Fundamentals of RadarFundamentals of Radar

Prof. N.V.S.N. SarmaProf. N.V.S.N. Sarma

OutlineOutline1.1. Definition and Principles of radarDefinition and Principles of radar2.2. Radar Frequencies Radar Frequencies 3.3. Radar Types and ApplicationsRadar Types and Applications4.4. Radar OperationRadar Operation5.5. Radar modesRadar modes

What is Radar?What is Radar?nn RADAR (Radio Detection RADAR (Radio Detection

And Ranging) is a way to And Ranging) is a way to detect and study far off detect and study far off targets by transmitting a targets by transmitting a radio pulse in the direction radio pulse in the direction of the target and observing of the target and observing the reflection of the wave.the reflection of the wave.

nn It’s basically radio echo It’s basically radio echo

What is Radar?What is Radar?nn RADAR (Radio Detection RADAR (Radio Detection

And Ranging) is a way to And Ranging) is a way to detect and study far off detect and study far off targets by transmitting a radio targets by transmitting a radio pulse in the direction of the pulse in the direction of the target and observing the target and observing the reflection of the wave.reflection of the wave.

nn It’s basically radio echo It’s basically radio echo

RADARRADAR OperationOperationRARAdio dio DDetection etection AAnd nd RRanginganging

Radar observables:• Target range• Target angles (azimuth & elevation)• Target size (radar cross section)• Target speed (Doppler)• Target features (imaging)

Antenna

TransmittedPulse

TargetCross

Section

Propagation

ReflectedPulse

(“echo”)

A radar operator view A radar operator view [4][4]

mile

statute nautical

1.609344 km 1.852 km

1,609.344 m 1,852 m

Brief history of radarBrief history of radar

nn Conceived as early as 1880 by Heinrich HertzConceived as early as 1880 by Heinrich Hertznn Observed that radio waves could be reflected off metal Observed that radio waves could be reflected off metal

objects. objects. nn RRadio adio AAid to id to DDetection etection AAnd nd RRangingangingnn 1930s 1930s

nn Britain built the first groundBritain built the first ground--based early warning system based early warning system called Chain Home.called Chain Home.

nn 19401940nn Invention of the magnetron permits high power transmission Invention of the magnetron permits high power transmission

at high frequency, thus making airborne radar possible.at high frequency, thus making airborne radar possible.

Brief history of radarBrief history of radar

nn CurrentlyCurrentlynn Radar is the primary sensor on nearly all military Radar is the primary sensor on nearly all military

aircraft.aircraft.nn Roles include airborne early warning, target Roles include airborne early warning, target

acquisition, target tracking, target illumination, acquisition, target tracking, target illumination, ground mapping, collision avoidance, altimeter, ground mapping, collision avoidance, altimeter, weather warning.weather warning.

nn Practical frequency range 100MHzPractical frequency range 100MHz--100GHz.100GHz.

Radar Frequencies

Radar Frequency Bands Radar Frequency Bands

Frequency

Wavelength 1 mm1 km 1 m 1 µm 1 nm

1 MHz 1 GHzIR UV

109 Hz

0 1 2 3 4 5 6 7 8 9 10 11 12

30 20 10 8 6 5 4 39 7

Allocated Frequency (GHz)

Wavelength (cm)

X-BandC-BandS-BandL-BandUHFVHF

Visible

1012 Hz

KuK

KaW

1.31.3 Airborne radar bands Airborne radar bands [1][1]

1.3.1 1.3.1 Airborne radar bands Airborne radar bands [1][1]

1.3.2 1.3.2 Airborne radar bands Airborne radar bands [1][1]

The RangeThe Rangenn Distance from the radarDistance from the radarnn Measured from time Measured from time

delay between transmitted delay between transmitted pulse and returned signal pulse and returned signal receivedreceived

The RangeThe Range

nn Remember, in general v=d/t and d=vt Remember, in general v=d/t and d=vt nn The range is just a distanceThe range is just a distancenn Since radio waves travel at the speed of light Since radio waves travel at the speed of light

(v = c = 300,000 km/sec )(v = c = 300,000 km/sec )range = crange = c••time/2time/2

nn Why divided by 2?Why divided by 2?

The RangeThe Range

nn The “2” is because the measured time is for a The “2” is because the measured time is for a round trip to and from the target. To determine round trip to and from the target. To determine the range, you only want the time to the object, the range, you only want the time to the object, so you take half !so you take half !

