. Europe has announced that, in association with Russia, the GLONASS system will be further developed. This would considerably enhance the reliability of the system and could bring GNSS close to meeting the needs of RNP. GNSS tends to be used:

As an integral part of a multi-sensor navigation system. As a Secondary and supplemental system in which the GNSS and its

augmentations may be used but another approved system must be available and useable at all times.

The integration of GNSS and an inertial reference system (IRS) is one means by which RNP can be achieved today. This provides the levels of:

Accuracy Integrity Availability Continuity

which are seen as the essential elements of RNP.

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The advantage of this hybrid system is that the IRS continuously determines vector velocities to a very high degree of accuracy. When these are mixed with the positional accuracy derived from GNSS, the resultant navigation performance is of a very high order. GNSS Applications GNSS with suitable augmentation will provide for:

Accurate en-route navigation including area navigation capabilities. Terminal area routing Precision approaches.

In general, the information is processed through the navigation computer of the FMC and is fed to the AFCS and to a display such as an EFIS or similar. GNSS is also being used in the Automatic Dependent Surveillance Broadcast (ADS-B). The system digitises the position information derived from the GNSS and broadcasts it as part of a data stream, which will include:

Aircraft identification/flight number Aircraft type Altitude Speed Heading Flight condition (climbing, turning levels, etc.).

This data will be renewed several times a second. If the data stream is linked to a communications satellite ATC will be given a continuous stream of “real time” data. Ideal for oceanic sectors, the same data stream transmitted through the transponder mode S could also be used by ATC for both:

Short range ATC, and Possibly TCAS warnings

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Chapter 19.

Area Navigation Systems Introduction In the early days of commercial air transport expansion a system of air routes was developed to provide a safe means of control and separation of aircraft. These routes, which became known as airways, were defined by radio navigation facilities sited at strategic distances apart and at significant navigational points such as:

Airway intersections Turning points, and FIR boundaries.

By following an airway, an aircraft is provided with a system of navigational checkpoints and, by being enclosed (by ATC) in a clear box of airspace, separation from other known aircraft in the vicinity. For many years the airway system has provided an adequate means of routing aircraft, in spite of the fact that navigation from departure aerodrome to destination is not normally the most direct route. In recent years however, a number of factors have led to a review of this situation. These include such elements as increasing congestion on the airways system. This is resulting in flow control and subsequent, frequently extensive, delays to flights. The development of improved and enhanced navigation and communication systems that permit an aeroplane’s position to be determined accurately and transmitted speedily to the responsible ATC unit. These enhanced ATC systems make it possible to provide aircraft with safe separation from other air traffic without the need to confine them to narrow corridors of airspace. Add to the above, the need to conserve costs which demand that the shortest route from departure to destination should be followed.

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To respond to these problems and enhanced capabilities, a system known as Area Navigation (RNAV) is being introduced. This is a system of navigation which is not dependant upon routing between points which are coincident with the position of a radio facility but is capable of providing navigational guidance along other non – airway routes marked by waypoints. A waypoint is a predetermined geographic position. This is defined in terms of latitude and longitude but, where appropriate, may also be defined as a radial and range from VOR/DME beacons. This is known as rho/theta (ρ/θ) system or by ranges from two DME beacons, known as rho/rho (ρ/ρ).

Subject to an aeroplane being properly equipped, area navigation will be available as follows:

Fixed Published RNAV Routes These can be nominated in a flight plan only if the aeroplane is fitted with an approved RNAV capability Contingency RNAV Routes Published routes useable by suitably equipped aeroplanes during specified times. Random RNAV Routes Unpublished routes. These may be flight planned within designated areas

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Area Navigation Concepts

JAR OPS 1 requires that an aeroplane’s navigation equipment should include “an Area Navigation System when area navigation is required for the route being flown”. There are three types of area navigation systems. Systems where the information is derived from airborne navigation equipment which is a self-contained system. A navigation system that is independent of any external information source. A typical self-contained navigational system will use the outputs from an inertial reference system (IRS). Inertial navigation is completely independent of any external visual or electronic reference. It simply updates the position of the aircraft by sensing its accelerations and integrating those, with respect to time, to establish distance and direction of movement from the start position. An integral navigation computer carries out all related navigation calculations. Externally referenced systems in which information from an external source (or sources) is required in order to provide navigational guidance. Hybrid systems that use information from a selection of self contained and externally referenced navigation systems Many commercial operators have been quick to realise the benefits of such a system and are not only specifying a suitable system for new aeroplanes but are, at considerable cost, actively retrofitting their older fleets with similar equipment. Most equipment installations on commercial aeroplanes will form an integral part of a comprehensive avionics package and will be capable of providing area navigation even when out of range of ground based navigation facilities. Such systems will normally be of the hybrid types. Most modern general aviation aeroplanes are fitted with a basic RNAV system (so called BRNAV) as standard. These are generally based on the rho/theta or rho/rho system using inputs from VOR/DME. However GNSS, the Global Navigation Satellite System, which is based on satellite navigation, will be increasingly utilised as the prime source for the required navigation information. Accuracy of RNAV Equipment There are two types of RNAV:

Basic RNAV The lateral track keeping accuracy of basic RNAV is ± 5 nm for 95% of the flight time

Precision RNAV The lateral track keeping accuracy of precision RNAV is ± 1 nm for 95% of the flight time

The track keeping accuracy is dependent on the navigation system error and Flight Technical Error. For obstacle clearance the Flight Technical Error is:

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Departure ± 0.5 nm (at the DER a value of ± 0.1 nm is assumed) Initial and Intermediate Climb ± 1 nm En-Route 2 nm

Basic RNAV VOR/DME based area navigation is a navigation and guidance system which uses basic signal inputs to compute track and distance to a waypoint. VOR bearings and DME slant ranging providing the required information. In some more sophisticated systems barometric altitude input may also be provided. A block diagram of a simple navigation system is shown below. The simple system, most commonly installed in general aviation aircraft, usually consists of a computer within which each waypoint is defined as a radial and range from a VOR/DME. Such waypoints are often called phantom or ghost stations. The computer’s memory is able to store a limited number of successive waypoints, normally a maximum of nine, so that the pilot can enter the planned route before departure. A more sophisticated system will utilise a navigational database stored either within the navigation computer or in an external storage unit. The navigational database contains all the necessary information regarding routes between airports, VOR/DME stations and waypoints. It is obviously important that this database is kept up to date. It will be updated every 28 days.

The Control Display Unit (CDU) is used to enter information into the computer and to display navigation information. In a basic system the navigation computer resolves the navigation problem. This receives a radial from the VOR receiver, DME distance from the DME interrogator, and altitude from the encoding altimeter. These parameters are used to establish the aeroplane’s current position.

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This is compared to the position of the next waypoint and an error signal is generated which is used to provide steering signals to a Course Deviation Indicator (CDI), Horizontal Situation Indicator (HSI) or other suitable display. A “distance to go” is also derived. In some aircraft installations, the computer may also send track correction signals (lateral steering commands) to the autopilot roll channel. Since the use of Area Navigation Systems permits waypoints to be accurately defined, determined and flown, the need to follow the ‘facility determined’ structure of the airways is removed and this permits direct routing and more effective use of the available airspace. The aircraft equipment will consist of the normal VOR/DME receivers, a navigation (course line) computer and a simple interface display.

Use of Basic RNAV When operating the course line computer, the pilot selects a VOR/DME station that is within the line of sight range of the desired waypoint. The radial and distance from the station to the desired waypoint is then manually entered. This can be repeated for a number of waypoints (if the equipment permits). Once the waypoint information is stored, the pilot can select the sector (waypoint ‘from’ and ‘to’) and the course deviation indicator will act as if a VOR radial has been selected. You should note that it is possible to select sectors that do not connect successive waypoints. This allows waypoints to be bypassed. Start navigating in the same manner as you would when tracking a VOR station. Distance to go will appear in the normal display.

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With area navigation, the amount of CDI needle deflection does not vary with distance to the waypoint as it would when tracking inbound to a VOR. It always represents a constant distance off track for a given deflection. On a 5 dot CDI, one dot deflection equals one mile of deviation, regardless of the distance to the waypoint. VOR-DME based RNAV has several applications:

You can file a RNAV route flight plan You get the opportunity of tracking a direct route (subject to ATC restrictions and

requirements) You can navigate directly to an intersection, bypassing waypoints. You can set up a holding pattern when the ATC instructs you to “hold at present

position” You can locate and approach an airfield that is not equipped with navigation aids.

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The plate shown here is the Oklahoma RNAV approach procedure in which all waypoints are given both in terms of radial/distance from the Oklahoma City VOR/DME, and latitude/longitude. The chart is shown for information purposes only. When using the RNAV approach mode, the system sensitivity is multiplied by 4. This means that, on the CDI, each dot represents a linear displacement of 1/4 of a nautical mile. RNAV Limitations The VOR/DME based BRNAV suffers the dependant limitations of VOR/DME equipment. If this signal is lost and the Nav warning flags appear on the CDI or HSI you must use an alternative method of navigation. You may fly the area navigation route, using guidance signals to the waypoint, only as long as the aircraft is within the operational range of the appropriate VOR/DME station. The practical range limit is around 200 nautical miles from the associated VOR/DME at the highest altitudes normally used for civil aviation but remember that the range limit for a light aeroplane will normally be considerably lower because of operational altitude. The accuracy of the position information and any derived steering signals is affected by the same sources of error as the VOR/DME in use. Some systems use DME/DME navigation, with frequency scanning DME interrogators. Such systems provide a more accurate navigation. All other area navigation systems that are dependent upon a single type of source, whether self contained or facility dependent will be affected by the errors of that source. A hybrid system, in which the data from a number of sources is electronically compared and the best information is used, tends to provide a higher and more consistent degree of accuracy.

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Chapter 20.

Introduction to the Flight Management System (FMS) Introduction There is considerable pressure to make more effective use of the airspace in which we fly. We are, therefore, increasingly adopting the Area Navigation philosophy and the modern transport aeroplane is extremely well equipped to comply with the needs of an Area Navigation System. In simple terms, those fundamental needs have not changed since ancient times.

We must know where we are – current position. We must know where we are going to – destination.

If we cannot see our destination, we must be able to compute the direction and distance to that point and we must continuously monitor our progress to ensure that we are following the correct path. The problem is the same but the tools that can be applied to create a solution are powerful beyond the dreams of navigators, even of 50 years ago. The heart of this power is the modern Flight Management System (FMS). This chapter is an introduction into the FMS. The Role of FMS On modern aircraft, the FMS is an integrated automatic flight management system that provides, through precision control of engine power and flight path, optimum economy of flight. At the same time, flight deck workload is reduced and this can considerably enhance safety. The diagram on the next page illustrates the many tasks that can be performed by the FMS In such an integrated system, the FMS is interfaced with the Power Management Control System (PMCS) and the Automatic Flight Control System (AFCS) so that it manages both control of power and flight path, both vertically and horizontally, against a pre-planned flight path. The FMS consists of two units namely:

Command Display Unit (CDU) The crew’s interface with the system. Flight Management Computer (FMC) Handles all the complex calculations and memory items required.

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The FMC has a data storage capacity, rather like the hard disk on your PC, which is in two broad sections. One section is dedicated to aircraft performance data for take-off, climb, cruise, descent, holding, go-around and abnormal flight (e.g. engine-out) situations. This data has a comparatively ‘long life’. The Navigation database, as the second section is known, stores all the data relevant to the airline’s route structure. This will include:

Navigation Facilities Position, frequency, identification, type Waypoints Latitude, longitude, type (en-route etc.) Airports & Runways Designations, elevations, locations, etc. Terminal Procedures SIDs, STAR, holds, etc. Approach & Go-around Procedures Routes Airway identifier, magnetic course

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Company Routes Details of normal routings.

This data does need frequent up dating and must be renewed every 28 days. Using these two packages along with the variable inputs (such as current position, wind velocity from aeroplane’s navigation computer and Air traffic clearance) it is possible to generate, or modify, a flight plan to meet the current needs. Since the FMC has all the data required, the activities associated with following a precision RNAV (PRNAV) route in three dimensions can be easily accommodated. All it requires is that the system is told the route to follow, the preferred flight profile, and the ATC clearance. This is done through the CDU. The aeroplane’s navigation sensors feed information to the FMC and, from those sensors, the best position information can be derived and a solution to the “what direction?”, “How far?” questions can be evolved. The navigation sensors used will normally consist of a hybrid of inputs from facilities selected by the FMC. A hybrid combination provides the necessary Required Navigation Performance to comply with the needs of a precision RNAV (PRNAV). These sensors may include some or all of the following:

VOR/DME ILS IRS LORAN GNSS

At this time, none of these sensors alone can be depended upon to provide the reliability, integrity and accuracy necessary.

VOR/DME units are limited in range ability and the position accuracy deteriorates with range from the facility.

IRS suffers from accumulative position errors. LORAN does not provide reliable coverage nor is it worldwide. GNSS, as we have seen, still requires augmentation.

The FMC will evaluate the data from the available sources to derive the “best position”. In addition, using inputs of altitude, airspeed, temperature and Mach number from the air data computer along with engine parameters and fuel data, a complete control of the flight profile can be exercised. This will ensure that the flight will be optimised in terms of fuel efficiency.

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Use of FMS

The FMS is now such a critical item of equipment that it will normally be a dual fit. With a dual installation, there is flexibility in how it is used, as follows:

Master/Slave Operation In this, one of the units is used as the “input” unit. All data entered into that unit is shared with the second unit. The computers “talk” to each other and, as well as sharing data, they compare each other’s information. Each FMC retains control of its associated AFCS, auto-throttle and selection of radio navigation aids. Independent use in which the FMS units operate totally independently. This allows the pilots to operate with one unit displaying performance pages while the other displays navigation data. Alternatively, one may be used to revise or review activities without disrupting either the active flight plan or the commands of the other CDU. Single use when only one FMS is operational. Back-up when the FMC is suffering from some failure but there is still a limited FMS function.

Within those parameters the crew need to decide on how much control is given to the FMS. The options are normally Managed Guidance, in which the FMS performs the task of maintaining the pre-planned route, speed and altitude profiles, or the alternative is to elect to control some parameter, such as a heading or speed hold, through the use of the flight control panel. A typical CDU is shown below. Through this unit, the flight crew can:

Construct a detailed flight plan Select data pages to view Respond to FMC requests for data entry Change displayed data.

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FMS design varies slightly from manufacturer to manufacturer but all will have similar capabilities and systems.

