precision approach and landing systems

7/29/2019 Precision Approach and Landing Systems

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Precision Approach and Landing

SystemsApproachgenerally the phase flight immediately preceding

landing

Non-precision Approach

Only horizontal guidance provided

Precision Approach

Vertical and horizontal guidance provided


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Systems

Precision Approach Categories

Cat I: 200 Ft. ceiling 2600 Ft. visibility

Cat II: 100 Ft. decision height, 1200 Ft. RVR*

*Runway Visual Range

Cat III:

subcat a: 0 Ft. decision height, 700 Ft. RVR

subcat b: 0 Ft. decision height, 150 Ft. RVR

subcat c: 0 Ft. decision height, 0 Ft. RVR


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Systems

Note: The higher the category, the more stringent the requirements

for ground and air installations and pilot qualifications

e.g.

extra runway lighting (centreline), approach lighting redundant transmitters (hot spares)

autoland


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Runway Visual Range


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Approach and Runway Lighting


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SystemsPrecision Instrument Approach Systems

Instrument Landing System (ILS)

Microwave Landing System (MLS) Differential GLS (Local and WAAS)


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ICAO standard approach system

Developed in late 1940s

Over 110 installed in Canada


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Components Localizer - horizontal guidance.

Glide Path (or Glide Slope) - vertical guidance

Marker Beacons - along - track position fixes (obsolete)

DME (sometimes)

Approach Lighting


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Frequency (Localizer): 108MHz-112 MHz

Even 10thsgives 40 frequencies

As with VOR, interference from FM broadcast stations can be a

problem

Frequency (Glide Path): 329MHz335 MHz (paired with localizer frequencies as

specified by ICAO)

Note: Receiver automatically selects Glide Path frequency when Localizer

frequency is tuned


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Localizer

The localizer antenna array is located at some distance (usually about 1000 Ft.) offthe stop end of the runway


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Localizer

The antenna array radiates two signals each with its own antenna pattern. One

pattern is modulated with a 150Hz AM tone, the other with a 90 Hz tone

When the aircraft is on the left of the extended runway

centreline, the 90Hz tone predominates and when it is onthe right, the 150 Hz tone predominates.

When the aircraft is on course, the two tones are equal


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Glide Path

The Glide Path antenna is usually located about 1000 Ft down the runway from

the threshold and 400 Ft. off to the side


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Glide Path

The signal format is the same as for the localizer

but rotated 90.

In order to minimize the height of the antenna,the ground is used as a reflecting surface.


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Glide Path


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Glide Path

Note: Because the ground is used in the generation of

the Glide Path signal, variations in the groundconductivity and/or level will change the Glide Path

angle. One problem is snow, which raises the level of

the conducting surface.

During snow storms, the clearing of the area in front ofthe Glide Path antenna has second priority (after crash

routes)

NOTE: The glide path signal flares starting around

2600 Ft from threshold. Also, the structure (deviations)becomes quite large. Thus the Glide Path is not a

reliable source of vertical position information for

AUTOLAND systems


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Markers

Part of the ICAO specification for ILS includes a facility

called a marker.

Markers are almost extinct now, but their original function wasto provide an indication of the distance of the aircraft from

touchdown.


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Markers

The OUTER marker (modulated with Morse Code dash dash

dash etc.)is located about 4 NM from threshold and indicates

the start of the final descent

The MIDDLE marker (modulated with dash dot dash dot etc.) is

located about 2500 Ft from threshold and indicates the decision

point for Cat I approaches

The INNER maker (modulated with dot dot dot etc.) is about

1200 Ft from threshold and indicates the decision point for Cat

II approaches


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Markers

Outer Middle Inner

Markers all operate at a frequency of 75 MHz. Their radiation

patterns are narrow and pointed upwards so that the aircraftreceives the signal only when it is directly overhead.


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ILS Aircraft Installations

The aircraft receiver is relatively simple since all it has to do is

measure the amplitudes of the 90Hz and 150Hz modulations andprovide an error signal:

The sum of the modulations is also provided as an integrity check

Tuning Demodulation

90Hz

Filter

150Hz

Filter

+

+

+

-

Guidance Flag

Antenna


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Antennas:

Localizer:

Uses the same antenna as the VOR. Either a half-wave dipole or a

loop.


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Dash 8 LOC/VOR Antennas


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Antennas:

Glide Path:

Normally the Glide Path antenna is a dual loop mounted inside the

radome

Radar Antenna

Glide Path Antenna


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Antennas:

Glide Path:

On Long bodied aircraft the radome is not a good location

3


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Antennas:

Glide Path:

On 747s and other such aircraft, the Glide Path antennas are

mounted on the nose gear doors.

Another consideration is that, in the radome, the radar antenna may

influence the reception from the Glide Path antenna


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ILS

Accuracy:

Localizer:

Maximum error defined by ICAO at the point where the average

localizer course crosses the runway threshold

Cat I: 35 Ft. (0.29

for a 6000Ft. Runway)Cat II: 25 Ft. (0.20 for a 6000Ft. Runway)

Cat III: 10 Ft. (0.08 for a 6000Ft. Runway)

Glide Path:0.056 where is the Glide Path angle (0.168 for 3 GP angle)


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ILS

Irregularities:

The course lines are generated by antenna patterns which can bealtered by the presence of reflecting surfaces such as hangars and

other aircraft.

Thus the localizer and glide paths are never straight lines but the

deviations from nominal (called structure) are controlled bythe ICAO specification.

The allowable deviations decrease as the threshold is approached

4 NM2500 Ft


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ILS

Integrity:

The ground station includes monitors which will detect out oftolerance conditions and either switch transmitters or turn off the

transmission.