Radar Range MeasurementRadar Range Measurement

Target

• Target range = cτ2

where c = speed of lightτ = round trip time

RadarRadar

nn The range and the direction of the target The range and the direction of the target determine its location. This is the job for many determine its location. This is the job for many radar radar applications such as applications such as air traffic controlair traffic control. .

How Strong Is It?How Strong Is It?

nn The strength of the received echo can also be The strength of the received echo can also be measuredmeasured

nn This will vary with the distance of the target, its This will vary with the distance of the target, its size, its shape and its composition size, its shape and its composition

Types and Uses of RadarTypes and Uses of Radar

nn Search radars scan a large area with pulses of short radio waves Search radars scan a large area with pulses of short radio waves

nn Targeting radars use the same principle but scan a smaller area Targeting radars use the same principle but scan a smaller area more often more often

nn Navigational radars are like search radar, but use short waves that Navigational radars are like search radar, but use short waves that reflect off hard surfaces. They are used on commercial ships and reflect off hard surfaces. They are used on commercial ships and longlong--distance commercial aircraftdistance commercial aircraft

Types and Uses of RadarTypes and Uses of Radarnn Mapping radar scans a large regions for Mapping radar scans a large regions for remote sensing remote sensing

and and geographygeography applications applications

nn Wearable radar Wearable radar which is used to help the visually which is used to help the visually impaired as a substitute to eye.impaired as a substitute to eye.

nn Air traffic controlAir traffic control uses radar to reflect echoes off of uses radar to reflect echoes off of aircraftaircraft

nn Weather radar uses radar to reflect echoes off of cloudsWeather radar uses radar to reflect echoes off of clouds

Types and Uses of RadarTypes and Uses of Radar

nn Weather radars use radio waves with horizontal, dual (horizontal Weather radars use radio waves with horizontal, dual (horizontal and vertical), or circular polarizationand vertical), or circular polarization

nn Some weather radars use the Some weather radars use the Doppler effect Doppler effect to measure wind to measure wind speedsspeeds

Incoherent Scatter RadarIncoherent Scatter Radar-- A Radar A Radar Application Application

nn Used to study the Earth's Used to study the Earth's ionosphere ionosphere and its and its interactions with the upper atmosphere, the interactions with the upper atmosphere, the magnetosphere, and the solar wind magnetosphere, and the solar wind

Two Basic Types of RadarTwo Basic Types of Radar

nn Continuous WaveContinuous Wave

nn Pulse TransmissionPulse Transmission

Continuous Wave RadarContinuous Wave Radarnn Employs continual Employs continual

RADAR transmissionRADAR transmission

nn Separate transmit and Separate transmit and receive antennasreceive antennas

nn Relies on the “DOPPLER Relies on the “DOPPLER SHIFT”SHIFT”

Doppler Frequency ShiftsDoppler Frequency Shifts

Motion Away:Echo Frequency Decreases

Motion Towards:Echo Frequency Increases

Continuous Wave Radar Continuous Wave Radar ComponentsComponents

Discriminator AMP Mixer

CW RFOscillator

Indicator

OUTOUT

ININ

Transmitter Antenna

Antenna

Pulsed radarPulsed radarnn A pulsed radar is characterized by a high power A pulsed radar is characterized by a high power

transmitter that generates an endless sequence of transmitter that generates an endless sequence of pulses. The rate pulses. The rate at which the pulses are repeated is at which the pulses are repeated is defined as the defined as the pulse repetition frequencypulse repetition frequency..

nn Parameters:Parameters:nn pulse width, pulse width, ττ, , usually expressed in usually expressed in µµsecsecnn pulse repetition frequency, pulse repetition frequency, PRFPRF, , usually in kHzusually in kHznn pulse period, pulse period, TTpp = 1/PRF, = 1/PRF, usually in usually in µµsecsec

Pulse TransmissionPulse Transmission

Pulse TransmissionPulse Transmissionnn Pulse Width (PW)Pulse Width (PW)

nn Length or duration of a given pulseLength or duration of a given pulsenn Pulse Repetition Time (PRT=1/PRF)Pulse Repetition Time (PRT=1/PRF)

nn PRT is time from beginning of one pulse to the PRT is time from beginning of one pulse to the beginning of the nextbeginning of the next

nn PRF is frequency at which consecutive pulses are PRF is frequency at which consecutive pulses are transmitted.transmitted.