At the top of the CDU is the data screen. This is a flat CRT normally providing up to 14 lines of characters, each line providing space for up to 24 characters. Small sized characters are either default or predicted values. The crew can change these if the data originates from the computer. Large size characters show data entered by the crew. The bottom line of the data screen provides for three activities. The left hand half is a scratch pad on which data entered by the pilot is shown. The next ten characters are used by the FMS to pass messages to the crew, while the last two characters display “up” or “down” arrows to show the direction of any necessary scroll (movement) up or down the screen. At the end of each line on the CRT is a “line key”. When the FMC requires data, a question mark (?) will appear at the relevant line. For example, the FMC will need to know the start position of the aeroplane in terms of gate number, so a question mark will appear. You can now type the gate number in, using the alpha numeric keypad. The details you type will appear on the scratch pad and, once you have verified them, can be transferred to the correct line by pressing the adjacent line key. If an arrow appears against a line it normally indicates an optional activity for the flight crew. This could be either functional (e.g., aligning the IRS, deselecting a GNSS satellite) or display (e.g., selecting another page of data). If you choose the option all you need to do is press the “line key”. Function keys are also located below the CRT.

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Output Information Information derived from the FMS may be linked directly to the AFCS so that the aeroplane may be flown through or by the FMS. Displays of the information will normally be presented on the aeroplanes EFIS system with a map shown on the nav section showing planned route, active sector, waypoints etc. Details of attitude, speed, vertical speed, and etc. will appear on the PFD as normal. Full details of EFIS are covered in the ‘Instruments’ section of these notes. Another output of the FMS will increasingly be fed to the data transmission system for use in the ADS – B system which we will look at in the next chapter.

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Chapter 21.

An Overview of CNS/ATM Introduction This chapter looks at trends in the Communication, Navigation and Surveillance / Air Traffic Management (CNS/ATM, previously referred to as FANS) and provides an overview of each. Communications Current activity in Europe, as elsewhere in the World, shows a much greater sensitivity to the economic impact that ATM procedures have on aircraft operations. Much consideration is being given to the creation of new (or modification of existing) procedures which will lead to more efficient fleet-wide operations. These services span oceanic, enroute and terminal regimes. Most of the changes are dependent upon effective Data Link facilities rather than increased complexity of the FMS. Air Traffic Management and FANS 1 The integration of future Air Traffic Management ground systems with the Communication-Navigation-Surveillance functions in aircraft avionics will enhance the ability to provide concise scheduling, optimised time control and well-defined, reduced separation of air traffic. It is likely that aircraft equipped with ATM-compatible avionics will benefit by receiving clearances as filed for the scheduled landing time along with minimising delays caused by weather and other aircraft. CNS-ATM is currently in the early stages of system definition. The first step in the airborne side of the equation (CNS) is the FANS 1 concept. Aircraft equipped with FANS 1 are already receiving benefits on specific routes. The avionics functions required for FANS 1 are:

Controller/Pilot Data Link Communication (CPDLC, also called TWDL or, offering easier comprehension of what is meant, ATC Comm Data Link),

Required Time of Arrival (RTA), GNSS input for time and position, Required Navigation Performance (RNP), Automatic Dependent Surveillance (ADS).

These capabilities will result in a fundamental change in the way airspace is managed today. The automatic transmission of ground-to-air and air-to-ground clearance messages, flight planning data, along with knowledge of the aircraft's intent, its scheduled arrival time and a negotiated RTA at the metering fix will provide the ground ATM systems with unprecedented amount of detailed information for each aircraft. The airspace will become better managed, that is the intervention of the manager will be an exception rather than the rule. It will not be controlled and will not require the intervention of the air traffic controller on a routine basis.

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Operators will be able to realise the benefits associated with reduced aircraft separation and the ability to fly preferred (or direct) routings provided their aircraft are properly equipped to the requisite RNAV level. Controller/Pilot Data Link Communication (CPDLC) The definition and design of Data Link systems for communications between aircraft and Air Traffic Control (ATC) agencies is already at a very advanced state. To this end, manufacturers have worked extensively with regulatory authorities, airlines and service providers to develop the Minimum Operational Performance Standards (MOPS) for airborne ATC implementations. Data Link communications systems are being designed to provide more efficient aircraft communications for ATC and Flight lnformation Services (FIS). Although these systems essentially replace normal voice radio communications, a voice radio backup is considered essential for ATC communications at this time. Flight plan data, including aircraft position and intent in the form of future waypoints, arrival times, selected procedures, aircraft trajectory, destination airport and alternates, will be transferred to the ground systems for traffic management. The data sent to the ground ATM system will aid in the process of predictions of where each aircraft will be at a given time. Conflict management will be made simpler for the ground equipment, based on the use of actual flight plan data, compared with making predictions on the ground. The ground system may decide that a re-clearance (for example, new flight plan) is required for one of the aircraft in a predicted conflict situation. Through the utilisation of the RTA function on board the aircraft, a profile negotiation capability is possible based on the available RTA and the current flight plan. The diagram below shows how the transmission of Data Link messages are exchanged between aircraft and the ground ATM systems. Some operators may elect to use another link to their Airline Operations Centre (AOC), flight operations, or flight planning services. This additional link may be used for monitoring or intervention of flight planning route data, management of alternate airports and/or weather data. All links are two-way and require no pilot intervention, except to monitor messages and to confirm and implement flight plan modifications. The monitoring may be accomplished via the MCDU.

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Aeronautical Telecommunications Network (ATN) The Aeronautical Telecommunications Network (ATN) provides the means for world-wide data communication between ground and airborne host computers within the aeronautical community. It is a data communications network designed to allow interoperability between aircraft, airlines and air traffic control facilities and authorities. ATN utilises satellite, VHF, Mode-S and other specific sub-networks to transmit data from one end system to another, linked by a common working protocol. When fully completed, the ATN will allow any ATN host computer to communicate with any other ATN host computer without having a direct physical link between them. International agreements for the ATN are currently being documented in the International Civil Aviation Organisation (ICAO), with regard to Secondary Surveillance Radar improvements, Collision Avoidance Systems Panel, and Standards and Recommended Practices (SARPs). These will establish communications protocols and will, by these means, ensure international interoperability services and operational requirements are met. The ATN protocols are designed using existing international standards as a basis. The ATN will pave the way for major improvement in FAA/CAA Air Traffic Management services. The use of the bit-orientated, transparent ATN protocols allows any type of application data to be sent (including messages, graphics and video). The aeronautical telecommunications industry is currently defining Air Traffic Services (ATS) applications that will operate over the ATN. These applications provide a means for ATM facilities to communicate with the avionics and the flight crew on Data Link equipped aircraft. Navigation Eurocontrol BRNAV and PRNAV Within the Eurocontrol region two forms (or levels) of RNAV are recognised viz. Basic RNAV (BRNAV) and Precision RNAV (PRNAV). BRNAV was made mandatory in 1998 and PRNAV is scheduled to be mandatory by 2005. The original BRNAV requirement of 99.999 percent availability has been removed. This availability requirement is now expected to be provided by the existing VOR route structure and an operating onboard VOR receiver. PRNAV requirements are intended to be met through the use of "RNP 1 ". Required Navigation Performance (RNP) The RNP concept was created by ICAO and defines a 95 percent confidence area of position. In the RNP concept, the ability to fly in a particular airspace is no longer defined by the equipment carried but instead by the ability of the equipment to meet defined accuracy, integrity and continuity of service requirements.

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Terminal Area Initiatives For some time there has been a sustained effort to get more benefits from the existing FMS equipment on board EFIS equipped aircraft. This effort has resulted in the definition of FMS procedures for the terminal area airspace (often referred to as slant E procedures). A considerable effort has been made on the use of Baro VNAV to provide additional final approach benefits. Actual benefits received will typically be negotiated with the local authorities and may vary depending on the ability to highlight situational awareness, coupled with a flight control system and an auto throttle. Surveillance Automatic Dependent Surveillance (ADS) Automatic Dependent Surveillance (ADS) allows ground facilities to track aircraft current and predicted status with minimal crew interaction. Crew interaction consists of selecting ON, OFF or EMERGENCY modes. All other operations are transparent to the crew. The general operation is that the ground application requests data and the aircraft application supplies the requested data at the required interval. Automatic Dependent Surveillance - Broadcast (ADS - B) Automatic Dependent Surveillance - Broadcast, or ADS-B is a system that transmits information about an aircraft's state and intent at a predefined interval for use by both ground-based air traffic control and other aircraft. The uses for this data are situational awareness and conflict resolution. It is expected that this information will be transmitted by using the Mode S transponder Data Link. All the information to be transmitted by ADS-B resides in the FMS function. Mode S Data Link We have already looked at the principle of operation of the aeroplane’s transponder and at the introduction of Mode S transponders. Modern units are now available with a capacity to transfer such data as:

Traffic information service Graphical Weather information Differential GPS uplinks Windshear alerts TCAS traffic resolution.

The next development for this will be to provide the capacity to communicate simultaneously with multiple ground stations while maintaining the capacity to conduct the TCAS function.

ATPL Radio Navigation ©Atlantic Flight Training 21-5

No doubt there will continue to be developments in these important subjects and, as a professional pilot, it will be your duty to keep up to date.

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ATPL Radio Navigation ©Atlantic Flight Training 22-1

Chapter 22.

Electronic Display Systems Reference: JAR 25 (Advisory Joint Material AMJ 25-11Electronic Display Systems) Glossary of Terms Abbreviation Decode Abbreviation Decode AC ACJ ADF ADI AFCS AFM AIR AMJ ARP AS CDI CRT DOT EADI ED EFIS EHSI EUROCAE

Advisory Circular (Published By The FAA) Advisory Circular Joint Automatic Direction Finder Automatic Direction Indicator Automatic Flight Control System Aeroplane Flight Manual Aerospace Information Report (SAE) Advisory Material Joint Aerospace Recommended Practice (SAE) Aerospace Standard (SAE) Course Deviation Indicator Cathode Ray Tube Department Of Transportation Electronic Attitude Director Indicator EUROCAE Document Electronic Flight Instrument System Electronic Horizontal Situation Indicator The European Organisation For Civil Aviation Equipment

FAA FAR HSI ILS INS JAA JAR JTSO MEL PFD RNAV ROM RTCA RTO SAE STC TSO VOR

Federal Aviation Administration Federal Aviation Regulations Horizontal Situation Indicator Instrument Landing System Inertial Navigation System Joint Aviation Authorities Joint Aviation Requirements Joint Technical Standing Order Minimum Equipment List Primary Flight Display Area Navigation Read Only Memory Radio Technical Commission For Aeronautics Rejected Take-off Society Of Automotive Engineers Supplemental Type Certificate Technical Standing Order VHF Omni-range Station

Introduction The contents of the reference provide guidance related to pilot displays and specifications for CRTs in the cockpit of public transport aircraft. This digest is meant as a reference document to supplement the FMS and EFIS notes found in the Instruments and General Navigation sections of the notes. This document has relevance to the exams and does provide the source for numerous questions in both examinations. The questions in the exams will refer to JAR 25 and to the Boeing 737-400 in the questions asked.

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General The document covers general information on the following General Certification Considerations:

Display functions Colour symbology Coding Clutter Dimensions Attention getting requirements

Display visual characteristics

Failure modes Instrument display and formatting Specified integrated display and mode considerations including:

Maps Propulsion parameters Warning and advisory checklist procedures Status displays

Initial electronic displays tended to follow the electromechanical display formats of the older style aircraft. As electronic displays evolve then significant improvements can be expected. The JAA allows for certification environments that will give flexibility yet still take into account flight safety. General Certification Considerations Display Function Criticality New designs of electronic displays allow designers to integrate systems that are simpler to operate. With this integration has come the automation of navigation, thrust, aeroplane control and related display systems. The above holds for “Normal Operations”, but what about the failure of a system? Certainly, “failure state” evaluation and determination may become more complex.

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Loss of Display Criticality of flight and navigation data displayed is evaluated in accordance with JAR 25. Normally pilot evaluation is used in determining the criticality of electronic displays and during test flying the test pilot will check:

The detectability of failure conditions The required subsequent actions, and That the actions taken are within a line pilot’s capabilities

NOTE For the rest of this document the word improbable means “REMOTE”

The following are listed as critical functions and the loss of information to both pilots must be improbable. Displaying hazardously misleading information must be extremely improbable.

Attitude Airspeed Barometric Altitude

As with the critical functions, loss of information to both pilots of the following essential functions must be improbable:

Vertical Speed Slip/Skid Indications Heading Navigation

Erroneous information being displayed to both pilots must also be improbable. Rate of turn information is a non-essential function. Navigation Information Where there is a relationship between navigation information and communicated navigation information then a non-restorable loss must be extremely improbable. For navigation displays “hazardously misleading” information on either pilot’s displays must be improbable. The term “hazardously misleading” has to be agreed for certification purposes and will depend on the type of installation and flight phase. Generally, to ensure that the display to both pilots is correct both raw navigation and multi-sensor data should be displayed.

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Propulsion System Parameter Displays The failure of a system in one engine must not adversely affect the accuracy of any parameter for the remaining engines. No single fault shall result in the permanent loss of display of more than one propulsion unit. Crew Alerting Display The alerting display should be compatible with the safety objectives associated with the system function for which it provides an alert. Flight Crew Procedures The display of hazardously misleading flight crew procedures must be improbable. Information Display The Basic T is generally retained in the glass cockpit. Information Display Colours Display features should be colour coded as follows:

Display Colour Warnings Flight Envelope and System Limits Cautions, Abnormal Sources Earth Engaged Modes Sky ILS Deviation Bar Flight Director Bar

Red Red Amber/Yellow Tan/Brown Green Cyan/Blue Magenta Magenta/Green

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Specified display features should be allocated colours from one of the following colour sets:

Colour Set 1 Colour Set 2 Fixed Reference Symbols Current Data, Values Armed Modes Selected Data, Values Selected Heading Active Route/Flight Plan

White White White Green Magenta** Magenta

Yellow* Green Cyan Cyan Cyan White

* The extensive use of the colour yellow for other than caution/abnormal information is discouraged ** In colour set 1, magenta is intended to be associated with those analogue parameters that constitute “fly to” or “keep centred” type information

Precipitation and turbulence areas should be coded as follows:

Precipitation Colour 0 – 1 mm/hr 1 – 4 mm/hr 4 – 12 mm/hr 12 – 50 mm/hr Above 50 mm/hr Turbulence

Black Green Amber/Yellow Red Magenta White or Magenta

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ATPL Radio Navigation ©Atlantic Flight Training 23-1

Chapter 23.