The receiver measures the modulations and sets a flag if the sum

of the modulations goes below a given threshold


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ILS

Integrity:

The ground station includes monitors which will detect out of

tolerance conditions and either switch transmitters or turn off the

transmission.

Response time is 10 seconds for Cat I and 2 seconds for Cat II

and III. Thus Cat II and III systems require that the standby

transmitter be on at all times (hot spare)

The receiver measures the modulations and sets a flag if the sum

of the modulations goes below a given threshold

ILS


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ILSFuture of ILS:

Threats:

FM broadcast stations

Relatively few channels available (40)

Susceptibility to interference

Limited Glide Path angle (5?)

Limited to straight in approaches

Strengths

Large number of ground and air installations

Meets 99+% of requirements

Guaranteed to be around until at least 2015

Microwave Landing System (MLS)


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Background

In the mid 1970s the US was running into ILS frequency

congestion problems in the North Eastern part of the country.(the 40 channel problem)

In an attempt to alleviate the situation, they proposed that ICAO

issue a SARP to specify a new type of landing aid that would use

microwave frequencies (specifically about 15 GHz)

In response, two techniques were proposed. The US and

Australia proposed a Time Referenced Scanning Beam (TRSB)

system and the British proposed a Doppler system.



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Background (continued)

Because there was very little difference between the two

systems and because there was perceived to be a great deal ofeconomic benefit to the winners, the selection process became

almost entirely political.

To no ones surprise, the US/Australian system was adopted.

Unfortunately, the FAA, which was given the job of introducing

the MLS into the civil aviation system, failed completely.

In 1994, the US government issued a statement that no further

work would be done on MLS and that GPS would be used instead.



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Background (continued)

This turned out to be premature. At the present time, only

WAAS-based procedures have been certified for use and they do

not quite meet Cat I requirements. LAAS for Cat II and III are

still in the pre-certification stage.

Thus the door is still open for MLS to stage a comeback.

Of interest is that NASA uses a 15 GHz TSB MLS for landing

the Space Shuttle. (Glide Path angle 19)



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Canadas Role

Several MLS facilities have been installed from time to time in

Canada over the years.

An experimental MLS was installed at Uplands Airport

Two MLSs were installed at airfields in the coal-mining area of

Alberta near Edson.

Two MLSs were installed at Toronto Island Airport for a few

years



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Frequency of Operation:

5.031 to 5.0907 GHz

300kHz spacing (200 channels)

Functions Provided (* options):

Azimuth (horizontal) guidance

Vertical guidance

Flare guidance* (extra accurate vertical guidance for the last

1000 Ft. or so)

Missed Approach guidance*



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Principle of Operation:

Angular position is determined by measuring the time of

detection of a beam which is being scanned at a

predetermined rate

ExampleAzimuth (horizontal):

The azimuth beam is shaped as follows:

Top View

Side View



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The beam is fan-shaped, that is, very narrow in the

measurement (azimuth) direction and fairly wide in the other

(vertical) direction meaning that it can be used at reasonably

high angles

Top View

Side View



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The beam is swept back and forth at a controlled rate

The amplitude of the sweep depends on the requirements

of the system but is nominally 40



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An aircraft thus detects the beam twice per period; once on the

to sweep and once on the fro sweep. The receiver measures

the time between the two detections or pulses.

t



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Principle of Operation - Time Multiplexing

To accommodate all of the required measurements (Az, El, Back

AZ and Flare), each is assigned a time slot in a cycle ofmeasurements which takes 115 milliseconds (ms). This is called

time multiplexing.



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Principle of Operation - Beam Timing:

-40

+40

0

T0

t

Dwell Time

Since the rate of scan and the dwell time are known, the

angle can be determined from the measurement of t.

)( 02 tTV

Where V is the scan rate = 0.02/s


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The other angle functions are provided in a similar fashion


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Azimuth Antenna Installation


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Elevation Antenna Installation


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Airborne Installation:

For the frequency of 5 GHz the wavelength is 6 cm

Thus the antenna (a wave monopole) is about 1.5 cm long

Receivers are very expensive due to the small market ( about

$25,000)


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Advantages over ILS

Less susceptible to siting (reflection) problems

Selectable glide path angles (up to 20) and azimuth

approach paths

Possibility of curved approachesMuch less susceptibility to interference

Many more channels available

Increases runway usability in IFR conditions

DisadvantagesExpensive

Not many ground stations to use it with


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The Future

Many countries in Europe are interested in MLS for Cat II and

Cat III operations because they are getting tired of waiting for

LAAS. They are facing much more interference from their FM

stations because they are permitted to use much higher powerthan in North America.

Four MLS facilities were installed at London Heathrow Airport

in 2003 and British Airways has equipped 60 Airbus A320

aircraft with receivers. (2003)


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Augmented GPS (WAAS, LAAS)

As was mentioned in the section on GPS, the position accuracycan be improved dramatically by the use of differential

techniques.

WAAS is capable of accuracies (95%) of

3.2m horizontally

6.0m vertically ~ 18 Ft. Cat I Glide Path gives 10 Ft.

This is not quite adequate for Cat I approaches. The vertical

error being the problem.

WAAS A h


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WAAS Approaches

About 525 LPV approaches (Lateral Precision with Vertical

Guidance) have been approved. Limits: 300Ft/3/4 mile vis

LAAS A h


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LAAS Approaches

LAAS promises to provide Cat II and III capability but no

approaches have been certified to date

P ibl P bl i h GPS A h


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Possible Problem with GPS Approaches

(See Notes)

precision approach and landing systems

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