nn PW can determine the radar’s minimum detection range; PW PW can determine the radar’s minimum detection range; PW can determine the radar’s maximum detection range.can determine the radar’s maximum detection range.

nn PRF can determine the radar’s maximum detection range.PRF can determine the radar’s maximum detection range.

nn Pulse repetition frequency (PRF)Pulse repetition frequency (PRF)nn a. Pulses per seconda. Pulses per secondnn b. Relation to pulse repetition time (PRT)b. Relation to pulse repetition time (PRT)nn c. Effects of varying PRF c. Effects of varying PRF

nn (1) Maximum range (1) Maximum range nn (2) Accuracy(2) Accuracy

nn Peak powerPeak powernn a. Maximum signal power of any pulsea. Maximum signal power of any pulsenn b. Affects maximum range of radarb. Affects maximum range of radar

nn Average powerAverage powernn a. Total power transmitted per unit of timea. Total power transmitted per unit of timenn b. Relationship of average power to PW and PRTb. Relationship of average power to PW and PRT

nn Duty cycleDuty cyclenn a. Ratio PW (time transmitting) to PRT (time of entire cycle, a. Ratio PW (time transmitting) to PRT (time of entire cycle,

time transmitting plus rest time)time transmitting plus rest time)nn b. Also equal to ratio of average power to peak powerb. Also equal to ratio of average power to peak power

Range vs. Power/PW/PRFRange vs. Power/PW/PRF

•Minimum Range: If still transmitting when return received RETURN NOT SEEN.

•Max Range: PRFPWPRTPW

PeakPowererAveragePow *==

As min Rh max Rh

PW

PRF

nn Describe the components of a pulse radar Describe the components of a pulse radar system.system.nn 1. Synchronizer1. Synchronizernn 2. Transmitter 2. Transmitter nn 3. Antenna 3. Antenna nn 4. Duplexer4. Duplexernn 5. Receiver5. Receivernn 6. Display unit6. Display unitnn 7. Power supply7. Power supply

Pulse Radar Block DiagramPulse Radar Block Diagram

Power

Supply

SynchronizerTransmitter

Display

Duplexer

(Switching Unit)

Receiver

Antenna

Antenna Bearing or Elevation

Video

Echo

ATRRF

TR

Pulsed radar architecture Pulsed radar architecture [1][1]

A labA lab--based pulsed radar based pulsed radar [4][4]

Pulsed modulation Pulsed modulation [1][1]

Pulsed Radar BandwidthPulsed Radar Bandwidth

In the frequency domain, the transmitted and In the frequency domain, the transmitted and received signals are composed of spectral received signals are composed of spectral components centered on the radar operating components centered on the radar operating frequency, f0, with a sin(x)/x shape. frequency, f0, with a sin(x)/x shape.

TheThe practicalpractical limitslimits ofof thethe frequencyfrequency responseresponse isisff00 ±± 11//ττ,, andand thereforetherefore thethe bandwidthbandwidth ofof thethereceiverreceiver mustmust bebe atat leastleast::

BWRx ≥ 2/BWRx ≥ 2/ττ

Pulsed radar average powerPulsed radar average power

nn Since a pulsed radar only transmits for a small portion Since a pulsed radar only transmits for a small portion of the time, the average power of the radar is quite low:of the time, the average power of the radar is quite low:PPavav = P= Ppeakpeak ττ / T/ Tpp

nn For example a pulsed radar with a 1 For example a pulsed radar with a 1 µµsec pulse width and a sec pulse width and a medium PRF of 4 kHz that transmits at a peak power of medium PRF of 4 kHz that transmits at a peak power of 10kW transmits an average power of:10kW transmits an average power of:PPavav = (10000 W) (0.000001 sec)= (10000 W) (0.000001 sec) (4000 /sec) (4000 /sec)

= _____= _____ W W = _____ dBW= _____ dBW

Pulsed radar range resolutionPulsed radar range resolution

nn The range resolution of a radar is its ability to The range resolution of a radar is its ability to distinguish two closely spaced targets along the same distinguish two closely spaced targets along the same line of sight (LOS). The range resolution is a function line of sight (LOS). The range resolution is a function of the pulse length, where pulse length, Lof the pulse length, where pulse length, Lpp = c= cττ..nn For example, a 1 For example, a 1 µµsec pulse width yields a pulse length of 0.3 sec pulse width yields a pulse length of 0.3

km.km.nn Two targets can be resolved in range if:Two targets can be resolved in range if:

LLpp < 2(R< 2(R22 –– RR11))

Pulsed radar range resolution Pulsed radar range resolution [4][4]

Pulsed radar range resolution Pulsed radar range resolution [4][4]

Pulsed radar range ambiguityPulsed radar range ambiguity

nn The PRF is another key radar parameter and is arguably The PRF is another key radar parameter and is arguably one of the most difficult design decisions.one of the most difficult design decisions.

nn The range of a target becomes ambiguous as a function The range of a target becomes ambiguous as a function of half the pulse period; in other words targets that are of half the pulse period; in other words targets that are further than half the pulse period yield ambiguous further than half the pulse period yield ambiguous range results.range results.

nn RRambamb = c / (2 PRF) = cT= c / (2 PRF) = cTpp / 2/ 2

4.44.4 Pulsed radar range ambiguity Pulsed radar range ambiguity [1][1]

Range ambiguity Range ambiguity

0 10 20 30

nn A target whose range isA target whose range is::nn R R << RRambamb = c / (2 PRF) = cT= c / (2 PRF) = cTpp / 2/ 2

PRF

Ramb

return time

Range ambiguity Range ambiguity

0 10 20 30

nn A target whose range isA target whose range is ::nn R R >> RRambamb = c / (2 PRF) = cT= c / (2 PRF) = cTpp / 2/ 2

PRF

Ramb

return time

Range ambiguity Range ambiguity

0 10 20 30

nn Which target is whichWhich target is which??

PRF

Ramb

?

Angle resolutionAngle resolution[4][4]

Pulse Vs. Continuous WavePulse Vs. Continuous Wave

Pulse EchoPulse Echonn Single AntennaSingle Antennann Gives Range, usually Gives Range, usually

Alt. as wellAlt. as wellnn Susceptible To Susceptible To

JammingJammingnn Physical Range Physical Range

Determined By PW Determined By PW and PRF.and PRF.

Continuous WaveContinuous Wavenn Requires 2 AntennaeRequires 2 Antennaenn Range or Alt. InfoRange or Alt. Infonn High SNRHigh SNRnn More Difficult to Jam But More Difficult to Jam But

Easily DeceivedEasily Deceivednn Amp can be tuned to look Amp can be tuned to look

for expected frequenciesfor expected frequencies

RADAR Wave ModulationRADAR Wave Modulation

G Amplitude Modulation–– Vary the amplitude of the carrier sine waveVary the amplitude of the carrier sine wave

G Frequency Modulation–– Vary the frequency of the carrier sine waveVary the frequency of the carrier sine wave

G Pulse-Amplitude Modulation–– Vary the amplitude of the pulsesVary the amplitude of the pulses

G Pulse-Frequency Modulation–– Vary the Frequency at which the pulses occurVary the Frequency at which the pulses occur

ModulationModulation

AntennaeAntennae

nn Two Basic Purposes:Two Basic Purposes:

nn Radiates RF EnergyRadiates RF Energy

nn Provides Beam Forming and FocusProvides Beam Forming and Focus

nn Must Be 1/2 of the Wave Length for the Must Be 1/2 of the Wave Length for the maximum wave length employedmaximum wave length employed

nn Wide Beam pattern for Search, Narrow for Wide Beam pattern for Search, Narrow for TrackTrack

Beamwidth Vs. AccuracyBeamwidth Vs. Accuracy

Beamwidth vs Accuracy

Ship A Ship B

Azimuth Angular Azimuth Angular MeasurementMeasurement

Azimuth Angular MeasurementRelative Bearing = Angle from ship’s heading.True Bearing = Ship’s Heading + Relative Bearing

NShip’s Heading Angle

Target Angle

Determining AltitudeDetermining Altitude

Determining Altitude

Slant Range

Altitude

Angle of Elevation

Altitude = slant range x sin0 elevation

Concentrating Radar Energy Concentrating Radar Energy Through Beam FormationThrough Beam Formation

nn Linear ArraysLinear Arraysnn Uses the Principle of wave summation (constructive Uses the Principle of wave summation (constructive

interference) in a special direction and wave cancellation interference) in a special direction and wave cancellation (destructive interference) in other directions.(destructive interference) in other directions.

nn Made up of two or more simple halfMade up of two or more simple half--wave antennas.wave antennas.

nn QuasiQuasi--opticalopticalnn Uses reflectors and “lenses” to shape the beam.Uses reflectors and “lenses” to shape the beam.