Boeing 737 - Electronic Flight Instrument System (EFIS) Introduction The EFIS provides displays for most of the aircraft navigation systems. Provision is made for:

Colour displays of pitch and roll Navigational maps Weather Radio altitude and decision height Autopilot ADF/VOR bearings ILS data Stall warning information

System Architecture The system comprises of the following components (See Diagram 1 – EFIS System Architecture):

Electronic Horizontal Situation Indicators (EHSI) Located directly in front of the Captain and First Officer Electronic Attitude Direction Indicators (EADI) Located directly in front of the Captain and First Officer EFIS Symbol Generators (SG) EFIS Control Panels EFI Transfer Switch

EFIS display units are provided for both the Captain and the First Officer. Left and right SGs provide video signals to drive the respective display units. An EFI transfer switch is provided to determine whether:

The left and right SGs drive the Captain’s and First Officer’s EADIs and EHSIs respectively

Or one SG drives all four displays

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EFIS Symbol Generator Two symbol generators receive inputs from various aircraft systems. The SGs respond to these inputs and then generate the proper visual displays for the respective EADI and EHSI The EFIS control panel provides the system control.

Diagram 1 – EFIS System Architecture

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Navigation Systems Inputs Symbol Generator 1 Symbol Generator 2 VHF/NAV DME Flight Management Computer Air Data Computer Auto Throttle Inertial Reference System Digital Flight Control System Stall Warning Computer Automatic Direction Finder Weather Radar Radio Altimeter Ground Proximity Warning System

VOR 1 and 2 DME 1 and 2 LOC 1 and 2 ADF 1 and 2 SWC 1 ADC 1 LRRA 1 G/S 1 and 2 FCC A and B IRS L and R MCP A/T

VOR 1 and 2 DME 1 and 2 LOC 1 and 2 ADF 1 and 2 SWC 2 ADC 2 LRRA 2 G/S 1 and 2 FCC A and B IRS L and R MCP A/T

EFIS Control Panel The EFIS control panel controls:

Display modes Ranges on the EADI and EHSI Selection of Decision Height Weather Radar on/off control

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Diagram 2 – EFIS Control Panel

EADI Controls The left hand side of the panel shown in Diagram 2 controls the EADI.

BRT Controls the brightness of the EADI display. DH REF The LCD displays the selected decision height (DH). The DH is also displayed on the EADI in the bottom right corner.

Underneath the DH REF display is the Decision Height Set Knob. This control has a range of –20 to +999 ft. The decision height defaults to 200 ft when power is applied. Turning the knob changes the DH.

RST This manually resets the DH alert on the EADI. The radio altimeter display changes from yellow to white.

ATPL Radio Navigation ©Atlantic Flight Training 23-5

EHSI Controls Using Diagram 2, the EHSI controls are to the right of the EFIS control panel.

RANGE Selects the range for the navigation (MAP, CTR MAP, PLAN) and weather data display on the EHSI, the Flight Management Computer and Weather Radar. Mode Select Switch This selects the mode of data to be displayed on the EHSI. BRT Two concentric knobs: Outer Control Adjusts the brightness of the HSI display Inner Control Adjusts the brightness of the weather radar display WXR The weather radar ON/OFF switch. When the switch is in the on position weather radar information is displayed on the associated EHSI in the MAP, CTR MAP, EXP VOR/ILS, FULL NAV or PLAN modes. Map Mode Display Selector Switches In the MAP, CTR MAP or PLAN modes these switches activate the display of the symbols listed below. Any or all of the switches can be activated at the same time. When selected on the switch is illuminated (white). VOR/ADF Displays VOR/ADF bearing data in the MAP and CTR MAP modes. The MAP mode displayed ADF Bearing Pointer is suppressed when this switch is selected. NAV AID Navigation Aid data is displayed, VOR, VORTAC etc. Only the high altitude navigation aids are provided by the FMC data base when the range scales 80 nm, 160 nm or 320 nm are selected. All navigation aids are displayed if any of the other range scales are used. ARPT Displays all airports which are stored in the FMC data base and which are within the viewable map area of the EHSI. RTE DATA Displays altitude constraints and the ETA for each active route waypoint.

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Electronic Attitude Direction Indicator The EADI has one basic display mode showing:

Aeroplane attitude Flight Director commands Various types of airspeed ILS Radio Altimeter

Across the top of the EADI, autothrottle and autopilot annunciations are provided when the autopilot is either “armed” or “engaged”. Turn and bank information is provided by an inclinometer (slip indicator) at the bottom of the EADI.

Diagram 3 – Typical EADI

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EADI General The EADI presents conventional EADI displays for:

Attitude Pitch Roll

Flight Director Commands Localiser and Glideslope deviation

In addition, the EADI displays information relating to:

Autoflight systems mode annunciations Airplane speed (VMO/MMO) Minimum speeds CAS Pitch Limit Mach Number Groundspeed Decision Height Radio Altitude

Attitude Display Attitude data is provided by the IRSs. The captain’s EADI uses the left IRS and the First Officer’s EADI uses the right IRS. The IRSs pitch and roll attitude information is valid through 360° of rotation in each axis. Mode Annunciations Mode annunciations for the A/T and the AFDS are displayed at the top of the EADI displays. Flight Director (F/D) Commands FD guidance commands from the selected FCC are displayed via split axis flight director command bars. The pitch and roll commands are displayed independently. Glideslope (G/S) and Localiser (LOC) Deviation Displays Glideslope and localiser deviation scales appear when a localiser frequency is tuned on the associated VHF NAV receiver. A valid signal is required before the deviation pointer is displayed.

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The normal localiser deviation scale is 1° per dot. When the course deviation is approximately 5/8° deviation (5/8 dot) and VOR/LOC is engaged, the scale automatically expands to indicate ½ degree deviation per dot. The scale remains expanded until:

After landing rollout, or Go-around with radio altitude greater than 200 ft

On a backcourse approach, the symbol generator reverses the polarity of the localiser deviation pointer on the EADI. The reversal occurs when the airplane track differs from the selected MCP course by more than 90°. When the frontcourse is set in the MCP display the EADI and EHSI course deviation display will agree on both a frontcourse and a backcourse approach. Additionally, the glideslope scale is not displayed for a backcourse approach. ILS Deviation Warning ILS deviation monitoring alerts the flight crew of excessive LOC or G/S deviations. This alerting function is operative during single or dual A/P channel ILS approach. The alerting system is armed when the airplane descends below 1500 ft radio altitude (RA) with the LOC or G/S captured. If the Captain’s or First Officer’s LOC deviation exceeds:

½ dot expanded scale ¼ dot standard scale

the respective LOC scale changes colour from white to yellow and the miniature runway stem flashes. If the Captain’s or First Officer’s G/S deviation exceeds 1 dot deviation, the respective G/S scale changes colour from white to yellow and the G/S pointer flashes. G/S deviation alerting will not be initiated below 100 ft RA, but continues below this altitude if the alert was triggered prior to descent below 100 ft RA. Each pilot’s alerting system self-tests upon becoming armed at 1500 ft RA. This self-test generates a 2 second LOC and G/S deviation alerting display on each EADI. Rising Runway Symbol The Rising Runway Symbol is an integral part of the LOC deviation display, and is positioned at the top of the LOC Deviation Pointer. The Rising Runway Symbol is displayed in addition to the RA display and gives an additional cue to the flight crew of the aircraft’s close proximity to the ground as the airplane descends below 200 ft RA. Full scale, vertical movement of the Rising Runway represents the last 200 ft of the radio altitude. Zero feet RA is indicated as the top of the Runway Symbol rises to the base of the Airplane Symbol.

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The requirements for display of the Rising Runway Symbol are as follows:

Valid ILS/LOC frequency selected Valid RA data RA less than 2500 ft

If any of the above conditions is not met the Runway Symbol will not be displayed. Attitude Comparator A yellow “PITCH” or “ROLL” alerting annunciation is displayed on both EADIs if either symbol generator detects a difference of more than 3° between the Captain’s or First Officer’s attitude displays. A short time delay is incorporated to minimise nuisance annunciations. Digital Radio Altitude and Decision Height When RA is less than 2500 ft, a digital display of radio altitude is depicted in the lower right hand corner of the EADI. At all other times the digital RA display is blanked. When a positive decision height has been selected on the respective EFIS Control Panel, the letters DH and the decision height are displayed just above the digital RA display of the associated EADI. When the airplane is below 1000 ft agl, a RA Dial is added to the radio altitude display and the digital DH display is replaced by a magenta pointer located on the radio altitude dial. When descending through the selected DH, a DH alert occurs. The RA dial and digital display and the DH pointer change colour to yellow, flash momentarily, then remains steady yellow as the airplane continues to descend. The DH alert is reset if any one of the following occurs:

The DH reset switch on the EFIS control panel is pressed The RA increases to DH + 75 ft The radio altitude is equal to zero feet (touchdown)

Following an electrical power interruption the DH value will default to 200 ft.

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Mach Display The current Mach Number from the respective Air Data Computer (ADC) is displayed if the following are satisfied:

Accelerating Mach Number is ƒ 0.40M Decelerating Mach Number previously above 0.4M and is still > 0.38M

Groundspeed Display A digital presentation of the current groundspeed is displayed. The groundspeed data is received from the FMC or IRS, the FMC being the primary source. The numeric range is from 0 to 999 knots. Pitch Limit Symbol The position of the Pitch Limit Symbol is a function of the stall warning computer. The Pitch Limit Symbol appears when the flaps are extended in any position. During take-off, the Pitch Limit Symbol is fixed at 15° pitch attitude until the stall warning computer commands a value greater than 15° (at approximately 100 knots). Above this speed, the position of the Pitch Limit Symbol is a function of the various inputs to the stall warning computer and is limited to a maximum of 30° of pitch. In general, the Pitch Limit Symbol is programmed so that stick shaker activation will coincide with a pitch attitude equal to the Pitch Limit Symbol indication. In a rapid pull up, the pitch attitude may exceed the Pitch Limit Symbol indication for a brief period of time without initiating the stick shaker warning. With a lightweight airplane the stick shaker may be activated by the low speed limit logic of the stall warning computer even though the Pitch Limit Symbol is positioned slightly above the airplane symbol. Speed Tape Scale A range of approximately 84 knots is displayed. Numbers are placed on the tape at 20 knot intervals from 40 knots to 420 knots. The speed tape scrolls up and down and current airspeed is indicated by the digital readout. Digital Airspeed Readout A digital readout of the current calibrated airspeed is located within the fixed airspeed reference pointer. The units digit “rolls” continuously based on the current fractional unit value

ATPL Radio Navigation ©Atlantic Flight Training 23-11

of the calibrated airspeed to emulate the rolling digit readout of a conventional electrical/mechanical airspeed indicator. Airspeed Trend Arrow A green arrow of variable length which points to the predicted airspeed that the aeroplane will achieve within the next 10 seconds. This prediction is based on the present airspeed and airspeed acceleration. The Airspeed Trend Arrow is not displayed unless its magnitude is greater than 4 knots. The Airspeed Trend Arrow is removed when its magnitude becomes less than 3 knots. Command Speed Displayed as a magenta, double line cursor located on the speed tape scale if the command speed is within the currently displayed Speed Tape range. Displayed as the numerical equivalent, above or below the Speed Tape scale, if the command speed is equivalent to the selected speed on the MCP or the FMC command speed, whichever is applicable. Max Operating Speed (VMO/MMO or Gear/Flap Placards) Represented by the high speed red and black barber pole. The position of the maximum operating speed symbol is a function of data supplied to the SG from the stall warning computer. The maximum operating speed is the lower of the:

Gear extended placard speed Flaps extended placard speed, or VMO/MMO

High Speed Buffet Margin The high speed buffet margin is represented by the bottom of a hollow yellow bar that extends from the bottom of the VMO/MMO symbol at high altitude. As the airplane climbs to altitudes above 25 000 ft, the yellow bar begins to extend to give an indication of the speed that would provide a 0.3G to high speed buffet margin. At lower altitudes the VMO/MMO speed is more limiting and the high speed buffet limit symbol is no longer visible. Since the stall warning computer uses FMC gross weight to calculate the high speed buffet margin speed, this display is not available if the FMC is unable to compute gross weight. Next Flap Placard Speed The same symbol is used to represent this speed as is used to represent the high speed buffet margin. If the airplane is in the air, and flaps are lowered the hollow yellow bar extends from the high speed end of the speed tape. The end of the hollow yellow bar represents the placard speed for the next normal flap position. Next flap placard speeds are displayed only for those flap positions that would normally be used during an approach and landing. The next flap placard symbol is blanked when current flap position equals the selected landing flap configuration on the FMC/CDU APPROACH REF page or when the flaps are being retracted.

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Flaps Up Manoeuvring Speed Indicated by a small green circle on the speed tape. This speed is an output of the stall warning computer and is based on the actual gross weight as computed by the FMC. It represents the best airspeed (climb or driftdown) for an airplane in the clean configuration. This function is not enabled until the flaps are up. Decision Speed Decision speed is depicted by a green “-1” located opposite the V1 speed on the speed tape if the V1 speed is within the displayed range. If the selected V1 speed is not within the displayed range, a green “V1” with the numeric value of the V1 speed is displayed at the high speed end of the speed tape. Before the V1 speed is displayed on the speed tape, the pilot must first enter the correct speed in the scratch pad on the FMC/CDU TAKE-OFF page and then line select this speed to the V1 prompt line. VR (Rotation) Speed VR speed is depicted by a green “-R” located opposite the rotation speed if the rotation speed on the speed tape if the VR speed is within the displayed range. The “-R” symbol is blanked if the rotation speed is not within the displayed range. Before the VR speed is displayed on the speed tape, the pilot must first enter the correct speed in the scratch pad on the FMC/CDU TAKE-OFF page and then line select this speed to the VR prompt line. VREF (Reference Speed) The VREF speed is represented by the “-R” symbol. The FMC/CDU APPROACH REF page displays VREF speed based on the current gross weight for three landing flap settings. The flight crew may select the FMC computed speed or manually enter another value into the field corresponding to the desired landing flap configuration. This speed will then be transmitted by the FMC and the SG will display the “-R” symbol opposite that speed on the speed tape. The FMC updates the computed VREF speeds as fuel is burned based on fuel totaliser inputs. A VREF value does not update once it has been selected for transmission to the speed tape. If the flight crew manually inserts a gross weight on the APPROACH REF page, the FMC computed VREF speeds will be based solely on the manually entered gross weight as long as the APPROACH REF page remains in view. A manually entered gross weight is not updated as fuel is burned off.

ATPL Radio Navigation ©Atlantic Flight Training 23-13

Minimum Flap Retraction Speed Indicated by a green “-F” on the right side of the speed tape. The stall warning computer computes this speed. It represents the speed that will provide the minimum manoeuvre speed (depicted by the end of the low speed yellow bar) for the next normal flap position; flap positions 5, 1 or UP only. The display will respond to the effects of extending the flight spoilers. Minimum Manoeuvring Speed Represented by the end of the low speed hollow yellow bar. If the airplane is at low altitude and is flown at this speed, a 0.3G manoeuvre margin to stick shaker is provided. This would allow for a 40° bank turn while manoeuvring in level flight. If the airplane is at high altitude and is flown at the minimum manoeuvring speed, a 0.3G manoeuvre margin to low speed buffet is provided as opposed to a 0.3G manoeuvre margin to stick shaker margin. Since the stall warning computer uses the FMC gross weight to calculate the minimum manoeuvre speed at high altitudes, this symbol is not displayed at high altitude if the FMC gross weight is not available. The display will reflect the effect of extending the flight spoilers. The hollow yellow bar is inhibited from take-off roll through the first flap retraction. Once the flaps are up, the display will be shown for the remainder of the flight, including subsequent flap extensions. Stick Shaker Speed Represented by the end of the low speed red and black striped barbers pole. This speed represents the airspeed at which the angle of airflow vanes will activate the stick shaker warning. The source of the stick shaker airspeed is the stall warning computer. The display will reflect the effect of extending the flight spoilers.

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EADI Symbols

Symbol Name Description

Airplane A white airplane attitude

symbol is provided by the EFIS SG

< Bank Indicator and Scale White

Sky/Ground/Horizon Line The sky is cyan (light blue) with yellow for the ground. The sky/ground movement and horizon line (white) are controlled by the IRS data

Pitch Scale A white scale (±90°) controlled by IRS data. 0° indicates the horizon line

Flight Director Command A magenta symbol produced

by FCC data. Displayed when the respective Flight Director (FD) switch is on and valid command steering is available or during automatic operation of the FD. Blanked off when the FD switch is off or when command steering becomes invalid

Localiser Pointer and Scale Expanded Localiser Scale and Pointer

A magenta pointer with a white scale and index The pointer indicates the localiser position The scale indicates the deviation When LOC is engaged and the deviation is slightly more than ½ dot the scale expands The pointer is blanked when the ILS localiser signal is too weak to be usable

ATPL Radio Navigation ©Atlantic Flight Training 23-15

Symbol Name Description

Glideslope Scale and Pointer

A magenta pointer with a white scale and index The pointer indicates the glideslope position and the scale deviation The pointer is not displayed when the glideslope signal is unusable or when the track and the front course on the Main Control Panel (MCP) differ by more than 90° (Back Course)

ALT Height Alert A white display when 500 ft < Radio Altitude (RA) < 1000 ft. Reset by the DH RST on the EFIS CP, or when RA > height alert value computed by EFIS SG

DH 200 1750

Decision Height and Radio Altitude

The decision height is displayed in green. The DH is the one selected on the EFIS CP when the RA is above 1000 ft agl Blank when the DH is negative Radio Altitude is displayed below 2500 ft agl in white The display is blanked above 2500 ft agl The symbol colour changes to yellow when below the selected DH on descent The symbol colour changes to white when passing DH +75 ft during a go-around, after touchdown or after pressing the RST switch on the EFIS CP

GS 250

Groundspeed The FMC/IRS values are displayed in knots and coloured white

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Symbol Name Description .765 Current Mach Number Displayed when the Mach

increases above 0.4M The Display is blanked when the Mach Number decreases below 0.38M

Pitch Limit Symbol A yellow symbol which

indicates the pitch attitude which will activate the stick shaker

Rising Runway A green symbol shaped like a runway at the bottom of the screen. Displayed when the localiser pointer is in view and the radio altitude is valid The symbol rises towards the aeroplane symbol when radio altitude is below 200 ft agl

Radio Altitude Dial

Diagram 4 – Radio Altitude

The digital display of Radio Altitude (white) is replaced with the dial when the aircraft is at or below 1000 ft agl. A DH pointer (magenta) replaces the digital DH display.

At or below 1000 ft agl the circumference of the dial is added to or removed dependent on whether the aircraft is climbing or descending

The display changes colour to yellow and flashes momentarily when the aircraft descends below DH

DH Alert is reset automatically if the aircraft Climbs 75 ft or more above the selected DH, or After the aircraft lands

DH Alert is manually reset if the RST switch on the EFIS Control Panel is pressed

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Speed Tapes

1. V1 Decision Speed

Displayed in green Displayed after manual entry on the FMC/CDU TAKEOFF REF page V1 is only displayed in this location during the initial take-off roll when the speed is

beyond the displayed range 2. FMC/MCP Command Speed

Displayed in magenta 3. VR Rotation Speed

Displayed in green Displayed after manual entry on the FMC/CDU TAKEOFF REF page

4. V1 Decision Speed

Displayed in green Replaces the digital display in the top right corner of the speed tape when V1 is

within the displayed range 5. Speed Tape Scale

Displayed in white Scrolls up or down in response to the ADC calibrated airspeed Range 45 – 420 knots

1 – V1 (decision Speed) - Green

2 – FMC/MCP Command Speed - Magenta 3 – VR (rotation speed) - Green 4 – V1 (decision speed) - Green

5 – Speed Tape Scale - White

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6. FMC/MCP Command Speed

Displayed in magenta Displayed in this position when the FMC/MCP Command Speed is above the

displayed range 7. Minimum Flap Retraction Speed

Displayed in green Displayed on the speed tape during take-off or go-around

8. Minimum Manoeuvre Speed

Displayed in yellow The top of the yellow bar indicates the minimum manoeuvre speed The display is inhibited from the take-off roll until first flap retraction

9. Stick Shaker Speed

Displayed in red and black The top of the barbers pole indicates the speed at which the stick shaker will

activate

6 – FMC/MCP Command Speed – Magenta 7 – Minimum Flap Retraction Speed – Green 8 – Minimum Manoeuvre Speed – Yellow 9 – Stick Shaker Speed – (Red and Black)

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10. Flaps up Manoeuvring Speed

Displayed in green Shown when the flaps are up

11. Rolling Digits Display

Displayed in white Indicates the current airspeed The position is fixed relative to the ADI position

12. Airspeed Trend Arrow

The tip of the arrow depicts the predicted airspeed in the next 10 seconds The prediction is based upon present airspeed and acceleration

13. FMC/MCP Command Speed

Displayed when the FMC/MCP Command speed is below the displayed range

10. Flaps up Manoeuvring Speed – Green 11. Rolling Digits Display - White 12. Airspeed Trend Arrow – Green 13. FMC/MCP Command Speed - Magenta

14. Max Operating Speed – Red and Black 15. High Speed Buffet Limit - Yellow

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14. Max Operating Speed

Displayed in red and black Indicates VMO/MMO

15. High Speed Buffet Limit

Displayed in yellow The bottom of the yellow bar indicates the speed that provides a 0.3G manoeuvre

margin to high speed buffet at high altitudes

16. Placard Speed

Displayed in red and black Indicates gear extended placard speed, or Flap extended placard speed for the selected flap position

17. Next Flap Position Placard Speed

Displayed in yellow The bottom of the yellow bar indicates the flap extended placard speed for the

next normal flap position Displayed during flap extension

18. VREF Speed

Displayed in green Indicates the VREF speed for the landing flap configuration as selected on the

FMC/CDU APPROACH REF page

16. Placard Speed – Red and Black 17. Next Flap Position Placard Speed – Yellow 18. VREF Speed - Green

ATPL Radio Navigation ©Atlantic Flight Training 23-21

EFIS – EADI Fault Displays Operation If the specific input data to a symbol generator is identified as “invalid” data, the associated data or parameter value, is blanked. In some cases yellow fault flags are displayed. The location, orientation, and letter characters associated with each of these flags are shown. The windshear warning annunciation is displayed in red.

Diagram 5 – EADI Fault Displays

EADI Failure Flags and Annunciations

1. Selected Speed Annunciation The command speed and displays are inoperative 2. V1 Inoperative Annunciation V1 display is inoperative 3. Speed Flag The speed tape is inoperative 4. Speed Limit Annunciation The displays associated with the stick shaker and

maximum operating speeds have failed 5. Mach Flag The Mach Number display has failed 6. Pitch Comparator Annunciation The Captain’s and First Officer’s pitch angle displays

differ by more than 3° 7. Attitude Comparator Function Flag The comparator function has failed 8. Localizer Flag The localizer deviation display on the EADI has

failed

ATPL Radio Navigation 28 October 2003 23-22

9. Windshear Warning Annunciation The ground proximity computer has detected a windshear condition

10. Roll Comparator Annunciation The Captain’s and First Officer’s bank angle displays differ by more than 3°

11. Radio Altitude Flag The radio altitude display has failed 12. Decision Height Flag The selected decision height display has

failed 13. Symbol Generator Fail Annunciation The selected symbol generator has failed 14. Attitude Flag The attitude display has failed 15. Flight Director Flag The flight director has failed EFIS Typical EHSI Centre Map, Map and Plan Displays The EHSI can display selected FMC Data in three different selectable formats:

CTR Map Map Plan

The format displayed depends upon the mode selected on the EFIS Control Panel (Diagram 6).

Diagram 6 – EHSI Mode Selector

General Each EHSI presents an electronically generated colour display of conventional HSI navigation data:

VOR/ILS Modes NAV Modes

Each EHSI is also capable of displaying the airplane’s flight progress on a plan view map:

Map Mode CTR Map Mode

ATPL Radio Navigation ©Atlantic Flight Training 23-23

Or the airplane’s flight plan on a plan view map orientated to True North:

Plan Mode Excluding operation in the FULL NAV, FULL VOR/ILS and PLAN Modes, each EHSI also serves as a weather radar display when the WXR Switch on the respective EFIS CP is on. During normal operation, each EHSI receives information from its own symbol generator. Each symbol generator receives data from a variety of aircraft systems to support the EHSI displays. EHSI Display Orientation The various displays on the EHSI are orientated to airplane heading (heading up). With heading up orientation, all displayed data is referenced to aircraft heading as shown at the 12 o’clock position on the compass rose. During normal operation, heading reference data is supplied to each EHSI from the respective IRS. Airplane track data is supplied by the FMC. If the FMC track data should become unreliable, the respective IRS automatically supplies track data. PLAN Mode The PLAN mode is a map display which may be used to view an FMC flight plan route, either in total for a short route, or waypoint by waypoint for a longer route. Primarily used before flight to set up the flight plan conditions. The upper part of the display describes the dynamic conditions of the airplane:

Track Selected and actual heading Distance ETA to the next waypoint

The lower part of the display is background data displaying the flight plan (Diagram 7).

ATPL Radio Navigation 28 October 2003 23-24

Diagram 7 – PLAN Mode

Features of PLAN Mode

PLAN is selected on the EFIS Mode Selector Switch A static map is displayed orientated to true north, only 70° of the compass rose is

shown The top of the EHSI is the same in MAP Mode The pilot can review the planned route by using the FMC/CDU LEGS page

CENTER STEP line select key Weather radar data is inhibited

MAP and CTR MAP Modes These modes display the aeroplane’s position with respect to the flight plan. The entire display is dynamic, rotating as the aircraft moves. The aircraft is represented by a triangle in the centre of the display. Waypoint and NAVAID symbols remain upright. Symbols which are rotated to maintain a proper orientation are:

ARPT (Airport) Holding pattern Procedural turn symbols

The displays are used to monitor the aeroplane position along the selected flight path.

ATPL Radio Navigation ©Atlantic Flight Training 23-25

Diagram 8 – Map Mode

Features of MAP Mode

MAP is selected on the EFIS Mode Selector Switch The MAP mode display shows 70° of the compass rose

The aeroplane is shown as a fixed symbol (⊇) at the bottom of the display superimposed on a moving map display

The basic map background includes Origin/destination airports Flight plan route Display of navaids in use

Optional background data is selected by using the Map Mode Display Selectors on the EFIS Control Panel, this data includes:

Off route navigation aids Off route airports Off route named waypoints Tuned VOR/ADF radials Flight plan route waypoints ETAs Altitude constraints

Weather radar returns are displayed when the WXR switch is ON

ATPL Radio Navigation 28 October 2003 23-26

Diagram 9 – Center Map Mode

Features of CENTER MAP Mode

CTR MAP is selected on the EFIS Mode Selector Switch The CENTER MAP Mode includes a 360° compass rose Displays the same data and symbols as the MAP mode The airplane symbol is displayed in the centre of the display so that MAP

information is displayed behind the aircraft NAV Mode Displays The NAV Modes display selected FMC data. The display is referenced to track and shows the aircraft position relative to:

The next waypoint, and The vertical and horizontal flight path The NAV modes can have 70° or 360° compass displays

ATPL Radio Navigation ©Atlantic Flight Training 23-27

Diagram 10 - Expanded Navigation Mode

Features of EXPANDED NAVIGATION Mode

EXP NAV is selected on the EFIS Control Panel Selector Switch The display shows lateral and vertical navigation guidance information similar to a

conventional HSI The FMC is the source of the navigation data Weather radar data is displayed when the WXR switch is on

ATPL Radio Navigation 28 October 2003 23-28

Diagram 11 – Full Navigation Mode Features of FULL NAVIGATION Mode

FULL NAV is selected on the EFIS Control Panel Selector Switch Displays the same as EXP NAV except:

Weather radar displays are not available The full compass rose is shown instead of the expanded compass rose Alternate symbols are used for the

Aeroplane symbol Course pointer

VOR and ILS Displays The VOR and ILS modes can be used with manually selected stations. The VOR mode is used en-route with VOR and DME station inputs. ILS is the landing mode and uses ILS and DME station inputs. Appropriate deviations are displayed in both modes. Heading displayed is magnetic below 73° north and 60° south latitude. At higher Latitudes, the heading is automatically referenced to true north Drift angle can be determined by the difference between the heading and track line. For the VOR and ILS systems, the EHSI has two different types of displays which are selectable on the EFIS Control Panel. The “FULL VOR/ILS” displays include a 360° compass rose with the airplane symbol in the center, and so are unable to display weather radar data. The “EXP VOR/ILS” displays include only a 70° compass arc with the airplane symbol at the bottom center. This display is used to more easily read the airplanes heading and track values. Also, the weather radar data can be displayed on these expanded displays. Whether the display presents VOR or ILS data depends upon the VOR or ILS frequency selection on the VHF NAV Control Panel. Except for the “FULL” and “EXP” display differences described above, the remainder of the navigational data shown is the same on both types of displays. A detailed description of all displayed symbols, parameters, and annunciations is provided later.

ATPL Radio Navigation ©Atlantic Flight Training 23-29

Expanded VOR Mode

Diagram 12 – Expanded VOR Mode

VOR/ILS is selected on the EFIS Control Panel Selector Switch With a VOR frequency selected, VOR navigation data is displayed This is orientated to the airplane heading The source of the navigation data is displayed as VOR 1 or VOR 2 in the lower

left corner of the EHSI The TO/FROM annunciation and the navigation source frequency is in the lower

right corner of the EHSI Weather radar return data and range arcs are displayed when the WXR switch is

on Full Rose VOR Mode

Diagram 13 – Full Rose VOR Mode

ATPL Radio Navigation 28 October 2003 23-30

FULL VOR/ILS is selected on the EFIS Control Panel Selector Switch The information displayed is the same as the expanded VOR mode with the

following exceptions Weather radar displays are not available A full compass rose is shown instead of the expanded compass rose A drift angle pointer replaces the track line The TO/FROM pointer is shown in addition to the TO/FROM annunciation Alternate symbols are used for airplane symbol and course pointer

Expanded ILS Mode

Diagram 14 – Expanded ILS Mode

VOR/ILS is selected on the EFIS Control Panel Selector Switch With an ILS LOC frequency selected, ILS navigation data is displayed orientated

to aircraft heading The source of navigation data (ILS 1/ILS 2) is shown in the lower left corner of the

EHSI The frequency of the navigation source is shown in the lower right of the screen Weather radar return data and range arcs are displayed when the WXR switch is

on

ATPL Radio Navigation ©Atlantic Flight Training 23-31

Full Rose ILS Mode

Diagram 15 – Full Rose ILS Mode

FULL VOR/ILS is selected on the EFIS Control Panel Selector Switch Data displayed is the same as the expanded mode with the following exceptions

Weather radar displays are not available A full compass rose is shown instead of the expanded compass rose A drift angle pointer replaces the track line Alternate symbols are used for airplane symbol and course pointer

EHSI Symbology The following symbols may be displayed on each EHSI depending on the EFIS Control Panel selection. The colours used are as follows.

Green Active or selected mode and/or dynamic conditions White Present status situation and scales Magenta Command information, pointers, symbols, fly-to conditions and

weather radar turbulence Cyan Non-active and background information Red Warning Yellow Cautionary information, faults and flags Black Blank areas and off conditions

ATPL Radio Navigation 28 October 2003 23-32

Data Name

Colour Applicable

Modes Remarks

Symbol Data Source

Airplane Symbol White

FULL VOR/ILS FULL NAV

Airplane position is indicated by the centre of the symbol which is the centre of rotation and translation for all the background and dynamic symbols. Source: EFIS SG

Full Compass Rose Display Colour: White (Low intensity display)

FULL VOR/ILS FULL NAV

Displays 360° of IRS compass data. Source: IRS

Full Compass Rose Display Colour: White (Low intensity display)

CTR MAP Displays 360° of IRS compass data. Source: IRS

Expanded Compass Rose Colour: White

EXP VOR/ILS MAP EXP NAV

Heading is from IRS 360° is available only 70° is shown

Airplane Symbol Colour: White

EXP VOR/ILS MAP EXP NAV CTR MAP

The apex of the white triangle indicates airplane position. The apex of the airplane is the centre of rotation and translation for all the background and dynamic symbols.

Heading Annunciator Colour: Green Heading Readout and index Colour: White (Heading index low intensity) M/TRU Annunciator Colour: Green

EXP VOR/ILS EXP NAV MAP CTR MAP PLAN

The number under the low intensity index pointer is a heading. Readout displays actual heading. Compass is referenced to magnetic north between 73°N and 60°S. Above these latitudes the compass is referenced to true north. Source: IRS

Heading Annunciator Colour: Green Heading Readout and index Colour: White (Heading index low intensity)

FULL VOR/ILS FULL NAV

Indicates number under pointer is a heading. Box displays actual heading. Compass is referenced to magnetic north between 73°N and 60°S. Above these latitudes the compass is referenced to true

ATPL Radio Navigation ©Atlantic Flight Training 23-33

Data Name Colour

Applicable Modes

Remarks Symbol Data Source

M/TRU Annunciator Colour: Green

north. Source: IRS

Magnetic Status Change Colour: Green Box around Green “M” annunciator

All Modes Box appears for 10 seconds at transition to magnetic reference display. Source: SG

True Enhancement Annunciator Colour: White box around Green TRU annunciator

All Modes Box surrounds “TRU” when display is referenced to true north unless yellow “TRU” annunciator (below) is displayed Source: SG

True Advisory Annunciator Colour: Yellow box around TRU annunciator

All Modes Yellow box surrounds “TRU” (Initially flashes for 10 seconds) when descent rate > 800 fpm for 2000 ft (landing phase) The yellow box remains until ascent rate > 500 fpm for 2000 ft (take-off phase) Then changes to the white enhancement annunciator above Source: SG

Selected Heading Marker and Line Colour: Magenta

All Modes Manually positioned by heading selector on the MCP. No dashed line on full displays Source: FCC Default: MCP

Drift Angle Pointer Colour: White

All Modes Indicates FMC/IRS computed drift angle Source: FMC Default: IRS

Present Track (Straight Trend Line) and Range Scale Colour: White

MAP NAV VOR ILS

Magnetic/true track which will result with present heading and winds. Displayed range is ½ the actual

ATPL Radio Navigation 28 October 2003 23-34

Data Name Colour

Applicable Modes

Remarks Symbol Data Source

CTR MAP selected range. Range marks are used in CTR MAP mode Range arcs (not shown) are used in the MAP mode and in the EXP VOR/ILS and EXP NAV modes with WX selected Track defaults to IRS for FMC not valid

Course Select Pointer and Vector Colour: Magenta

VOR Indicates selected VOR course when tuned to VOR source Source: FCC Default: MCP

Runway Heading Pointer Colour: Magenta White

ILS Indicates selected runway heading for front course when tuned to ILS source Source: FCC Default: MCP

Desired Track Pointer and Vector Colour: Magenta

NAV Indicates desired track angle from FMC or ANCDU

ALERT Course Change Annunciation Colour: Yellow

NAV Displayed 10 seconds before reaching waypoint. Removed at reaching waypoint Source: FMC or ANCDU

VOR Bearing Vectors and Identifiers Colour: Green

MAP CTR MAP

Displayed when VOR/ADF selector on EFIS CP is pushed, and associated DME not in agility mode Source: DAA

Course Deviation Scale Colour: Scale – White Bar - Magenta

VOR When VOR frequency tuned, VOR course deviation is displayed. One dot equals 5° deviation Computed in the SG Deviation = VOR Bearing minus selected course

Localiser Deviation

Scale Colour: Scale – White Bar - Magenta

ILS When ILS frequency tuned, ILS course deviation is displayed. One dot is approximately 1° deviation Source: DAA

ATPL Radio Navigation ©Atlantic Flight Training 23-35

Data Name Colour

Applicable Modes

Remarks Symbol Data Source

Cross Track Deviation Colour: Scale – White Bar - Magenta

NAV When NAV mode is selected, indicates deviation from desired track. One dot is equal to 2 nm Source: FMC

Glideslope Deviation Scale Colour: Scale – White Bar - Magenta

ILS Displays glideslope deviation in ILS mode. One dot is approximately 0.35°. NCD in backcourse Source: VHF NAV Unit

ADF Bearing Vectors and Identifiers Colour: Green

MAP CTR MAP

Displayed when: VOR/ADF selector on EFIS CP is pushed, or In “ADF” mode Source: ADF RCVR

Left and Right ADF Pointers Colour: Green

Reciprocal Left and Right ADF Pointers Colour: Green

VOR ILS NAV PLAN

Displayed in “ADF” mode and if “ON” switch on ADF CP is on Source: ADF RCVR

DME 13.5 DME Distance Colour: White

VOR ILS

Displays distance to tuned DME station Two resolutions: Resolution 1: Whole nm if distance >100 nm Resolution 2: tenths of a nautical mile if distance < 100 nm Source: DAA

VOR 1 (2) VOR

ILS 1 (2)

Selected Radio Nav Source Colour: Green

ILS

Displays selected Radio Nav mode based on the EFIS CP mode selection or frequency from DAA Radio Nav source is based on source select programme pins on EFIS SG

NAV FMC Selected Colour: Green

NAV Display indicated source of NAV DATA. Source: FMC

ATPL Radio Navigation 28 October 2003 23-36

Data Name Colour

Applicable Modes

Remarks Symbol Data Source

110.10 ILS Frequency Display Colour: Green

ILS Displayed when a valid ILS frequency is being received Source: DAA

116.80 or AUTO

VOR Frequency Colour: Green

VOR The VOR frequency is displayed when a valid VOR frequency is manually tuned The word AUTO is displayed when the VOR frequency is automatically tuned by the FMC Source: DAA

TO (FROM) TO/FROM Annunciator Colour: White

VOR Indicates whether the selected course will take the airplane TO or FROM the selected VOR station Source: EFIS SG

Wind Direction and Speed Colour: White

MAP CTR MAP NAV VOR ILS

Indicates wind speed in knots Wind direction is with respect to the display reference (magnetic or true north) Displayed when wind speed is ƒ 6 knots Source: FMC Default: IRS

Waypoint and ID Colour: White

MAP CTR MAP PLAN

Identifier listed below and right of the standard waypoint Source: FMC

BUGLE

Active Waypoint Colour: Magenta

NAV Name of active waypoint Source: FMC

Route Colour: see right

MAP CTR MAP PLAN

The active route is displayed with continuous MAGENTA lines between waypoints Inactive routes in CYAN with long dashes between waypoints Changes to the active route are displayed in white dashes between waypoints When the route change is executed on the CDU, the short dashes are replaced with a continuous magenta line Source: FMC

Bearing to Waypoint and Vector Colour: Magenta

NAV Indicates bearing to active waypoint Source: FMC

ATPL Radio Navigation ©Atlantic Flight Training 23-37

Data Name Colour

Applicable Modes

Remarks Symbol Data Source

27.5 NM Distance To Go (DTOGO) Colour: White

MAP CTR MAP PLAN NAV

Distance to an active waypoint: Resolution 1: Whole nm if DTOGO >100 nm Resolution 2: tenths of a nautical mile if DTOGO < 100 nm Source: FMC

0834.4z ETA Display Colour: White

MAP CTR MAP PLAN NAV

Indicates time of arrival at an active waypoint Source: FMC

FMC/IRU Position Difference Colour: White

MAP CTR MAP NAV

The SG computes the position difference between the FMC and each IRU If either position difference > 12 nm, the SG displays the magnitude and direction of both position differences The position difference arrow points to the IRU position L or R indicates which IRS present position the displayed position difference corresponds to.

; KTEB Airport and ID Colour: Cyan

MAP CTR MAP PLAN

Non-flight plan airports are displayed when ARPT switch selected Source: FMC

Route Data Colour: same as waypoint symbol

MAP CTR MAP PLAN

ALT and ETA for active waypoints (with ID) displayed when route data switch on EFIS CP is on Source: FMC

Navaids and ID: VOR DME/TACAN VORTAC Colour: Untuned – Cyan Tuned - Green

MAP CTR MAP PLAN

Non-flight plan navaids displayed when NAV AID switch on EFIS CP on. Source: FMC Select course source: FCC or MCP

ATPL Radio Navigation 28 October 2003 23-38

Data Name Colour

Applicable Modes

Remarks Symbol Data Source

Tuned Navaid and Selected Course Colour: Green

MAP CTR MAP PLAN

When navaid is manually tuned Selected course and its reciprocal are displayed Source: FMC

Non-Flight Plan Waypoint and ID Colour: White

MAP CTR MAP PLAN

Non-flight plan waypoints displayed when WPT switch on Source: FMC

Airport, Runway and ID Colour: White

MAP CTR MAP PLAN

Selected on FMC CDU Displayed when EHSI range is 80 nm, 160 nm or 320 nm Source: FMC

Runway and ID Colour: White

MAP CTR MAP PLAN

Selected on FMC CDU Displayed when EHSI range is 10 nm, 20 nm or 40 nm Source: FMC

Holding Pattern Colour: Inactive – Cyan Active – Magenta Modified - White

MAP CTR MAP PLAN

Displayed when a holding pattern is selected as part of the route Source: FMC

North Up Pointer Colour: Green

PLAN Display below track tape is referenced to True North Source: FMC

Vertical Deviation Scale and Pointer Colour: Scale – White Pointer - Magenta

MAP CTR MAP NAV

Displays vertical deviation from vertical profile during VNAV descent mode One dot equals 400 ft deviation For deviation greater than 440 ft pointer is at end of scale and, in addition, a digital readout appears below the scale if airplane is above flight path. If airplane is below flight path digital readout is above scale Source: FMC or ANCDU

Selected Reference Point (SRP) With ID and Radials Colour: Green

MAP CTR MAP PLAN

Displayed as a selected reference point (fix) via the FMC CDU Can be with any number of special map symbols (eg VOR, VORTAC, Airport etc) Source: FMC

ATPL Radio Navigation ©Atlantic Flight Training 23-39

Data Name Colour

Applicable Modes

Remarks Symbol Data Source

Selected Reference Point (SRP) With ID and Distance Circle Circle: Green

MAP CTR MAP PLAN

Circle representing selected distance displayed around SRP Selected distance indicated below circle Source: FMC

Range to Altitude Arc Colour: Green

MAP CTR MAP

Curved arc represents the point where MCP selected altitude will be reached if current vertical and lateral flight path angles are maintained Source: FMC

; T/D ; S/C ; T/C ; E/D

Altitude Profile Points Colour: Green

MAP CTR MAP PLAN

Calculated by FMC T/D – Top of Descent S/C – Step Climb T/C – Top of Climb E/D – End of Descent Source : FMC

Curved Trend Vector Colour: White

MAP CTR MAP

Segmented curve predicts the directional trend It is calculated by the SG Based on present position, groundspeed and cross track acceleration It shows the predicted position at the end of 30, 60 and 90 seconds The number of segments represented: Range > 20 nm - 3 segments Range 20 nm – 2 segments Range 10 nm – 1 segment

Weather Radar Returns Colour: As for AWR

MAP CTR MAP EXP NAV EXP VOR EXP ILS

Multi-coloured returns are presented when EFIS CP WXR ON switch is pushed Most intense areas are displayed in red, turbulence in magenta Source: WXR XCVR

VAR/WX VAR/WX + t VAR/MAP

TEST

Weather Radar Mode Colour: Green

MAP CTR MAP EXP NAV EXP VOR EXP ILS

Displayed when weather radar is turned on VAR/ is displayed when variable gain is selected Source: WXR XCVR

ATPL Radio Navigation 28 October 2003 23-40

Data Name Colour

Applicable Modes

Remarks Symbol Data Source

+ 12 Weather Radar Antenna Tilt Colour: Green

MAP CTR MAP EXP NAV EXP VOR EXP ILS

Displayed when weather radar is turned on Source: WXR XCVR

EHSI System Failure Flags and Annunciation Whenever specific input data to a symbol generator is identified as “invalid” data, the associated symbol, or parameter value, is blanked. In all cases flags and annunciations are in yellow.

Diagram 16

HDG The heading data has failed

ATPL Radio Navigation ©Atlantic Flight Training 23-41

Range Disagreement Annunciations Three sets of words can be used: WXR RANGE DISAGREE Indicates that the selected range on the EFIS Control Panel is different than the WXR display (as above) MAP RANGE DISAGREE Indicates that the selected range on the EFIS Control Panel is different than the MAP display range (see later diagrams) WXR/MAP RANGE DISAGREE Indicates that the selected range on the EFIS Control Panel is different than the MAP and WXR display range (see later diagrams) Weather Annunciations WXR FAIL Indicates weather radar has failed (no weather data displayed) WXR WEAK Indicates weather radar calibration fault WXR ATT Indicates loss of attitude stabilization for antenna WXR STAB Indicates weather radar stabilization is off WXR DSPY Indicates loss of Display Unit cooling or an overheat condition of the HSI Weather Radar display is blanked VTK Vertical Track Flag indicates a failure of the FMC vertical track data VOR or LOC Displayed if the EXP VOR/ILS mode is selected XTK Cross track deviation flag displayed if EXP NAV Mode is selected XXXXX Possible annunciations are:

RT ANT CONT ATT WEAK, and/or STAB

Only when WXR TEST has failed

ATPL Radio Navigation 28 October 2003 23-42

Diagram 17

Diagram 18

MAP Indicated if MAP Mode is selected

ATPL Radio Navigation ©Atlantic Flight Training 23-43

Diagram 19

VOR 1 VOR Flag indicates failure of a VOR Display on the EHSI. Displayed if the MAP or CTR MAP Mode is selected and the VOR/ADF map switch is on

Diagram 20

EXCESS DATA The refresh rate of the MAP Display has dropped below limits. The display may flicker at lower refresh rates.

ATPL Radio Navigation 28 October 2003 23-44

Instrument Transfer Switching During normal operation, each pilot’s EFIS display utilises independent IRS and SG inputs. The EFI Transfer switch determines the SG source for both displays.

With the EFI transfer switch in the normal position the Number 1 SG provides display symbols for the Captain’s EFIS displays, the Number 2 SG provides display symbols for the First Officer’s EFIS displays.

If the EFI transfer switch is in the BOTH ON 1 position, both sets of displays utilise the Number 1 SG

If the EFI transfer switch is in the BOTH ON 2 position, both sets of displays utilise the Number 2 SG

The IRS transfer switch selects the IRS that supplies inputs to the respective SG as well as other airplane systems.

With the IRS transfer switch in the normal position, the left IRS provides inputs to the Number 1 SG and the right IRS provides inputs to the Number 2 SG

If the IRS transfer switch is positioned to BOTH ON L, the left IRS provides data to both SGs

If the IRS transfer switch is positioned to BOTH ON R, the right IRS provides data to both SGs

Light Sensing and Brightness Controls There are two sets of ambient light sensors that automatically adjust the brightness of the EADI and EHSI displays. The Captain’s and First Officer’s displays are independently adjusted. Two remote light sensors, located on the instrument glare shield, adjust the brightness of the associated EADI and EHSI as a function of the light coming through the forward windows. Two integral light sensors, located in the EADI and EHSI instrument bezels, work in parallel to adjust the brightness of the EADI and EHSI displays as a function of ambient light shining on the face of either display. Manual adjustment of the display brightness, above and below the brightness level set by the automatic system, is accomplished by adjusting the brightness controls on the associated EFIS Control Panel.

ATPL Radio Navigation ©Atlantic Flight Training 24-1

Chapter 24.

Boeing 737 - Flight Management Computer (FMC)

General The Flight Management System (FMS) is comprised of four component systems:

Flight Management Computer System (FMCS) Autopilot/Flight Director System (AFDS) Autothrottle (A/T) Inertial Reference Systems (IRSs).

ATPL Radio Navigation 28 October 2003 24-2

Each of these components is an independent system, and they can either be used individually or in various combinations. However, the term “FMS” refers to the concept of joining these independent components together into one integrated system which provides continuous:

Automatic navigation Guidance, and Performance management.

In essence, the FMS is capable of four dimensional area navigation (Latitude, Longitude, altitude, time) while optimizing performance to achieve the most economical flight possible. The integrated FMS provides centralized cockpit control of the airplane’s flight path and performance parameters. The Flight Management Computer, or FMC, is the heart of the system, performing navigational and performance computations and providing control and guidance commands. The automatic management features of the FMC eliminate many routine tasks and manual computations previously performed by the pilots. However, the pilots must still monitor the FMC to ensure compliance with the planned route of flight.

ATPL Radio Navigation ©Atlantic Flight Training 24-3

Operation Overview The two Multi-Function Control Display Units (MCDUs) allow the crew to enter the desired flight plan routing and performance parameters into the FMC. FMC navigational and performance computations are then displayed on the MCDUs for reference or monitoring. Related FMC commands for lateral and vertical navigation may be coupled to the AFDS and A/T through the Mode Control Panel (L NAV and V NAV). The IRSs and other airplane sensors provide additional required data. MCDUs also permit interface with the ACARS system. When radio updating is not available, the FMC uses the IRS position as a reference. This mode of navigation is referred to as IRS NAV ONLY, and a message is displayed to warn the flight crew that navigation accuracy may be less than required. During IRS NAV ONLY operation, the FMC applies an automatic correction to the IRS position to determine the most probable FMC position. This correction factor is developed by the FMC by monitoring IRS performance during periods of radio updating to determine the IRS error. Flight crews should

ATPL Radio Navigation 28 October 2003 24-4

closely monitor FMC navigation during periods of IRS NAV ONLY operation especially when approaching the destination. The accuracy of the FMC navigation should be determined during the descent phase of flight by using radio navaids and radar information if available. Inaccurate radio updating may cause the FMC to deviate from the desired track. The crew may select any degree of automation desired. This can mean simply using the CDU Data Displays for reference during manual, flight, or using conventional autopilot functions, or selecting full FMS operation with automatic flight path guidance and performance control. Even with full FMS operation, management and operation of the airplane is always under the total control of the flight crew. Certain functions can only be implemented by the pilots, such as:

Thrust initiation Take-off Altitude selection ILS tuning Airplane configuration, and Landing rollout.

The flight crew should monitor FMC navigation throughout the flight to ensure that the desired route of flight is being accurately followed by the automatic systems. The following schematic presents a simplified depiction of how the various FMS components interrelate. The Flight Management Computer System with its Control Display Units (FMC/CDU) provides the pilots with a flight management tool which performs navigational and performance computations. Computations related to lateral navigation include items such as:

Courses to be flown ETAs, and Distance to go.

For vertical navigation, computations include items such as:

Fuel burn data Optimum speeds, and Recommended altitudes.

ATPL Radio Navigation ©Atlantic Flight Training 24-5

When operating in the Required Time of Arrival. (RTA) mode, the computations include:

Required speeds Take-off times, and Route progress information.

In addition, the FMC also provides control and guidance commands which can be coupled to the AFDS and A/T. This allows integrated FMS operation with automatic lateral and vertical navigation from initial climb to final approach. CDU Function CDU Page Display General

The page displays a single page of FMC data, as selected by a function and mode key or a line select key. Ambient Light Sensor Automatically adjusts the Display contrast for ambient light conditions.

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Brightness Control Rotating the switch adjusts the brightness of the display. Function and Mode Keys

Press Mode key selects the appropriate page of FMC data for the display. EXECute Key Executes the display data.

Line Select Keys There are 6 keys either side of the screen (Left 1L – 6L, Right 1R – 6R. Each key is associated with its adjacent data line. (See diagram below).

Press If the scratch pad is blank, appropriate moveable data on the line is copied into the scratch pad. If appropriate data already exists in the scratch pad, then that data transfers up the line (if modifiable) and replaces any previous data. If the line has the access prompt symbol < or > the display changes to the indicated new page, or the indicated function is accomplished. If the line is a procedure, the procedure is selected.

ATPL Radio Navigation ©Atlantic Flight Training 24-7

Keyboard

SP Key When pressed a space is inserted into the scratch pad ± Key Keys a “-“ character into the scratchpad. Changes to + with a second press. Delete Key When pressed the word “DELETE” appears in the scratch pad (if previously blank) Subsequent line selection to a deletable line deletes that line (in some cases execution is also required) CLR Key Used to clear scratch pad data from this CDU only, or to clear CDU Messages.

Press (Momentary) Clears the last character keyed-in, or clears a message Press (Hold) Clears all the characters

Slash (/) Key When pressed keys a “/” character in the scratch pad. Used to separate data entries for speed/altitude etc. Main Keyboard When pressed the alpha/numeric characters are keyed into the scratch pad.

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CDU Page Display

PERF INIT Page title 1/2 Top Right of Screen The first digit identifies the page number (from within a group of related pages) that is presently displayed. The second digit identifies the total number of related pages (if any) which are available for display using the PREV PAGE or NEXT PAGE keys.

Box Prompts Identify a line where a data entry is required for FMC operation – – – /– – Dash Prompts Identify a line where a data entry optional. Such entries help to optimize FMC computations. 2L Data Line Title/ Data Line (“line”) The title when displayed identifies the type of data displayed on the line below. _ _ _ _ _ _ _ _ _ _ _ Display Division Page data is displayed above the dashed line. Various prompts and the scratch pad are located below the line. Scratch Pad The bottom line of the display. Displays CDU messages, Keyboard entries, or data being transferred between lines or pages within the line select keys

Scratch pad entries are not affected when a new page is selected

ATPL Radio Navigation ©Atlantic Flight Training 24-9

Page Status

Indicates the active or modified status of certain route or performance data (“active” route shown) If the page data is inactive, page status is blank. Function and Mode Keys

CLB The Climb Mode Key when pressed displays various climb pages which allow

evaluation and/or change of the climb mode. CRZ The Cruise Mode Key when pressed displays various cruise pages which allow

evaluation and/or change of the cruise mode. DES The Descent Mode Key when pressed displays various descent pages which allow evaluation and/or change of the descent mode. EXEC The Execute Function Key which is the FMC command key

Illuminated (White) Illumination of the integral light bar indicates that data entered on a page may now be activated. The data can consist of required entries, or be a proposed modification to the active route or performance data. If a modification, the original (unmodified) data is still active.

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Press Activates the data or modification and extinguishes the light bar

PREV PAGE or NEXT PAGE The previous page or next page function keys. When

pressed the display changes to the next lower or next higher page number within a multiple page group of related pages.

N1 Limit Mode Key When pressed displays a page with engine N1 limit values.

Allows the pilot selection of thrust limits if automatic selection is not desired. The active limit is always available for display on the N1 indicators and for use by the autothrottle.

INIT REF Initialisation/Reference Mode Key when pressed displays pages used for initializing the FMC and IRSs, plus other pages containing various categories of reference data.

RTE The route mode key when pressed displays pages used to enter or revise

the:

Origin Destination Departure runway Each route segment

of the flight plan route. Also displays procedures selected on the Departures/Arrivals pages. The route is entered manually or as a stored company route.

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MENU KEY Provides access to sub-systems:

FMC ACARS etc

DEP/ARR Departure/Arrival Mode Key. Displays the pages for listing terminal area

procedures for a selected airport. Permits the selection of departure and arrival/approach procedures for entry into the flight plan route.

HOLD Displays the pages for any previously defined holding patterns, or allows

initial development of holding patterns for entry into the active route. Also used to exit a holding pattern.

PROG The Progress Mode Key when pressed displays pages with current dynamic

data concerning progress along the active route. Includes ETAs and fuel remaining estimates for the next two waypoints, destination and the next altitude change point.

Also provides information on wind, temperature, TAS, Route Tracking and the status of the IRSs and DME/VHF Nav Radios. Allows access to the RTA PROGRESS page for the initialization and monitoring of the RTE Mode.

LEGS Legs Mode Key displays pages containing lateral and vertical details for each leg of the flight plan route. Allows revision of individual waypoints and certain speed/altitude crossing restrictions. FIX The Fix Mode Key displays pages used for determining:

ETA at Distance to Altitude at

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The intersection of the active route and a radial or distance from any waypoint/fix. Also allows determination of present radial and distance from any waypoint/fix

Lights

Menu Page Displayed if the FMC has failed. Message Light (White)

Illuminated (White) The FMC contains an Alerting or Advisory Message for display in the scratch pad. If non-message data is presently displayed in the Scratch Pad, pressing the CLR Key will display the message. Press the CLR key to clear the message and extinguish the light.

Fail Light Amber. When illuminated the FMC test is in progress.

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Call Light White. When illuminated a user other than the FMCS is requesting control of the CDU. System Components The single FMC contains stored data bases of navigation and in-flight performance information. This stored data is an “electronic flight bag” which contains information similar to (but more refined than) the pages of Operations Manual Performance Data and the navigation publications that are carried in a pilot’s flight bag. The two identical, independent CDUs provide the means for the FMC and flight crew to “talk” to each other in familiar Air Traffic Control language. The crew may enter data into the FMC using either CDU, although simultaneous entries should be avoided. The same FMC data and computations are available for display on both CDUs. However, each pilot has independent control over what is actually being displayed on an individual CDU. Data Bases Standard information stored in the permanent navigation data base includes:

The location of and other facility information for selected airports Runways, and VHF navaids

Other data that may be stored (at the airline’s option) includes:

The company route structure Airways Waypoints SIDS, STARS, and approach procedures.

This “permanent” navigation database contains two sets of time-sensitive data:

Active data which is applicable to an initial 28 day time period, and Alternate data which is applicable to the time periods below.

During pre-flight, the pilots select the data to be used via the CDU IDENT page. Updates to the navigation database are accomplished by maintenance personnel using a magnetic-tape cassette, and are normally accomplished on a 28 day cycle corresponding to the revision date of navigation publications. If the permanent data base does not contain all of the required flight plan data, additional airports, navaids, and waypoints can be defined by the crew and stored in either a supplemental or a temporary navigation data base. Use of these additional databases provides world-wide navigational capability, with the crew manually entering desired data into

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the FMC via various CDU pages. Information in the supplemental nav database is stored indefinitely, requiring specific crew action for erasure. The temporary nav database is automatically erased at flight completion. The performance database contains information on:

Climb and cruise performance Thrust limits Maximum and minimum altitudes Maximum and minimum airspeeds for various configurations, and Drag characteristics.

Maintenance personnel can refine the data by entry of correction factors for individual airplane drag and fuel flow characteristics. Operation The CDUs are used during preflight to manually initialize the IRSs and FMC with dispatch information such as:

Present position Flight plan routing Zero fuel weight, and Planned cruise altitude.

These CDU entries and the databases then form the starting point for FMC computations. The FMC compares this planning information with actual data from a number of other sources (including required IRS inputs). These other inputs provide dynamic real-time information on:

Flight progress Airplane environment, and Systems status.

The FMC then continuously computes:

Airplane position ETAs Thrust (N1) targets and Limits and optimum/required speeds, and Recommended altitude for the selected vertical profile.

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Computations are displayed on the CDUs, while associated outputs are used to provide:

Roll Pitch, and Airspeed commands

to the AFDS (L NAV and V NAV modes) and thrust commands and limits to the A/T. FMC N1 Limits may be automatically displayed on the N1 Indicators. The N1 limit or reduced mode which is being used by the A/T is annunciated on the Thrust Mode Annunciator. During V NAV operation, the FMC target airspeed may be automatically displayed on the Mach/Airspeed Indicators and EADI speed tapes. Also, lateral and vertical navigation data may be displayed for reference on the HSIs. With LNAV and V NAV engaged, the CDU displays allow the crew to monitor proper FMS operation and flight progress. With L NAV and V NAV disengaged, the displays are used for reference, allowing the crew to fly the selected route/profile either manually or with conventional autoflight modes. The CDUs are also used to:

Provide “what if’ previews of flight plan options Make revisions to the flight plan, and Provide reference data.

Lateral Navigation Lateral outputs from the FMC are normally referenced to a direct great circle course (either on or off airways), but can be referenced to a fixed heading or course when flying a published procedure. FMC navigational computations are based upon an “FMC position” which is established using radio inputs and/or IRS present position. The FMC position may be based upon IRS data only; however, available DME inputs are normally used to refine and update the FMC position. Just prior to take-off, the crew may set the FMC position to a point on the departure runway via the CDU TAKEOFF REF page. Activation of the TO/GA button updates the IRS to this position. It should be noted that radio updating does not occur on the ground. Consequently, navigation position error can accumulate in the FMC during transit or through flight stops. Fast realignment of the IRSs with a new present position removes the errors. The errors will also be removed after take-off when radio updating again becomes available. It should also be noted that the FMC is not certified as a “sole source of navigation” system. It is certified to navigate accurately in conjunction with an accurate radio navaid environment.

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Vertical Navigation Vertical outputs from the FMC are normally referenced to the best economy profile for:

Climb Cruise Descent and holding.

Computation of optimum speeds for the economy profile is based upon a company specified Cost Index, which is the ratio of other operating costs compared to the cost of fuel. Changing the Cost Index changes the computed optimum speeds. If desired, the crew may manually select any speed profile other than economy. Required Time of Arrival (RTA) Navigation The RTA navigation mode is designed to assist the pilot in complying with a required time of arrival at a designated waypoint such as the final approach fix, holding fix or airport. After the appropriate waypoint and RTA are input to the FMC via the CDU, the FMC will compute:

A recommended take-off time Speeds required to comply with the RTA, and Progress information as the flight takes place.

Speeds are automatically adjusted for in-flight winds and route changes by the FMC adjusting the Cost Index. If the RTA is unobtainable under present routing and/or environmental conditions, the FMC will so advise by displaying an appropriate message for the most economical operation. The recommended take-off time should be met, as a later take-off will result in a higher cost index than originally planned. Radio Tuning The dual frequency scanning DME radios are automatically tuned by the FMC. The stations to be tuned are selected based upon the best available signals (in terms of geometry and strength) for updating the FMC position, unless a specific station is required by the flight plan. Radio position is determined by the intersection of two DME arcs. If the DME radios fail, or if suitable DME stations are not available, FMC navigation is based upon IRS position information only. The two VHF NAV Radios are used by the FMC for LOCALIZER updating during an ILS approach and by the crew for navigation monitoring. The FMC is designed to automatically reject unreliable navaid data during FMC position updating. However, in certain conditions, navaids which are in error may satisfy the “reasonableness criteria” and provide the FMC with an inaccurate radio position. One of the most vulnerable times is when a radio position update occurs just after takeoff. This is usually

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manifested in an abrupt heading correction after engaging L NAV. The position shift can be seen on the EHSI map which will shift the desired track and runway symbol to a position significantly different from that displayed during the ground roll. If the flight crew observes either of these indications, and an extended period of IRS NAV ONLY flight follows, the FMC position should be carefully monitored.

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Electrical Power The FMC and both CDUs are powered indirectly by transfer bus No. 1. If electrical power is lost for less than ten seconds:

L NAV and V NAV disengage All entered data is retained by the FMC, and When power is restored the FMC/CDU resumes normal operation.

If power is lost for ten seconds or more:

While on the ground All pre-flight procedures and entries must be re-accomplished when power is restored In-flight LNAV and VNAV disengage and all entered data is retained by the FMC. When power is restored the MOD RTE LEGS page is displayed with the Advisory Message “SELECT ACTIVE WPT/LEG”

A “Software Restart” which results from the FMC entering an impossible computational state, such as division by zero, will appear to the aircrew to be a temporary loss of electrical power. The CDU will momentarily blank and then display “FMC’. This is followed quickly by the display of the MOD RTE LEGS page with box prompts in the Active Waypoint line. The message “SELECT ACTIVE WPT/LEG” will be displayed. In some cases, multiple Software Restarts inflight will result in FMC failure. The FMC may be reinitialized by removing AC power for more than 10 seconds after landing. Terminology The FMC data bases contain information from which the crew makes selections or modifications to their specific flight plan. The terminology below is used to describe the status of this information and methods of crew interaction with the FMC/CDU. Executing Pressing the EXEC key when it is armed (Light Bar illuminated). Often written as EXECuting. Inactive Route or performance information which is not being actively tracked by the FMC. Such information cannot be engaged to the AFDS or A/T. Inactive data is made active by EXECuting.

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Active Route or performance information which is being actively tracked by the FMC and which may be engaged to the AFDS (L NAV and/or V NAV) or Alt. Often written as ACTive. Within the above general context, the term “active” may also be used to specifically identify the current leg or route segment being flown, or the upcoming waypoint being flown towards; for example, when flying from ABC to DEF (the active Leg), the active waypoint is DEF. Page Status The Page Title line indicates whether certain route and performance pages are inactive, active, or modified. For example:

Inactive Title Active Title RTE ECON CLB

ACT RTE ACT ECON CLB

Modification Changes which are being proposed to active route or performance pages, as indicated by the Page Status. Modified pages again become ACTive after EXECuting. Often written as MODified. For example:

Modified Title New Active Title MOD RTE MOD ECON CLB

ACT RTE ACT ECON CLB

Initialization The process of checking the effective dates of the navigation database and entering preflight data into the FMC and IRSs via the CDU. Line Select The act of pressing a Line Select Key. Enter To enter data on a page is to line select that data from the Scratch Pad up to the desired line. The data is initially entered in the Scratch Pad by a Keyboard entry, or by copying existing page data into the Scratch Pad by Line selection.

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Access To access a page is to cause that page to be displayed on the CDU. Propagate Certain types of data (for example, cruise attitude) are used on more than one page. Generally, such data need only be entered one time; the FMC then automatically “propagates” (duplicates) the entry to other applicable pages. Page Concepts General The FMC stores its navigation and performance information on electronic “pages”. These pages are displayed on the CDU through use of the Mode Keys. Pressing a key provides access to all pages of data which are functionally related to that mode; this may be a single page, or multiple pages. Access Prompts on the displayed page serve as an “index” that provides access to other related pages (if any), or perhaps to a separate Index Page. Consider the following Display:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ <INDEX TAKEOFF>

In this example, the complete index to other related pages is of such Length that a separate index listing must be accessed by line selecting <INDEX. However, one of the related pages concerning takeoff data can be accessed directly by line selecting TAKEOFF. Occasionally, a page may be selected by pressing either the Mode Key or a Line Select Key. Consider the following display:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ROUTE>

In this example, page 1 of route data can be accessed by either line selecting ROUTE, or by pressing the RTE Mode Key. Also, line selecting an Access Prompt may occasionally be used to perform a function rather than to select another page. Consider the following display:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ACTIVATE>

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In this example, line selecting ACTIVATE> arms the execution function, causing the Light Bar In the EXEC key to illuminate. Page Sequence Logic Although the FMC contains many displayable pages, proper page selection and execution are not difficult. Automatic display of some pages by phase of flight, as well as access prompts on many displays, provide assistance with the proper sequence of steps to initialize, activate, and fly the desired flight plan. For example, the diagram on the next page shows how the FMC guides the crew through the required pages of a normal preflight. Upon initial power application, the IDENT page normally appears. If the IDENT page does not appear, then it may be accessed via the INIT/REF INDEX page, as shown at the bottom of the diagram. After checking the displayed data, line select key 6R is pressed in order to display the next logical page, POS INIT. The crew continues with each page, checking and entering data as required, then line selecting 6R, until preflight is complete. If a Standard Instrument Departure (SID) must be entered into the route, press the DEP/ARR mode key for access to the DEPARTURES page. Following selection of the SID, line selection of 6R returns the display to the RTE page. When the EXEC key Illuminates, It must be pressed before continuing in order to activate the entered data. The RTE page requires line selection of the ACTIVATE prompt before the EXEC key will Illuminate. This two-step procedure protects the crew from inadvertent activation of unintended data.

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The FMC/CDU is designed to automatically preserve the most capable modes of navigation and guidance that can be maintained with the equipment and navigation aids available. If an error or system failure results in reduced capability (downmoding), then the FMC may generate a crew message for display in the CDU Scratch Pad. If other system inputs to the FMC should fail, affected CDU Displays are blanked to prevent the display of misleading or erroneous data. For example, loss of the total fuel input causes all performance-related data to be blank.

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The messages and FMC internal responses provide for an orderly transition from full FMC -guided flight to less automated capability, if required. CDU Messages There are two categories of CDU messages. Alerting Messages have the highest priority and identify a condition which must be acknowledged and corrected by the crew before further FMC-guided flight is advisable or possible. Advisory Messages have lower priority and inform the crew of CDU entry errors or system status. The generation of any message causes the white CDU MSG light to illuminate. Alerting Messages also illuminate the amber FMC Alert Light on each pilot’s instrument panel. Placing the test switch to position 1 or 2 also illuminates the lights. Pressing the switch extinguishes the light.

If the Scratch Pad is empty, any message is displayed immediately when generated. Some messages will displace an existing Scratch Pad entry and are also displayed immediately when generated. Other messages will not be displayed until the Scratch Pad has been cleared; however, the MSG Light will still be illuminated. A new entry in the Scratch Pad overrides any displayed message. Messages caused by CDU entry errors are displayed only on the associated CDU; other messages are displayed on both CDUs. When multiple messages have been generated, they will be “stacked” for display in priority sequence, or in the order of their occurrence if of the same priority. As each message is cleared, the next message in the stack is displayed. Most messages are cleared with the CLR key on the CDU, or by correcting the condition. Other messages are cleared by changing the displayed page; this will delete the entry which caused the message. Waypoint The term “Waypoint” may refer to either a specific waypoint name or, in a generic sense, to any of the below definitions.

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Waypoint Identifiers Stored waypoint identifiers may be entered manually on either the RTE or RTE LEGS pages, or they may be entered automatically as part of a company route designation. The following are valid CDU entries for published waypoint identifiers stored in the permanent navigation database (five characters maximum):

Waypoint identifier (waypoint name) Navaid identifier Runway number Airport ICAO identifier

Created Waypoints If the permanent nav database does not contain the desired stored waypoint(s), then new (previously unstored) “Created Waypoints” can be defined by the crew. On the RTE or RTE LEGS pages. Created Waypoints are keyed into the Scratch Pad as any of the following:

Place Bearing/Distance (for example, COV150/50, where “Place’ is any identifier already stored in either the permanent, supplemental, or temporary nav data base (this Scratch Pad entry could also be a transfer from the FIX INFO page).

Place Bearing/Place Bearing (for example, COV080/TNT190), the intersection of bearings from two different “Places’.

Along-Track Displacement (for example, COV/-10), the distance either side of an existing flight-plan waypoint.

Latitude and Longitude (for example, N5732.8W00410.3). When the Scratch Pad entry is line selected into the route, the FMC assigns a sequential identifier number to Created Waypoints. For the first three types of waypoints, the FMC uses the first three Letters of the reference waypoint, followed by the sequential number of times that reference has been used. For example, COV150/50 would be assigned the identifier COV01. Then COV080/TNT190 would be assigned COV02. Latitude/Longitude entries use WPT as the first three letters. The waypoints are automatically stored in the temporary nav database for one flight only. On the NAV DATA pages, entry of the FMC-assigned identifier on the WPT IDENT line provides a display of the parameters originally keyed-in to define that waypoint.

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Alternatively, Created Waypoints can also be initially defined using crew-assigned identifiers on either the SUPP NAV DATA or REF NAV DATA pages. This method allows “waypoints” to be defined in any of three FMC categories:

Waypoints Navaids, or Airports.

Entries defined on the SUPP NAV DATA pages (accessible on the ground only) are automatically stored in the supplemental nav database until deleted by the crew. Entries defined on the REF NAV DATA pages are automatically stored in the temporary nav database for one flight only. The supplemental and temporary databases share storage capacity for:

40 Navaids, and 6 Airports,

The entries being stored in either nav base on a “first come, first served” basis. For the Waypoint category, exclusive storage is reserved in the temporary database for 20 entries (including those created on the RTE or RTE LEGS pages). An additional 20 Waypoints (up to a maximum of 40) can be stored in either the temporary or supplemental data base on a “first come, first served’ basis. Conditional Waypoints The preceding waypoints all refer to geographically fixed positions. Waypolnts which are not geographically fixed are called Conditional Waypoints, and are imbedded within stored procedures and displayed on the CDU in parenthesis. They cannot be entered manually. Conditional Waypoints are displayed as any of the following:

(1500) altitude condition (COV.100) VOR radial crossing condition (COV-l0) DME crossing condition (INTC) intercept course to next waypoint (VECTOR) maintain heading indefinitely

When (VECTOR) is the active leg, the FMC does not automatically sequence to the next waypoint. The next waypoint becomes active only upon EXECution of the procedures for proceeding direct to a waypoint or intercepting a leg to a Waypoint. When any storage category is full, entries which are no longer required should be deleted by the crew to make space for additional new entries. Created Waypoints cannot be stored in the database Runway Category.

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Intentionally Left Blank

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Chapter 25.

Boeing 737 - Navigation Equipment and Flight Management Inertial Reference System

IRS Mode Selector Controls the operating mode of the respective inertial reference system (left or right)

OFF Alignment is lost. All electrical power is removed from the system after a 30 second shutdown cycle ALIGN Used for initial alignment. The aircraft must be parked.

From OFF to ALIGN initiates the alignment cycle. The selector may be moved to NAV during the cycle From NAV to ALIGN automatically updates alignment and zeroes groundspeed error (fast realignment). Present position should be manually updated, but is not required. Return the selector to NAV.

NAV Navigation – detented position. The system enters the NAV mode after completion of the alignment cycle and entry of present position.

Provides full IRS data to airplane systems for normal operations.

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ATT Attitude – a back up mode. Provides only attitude and heading information.

Attitude Information is Invalid Attitude flag in view until ALIGN light

is extinguished. Heading Information is Invalid Heading flag in view until the

actually magnetic heading is manually entered and the ALIGN light extinguished.

Position and groundspeed information is lost until the IRS is aligned

on the ground. The selector must be cycled through OFF before reselecting ALIGN or NAV.

Align Light (White)

Illuminated (Steady) The respective IRS is operating normally in either the: ALIGN mode Initial ATT mode The shutdown cycle

Illuminated (Flashing) Alignment cannot be completed due to IRS detection of one of the following errors:

Significant difference between previous and entered positions or an unreasonable present position entry

No present position entry Extinguished IRS is not in the ALIGN mode:

Mode selector in NAV. Alignment completed. Full IRS data is available.

Mode selector in ATT. Attitude information is available. Heading information is also available following entry of the initial magnetic heading.

Fault Light (Amber)

Illuminated A system fault which affects the respective IRS ATT and/or NAV modes has been detected.

On DC Light (Amber)

Illuminated The respective IRS is operating on DC power from the switched hot battery bus (AC power not normal).

If on the ground, the ground call horn in the nose wheel well sounds providing an alert that a battery drain condition may exist. Momentary illumination is normal during alignment self test (DC power normal).

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DC Fail Light (Amber)

Illuminated DC power for the respective IRS is not normal. If the other lights are extinguished the IRS is operating normally on AC power.

Inertial Reference System General Two independent Inertial Reference Systems (IRS) are installed consisting of:

Inertial Reference Units (IRU) Mode selectors IRS Display Unit

Each IRS has three sets of laser gyros and accelerometers that replace the conventional mechanical gyros and compass system. The IRSs are the airplanes sole source of attitude and heading information, except for the standby attitude indicator and the standby magnetic compass. In their normal navigation mode, the IRSs provide:

Attitude True and magnetic heading Acceleration Vertical speed Groundspeed Track Present position Wind data

IRS outputs are independent of external navigation aids. Alignment Alignment on the Ground An IRS must be aligned and initialised with the airplane present position before it can enter the NAV mode. The position is normally entered through the FMC CDU during alignment. If the position cannot be entered through the FMC CDU, it may be entered through the IRS Display Unit. The airplane must remain stationary during alignment.

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Normal alignment between 70°12’N and 70°12’S is initiated by rotating the IRS Mode Selector from OFF directly to NAV. The IRS performs a short DC power test, during which the ON DC Light illuminates. When the ON DC light extinguishes and the ALIGN light illuminates, the IRS has begun the alignment process. Airplane present position should be entered at this time. The IRS will automatically enter the NAV mode after approximately 10 minutes, and the ALIGN light will extinguish. High latitude alignment, at latitudes between 70°12’ and 78°15’, require an extended alignment time. The mode selector must be left in the ALIGN position for 17 minutes, then rotated to the NAV position. The IRS will then immediately enter the NAV mode. Magnetic variation between 73°N and 60°S is stored in each IRS memory. The data corresponding to the present position are combined with true heading to determine magnetic heading. If magnetic information is unavailable, special navigation equipment is required to provide true heading to the EHSIs. Fast Realignment on the Ground During transit or through-flight stops with brief ground times, a 30 second realignment and zeroing of groundspeed error may be performed by selecting ALIGN from NAV while the airplane is parked. Present position should be simultaneously updated by manually entering latitude and longitude prior to reselecting NAV. If the airplane is moved during alignment or fast realignment (ALIGN light illuminated), the IRSs automatically begin the full 10 minute alignment process over again. Loss of Alignment If an IRS loses both AC and DC power, the alignment is lost. Alignment can also be lost if the mode selector is moved out of the NAV position. If alignment is lost during flight, the navigation mode (including present position and groundspeed outputs) is inoperative for the remainder of the flight. Selecting ATT allows the attitude mode to be used to re-level the system and provide ADI attitude. The attitude mode requires approximately 30 seconds of straight and level unaccelerated flight to complete the re-level process. Some attitude errors may occur during acceleration, but will be slowly removed after acceleration. The attitude mode can also provide heading information, but to establish compass synchronisation the crew must manually enter the initial magnetic heading. Thereafter, drift of the IRS heading will occur (up to 15° per hour). When in ATT mode, an operating compass system must be periodically cross checked and an updated heading entered in the IRS as required.

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IRS Display Unit

System Display Selector Displays the left (L) or right (L) IRS for the data displays. Display Selector Selects the desired function or data for the data displays. Displays are for the IRS selected with the System Display Selector.

TEST Spring loaded to TK/GS. Used only during alignment. All lights in the data displays and on the mode selector unit momentarily illuminate; followed by a 10 second internal self test. Positioning the Master Lights Switch on the centre instrument panel to TEST illuminates all lights in the data displays and on the mode selector unit. TK/GS Track and groundspeed. The left window displays present true track

(course). The right window displays present in-flight wind speed (knots).

PPOS Present position. Present latitude and longitude are displayed. WIND The left window displays present in-flight true wind direction. The right window displays present in-flight wind speed (knots) HDG/STS Heading status. The left window displays true heading. The right

window displays any applicable maintenance status codes (last two digits).

During IRS alignment, the right window also displays the minutes remaining until alignment is complete. The window displays 7 (at the third digit) until the time remaining reaches 6 minutes. The display then counts down in one minute intervals.

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Brightness Control In the centre of the display selector. Adjusts the brightness of the data displays.

Data Displays Two windows display data for the IRS selected with the System Display Selector. The display selector normally determines the type of data displayed. Keyboard entry of present position or magnetic heading will override the selected display. The last digit of each window is for a decimal place. Enter Key

Illuminated The integral cue lights illuminate when N, S, E, W or H entries are being keyed. When keying is complete the key is pressed. Press The cue lights extinguish and the keyed data is simultaneously

entered into each IRS following completion of a valid self test for data sensibility. The data displays are again controlled by the display selector.

Keyboard Provides for manual IRS entry of present position or magnetic heading. The keyboard functions independently from the display selector position and the L or R position of the System Display Selector. Alpha Keys

Press The data displays are controlled by the keyboard when the N, S, E or W (latitude/longitude) or H (heading) key is pressed. Arms the keyboard for numeric entries.

Numeric Keys

Press Permits manual entry of present position (latitude/longitude) when either ALIGN light is illuminated. Permits manual entry of present magnetic heading when either mode selector is in ATT.

Clear Key

Illuminated The integral cue lights illuminate following an ENT operation if the self test determines the data to be of an unreasonable value (entry not accepted by the IRSs).

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Press Extinguishes the cue lights. If the cue lights are already extinguished, pressing CLR clears the associated data display of data keyed in but not yet entered (or not accepted). The data displays are again controlled by the display selector.

Instrument Transfer Switch/Instrument Transfer Switch Light

IRS Transfer Switch Should either IRS fail, the IRS Transfer Switch is used to switch the flight instruments attitude and heading source to the functioning IRS.

Instrument Transfer Switch Light (Amber) When illuminated, one or both of the Instrument Transfer Switches has been moved from the normal position.

DME System A dual scanning DME is installed. This consists of two DME interrogators, each of which rapidly alternates between the manually tuned frequency (VOR/ILS) and an automatically tuned frequency for FMC use

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Radio Distance Magnetic Indicator

1 DME Indicators 300 nm maximum search for all DME stations

Warning Flags

Warning Flag – Electrical power lost

DME receiver powered, but not receiving a DME station, or during Agility-Tuning

2 Bearing Pointer Number 1 Warning Flag in View

VOR Mode Power failure VHF NAV Signal unreliable ADF Mode Power Failure

3 Heading Warning Flag In View Selected compass signal is invalid Power failure 4 Bearing Pointer Number 2 Warning Flag in View

VOR Mode Power failure VHF NAV Signal unreliable ADF Mode Power Failure

ATPL Radio Navigation ©Atlantic Flight Training 25-9

5 Bearing Pointers Signals to the VOR Bearing Pointers are not affected by the VHF NAV Transfer Switch.

Narrow Pointer Uses signals from the VHF NAV receiver Number 1, or ADF receiver number 1 Wide Pointer Uses signals from the VHF NAV receiver Number 2, or ADF receiver number 2

6 ADF/VOR Bearing Pointer Switches When pressed, selects the ADF or VOR for the bearing pointer

VOR/ILS Navigation Two navigation receivers and control panels are installed. The panels are used to tune related VOR and ILS frequencies. VOR/ILS information is displayed on the RDMI and the EHSI VOR/ILS mode. The VHF NAV Transfer Switch is used to switch the EHSI to a functioning receiver in the event of a failure of the number one or the number two navigation radio, or the loss of navigation information to an EFIS display.

ATPL Radio Navigation 28 October 2003 25-10

VOR Test Switch With a VOR frequency tuned and a course of 000 selected:

Press The course deviation bar centres VOR Bearing Pointer indicates 180° TO-FROM Ambiguity Indicator shows a FROM indication

ILS Test Switch With an ILS frequency tuned, pressing switches gives test indications Frequency Selector Manually selects the desired frequency Frequency Indicator Indicates the frequency selected by the frequency selector or by the automatic tuning system. Mode Selector Switch Selects between automatic and manual tuning of the frequencies Marker Beacons Each pilot has a set of marker beacon lights that show outer, middle and airways beacon passage. Both sets are operated by one marker beacon receiver.

ATPL Radio Navigation ©Atlantic Flight Training 25-11

The high-low switch is used to adjust the sensitivity of the receiver.

Airways (White) Illuminates over an inner or airways marker beacon Middle (Amber) Illuminates over a middle marker beacon Outer (Blue) Illuminates over an outer marker beacon Marker Beacon Switch

High High sensitivity of the receiver Low Low sensitivity of the receiver

ADF Navigation Two ADF receivers are installed. The ADF bearing signals are sent to the pointers on the EHSI and RDMI. The audio is heard by using the ADF Receiver Control on an audio selector panel. An automatic direction finding (ADF) system enables automatic determination of magnetic and relative bearings to selected facilities. If heading or track information is lost or invalid, EHSI ADF bearing pointers will not be displayed, and RDMI ADF bearing pointers will not display correct magnetic bearings. Relative bearings indicated by pointers may be correct, if the receiver is operating.

ATPL Radio Navigation 28 October 2003 25-12

Frequency Indicators Two windows showing the frequency selected for ADF 1 and ADF 2 ADF Mode Selector Two selectors for each ADF

OFF No electrical power to the receiver ANT Audio reception optimised. No ADF bearing sent to the RDMI. ADF Audio reception is possible. ADF bearing sent to RDMI TEST Bearing pointer indicates 45° left of lubber line for a valid test.

ADF Gain Control Adjusts the receiver gain of the respective ADF. Tone Switch Position 1 or 2 adds tone to the respective ADF receiver.

Central position disables tone.

SSR Transponder Selections Two ATC Transponders are installed and controlled by a single control panel. The ATC Transponder System transmits a coded radio signal when interrogated by ATC ground radar. Altitude reporting capability is also provided allowing altitude information from a selected ADC to be transmitted to the ATC radar facility.

ATPL Radio Navigation ©Atlantic Flight Training 25-13

ATC Identification Code Selectors Select the ATC code ATC Code Indicator Displays the code selected by the code selectors “F” will appear if a fault in the transponder system is detected Altitude Reporting Switch Enables altitude reporting Identification Pushbutton Transmits the identification signal Transponder Select Switch

STBY Transponder mode disabled L or R System connected to Number 1 or Number 2 ADC AUTO Some systems have an AUTO system that activates the transponder when an air/ground switch in the main landing gear shows the aircraft to be airborne. The transponder is deactivated when the aircraft is shown to be on the ground.

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Weather Radar The weather radar is an X band colour radar system used for:

Weather detection and analysis Ground mapping

The system detects and locates various types of precipitation bearing clouds along the flight path of the aircraft, and gives the pilot a visual indication of their intensity by using colour contrast. The radar indicates the clouds rainfall intensity against a black background.

Areas of heaviest rainfall appear in red The next level in yellow The least rainfall in green

In map mode the radar displays surfaces in red, yellow and green (most reflective to least reflective) These displays enable identification of:

Coastlines Hilly or mountainous regions Cities or large structures

Ground mapping mode can be useful in areas where ground based navigation aids are limited. The WX/T mode displays normal precipitation and precipitation associated with turbulence. When the radar detects a horizontal flow of precipitation, with velocities of 5 or more metres per second, towards or away from the radar antenna, that target display also becomes magenta. This magenta area is associated with heavy turbulence. The detection of turbulence is automatically limited to 50 nm range regardless of the selected range. The IDNT mode activates the ground clutter reduction circuits. Signals that are determined to have a high probability of originating from ground returns will be automatically removed from the display. Some portions of weather targets may be removed as well. The IDNT mode is provided for analysis by the pilot and is not for continuous use.

ATPL Radio Navigation ©Atlantic Flight Training 25-15

EFIS Control Panel – Top Right Section

Range Selector Selects the desired range in nautical miles for:

HSI MAP Plan, and Weather radar displays

Weather Radar Switch Press to activate the weather radar display

TFR Activates the radar display on the left or right HSI WX/T Activates display of detected precipitation and turbulence within 50 nm WX Activates display of detected precipitation MAP Activates display of detected ground returns IDNT Activates ground clutter suppression in WX mode, normal operation position would be off

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TEST Displays maintenance test pattern GAIN Rotating gain control. Manually sets receiver sensitivity to enhance

Ground WX WX/T, and MAP

TILT Manually controls antenna tilt position from 15° up to 15° down. GAIN UCAL Illuminates when gain is improperly set. Radio Altimeters The low range radio altimeters provide indication of airplane height above the ground up to 2500 ft absolute altitude. The radio altitude is indicated on each pilot’s EADI. When the Captain’s RA is inoperative:

All modes of the GPWS are inoperative Autopilot A channel should not be selected for approach Autothrottle automatic retard during landing flare is inoperative.

When the First Officer’s RA is inoperative:

Autopilot B channel should not be selected for approach