Broadside Endfire Array

Reflector ShapeReflector Shape

nn Paraboloid Paraboloid -- Conical Scan used for fire Conical Scan used for fire control control -- can be CW or Pulsecan be CW or Pulse

nn Orange Peel Paraboliod Orange Peel Paraboliod -- Usually CW and Usually CW and primarily for fire controlprimarily for fire control

nn Parabolic Cylinder Parabolic Cylinder -- Wide search beam Wide search beam --generally larger and used for longgenerally larger and used for long--range range search applications search applications -- PulsePulse

Wave Shaping Wave Shaping --QuasiQuasi--Optical SystemsOptical Systems

Reflectors Lenses

Parabolic (dish) antennaParabolic (dish) antenna

nn Early airborne radars typically Early airborne radars typically consisted of parabolic reflectors consisted of parabolic reflectors with horn feeds.with horn feeds.nn The dish effectively directs the The dish effectively directs the

transmitted energy towards a target transmitted energy towards a target while at the same time “gathering while at the same time “gathering and concentrating” some fraction of and concentrating” some fraction of the returned energy.the returned energy.

Planar (phased) array antennaPlanar (phased) array antenna

nn Recent radars more likely Recent radars more likely employ a planar arrayemploy a planar arraynn It is electronically steerable as a It is electronically steerable as a

transmit or receive antenna using transmit or receive antenna using phase shifters.phase shifters.

nn It has the further advantage of It has the further advantage of being capable of being integrated being capable of being integrated with the skin of the aircraft with the skin of the aircraft (“smart skin”).(“smart skin”).

Radar antenna beam patternsRadar antenna beam patterns

nn The main lobe of the radar antenna beam is central to The main lobe of the radar antenna beam is central to the performance of the system.the performance of the system.nn The side lobes are not only wasteful, they provide electronic The side lobes are not only wasteful, they provide electronic

warfare vulnerabilities.warfare vulnerabilities.

Airborne radar modesAirborne radar modesnn Airborne radars are designed for and used in many Airborne radars are designed for and used in many

different modes. Common modes include:different modes. Common modes include:nn airair--toto--air searchair searchnn airair--toto--air trackingair trackingnn airair--toto--air trackair track--whilewhile--scan (TWS)scan (TWS)nn ground mappingground mappingnn continuous wave (CW) illuminationcontinuous wave (CW) illuminationnn multimodemultimode

AAirir--toto--air search air search [1][1]

AirAir--toto--air tracking air tracking [1][1]

AirAir--toto--air trackair track--whilewhile--scan scan [1][1]

Ground mapping Ground mapping [1][1]

Continuous wave illuminationContinuous wave illumination

Multimode Multimode [1][1]

InIn--class exercisesclass exercises

Quick response exerciseQuick response exercise

nn Given a 10.5 GHz intercept radar and a transmitter Given a 10.5 GHz intercept radar and a transmitter capable of providing a peak power of 44 dBW at a capable of providing a peak power of 44 dBW at a PRF of 2 kHz:PRF of 2 kHz:nn What pulse width yields an average power of 50W?What pulse width yields an average power of 50W?nn What is the bandwidth in MHz and in % of this signal?What is the bandwidth in MHz and in % of this signal?

Pulsed radar calculationsPulsed radar calculations

nn Design the pulse parameters so as to achieve maximum average Design the pulse parameters so as to achieve maximum average power for an unspecified Ku band pulsed radar given the power for an unspecified Ku band pulsed radar given the following component specifications and system requirements:following component specifications and system requirements:nn the receiver has a bandwidth of at least 0.5% across the bandthe receiver has a bandwidth of at least 0.5% across the bandnn the required range resolution is 50m the required range resolution is 50m nn The required range ambiguity is 25 kmThe required range ambiguity is 25 kmnn For cooling purposes, ensure that the duty cycle of the transmitter does For cooling purposes, ensure that the duty cycle of the transmitter does

not exceed 0.2%not exceed 0.2%

ReferencesReferences1)1) Moir & Seabridge, Moir & Seabridge, Military Avionics SystemsMilitary Avionics Systems, American Institute of Aeronautics & , American Institute of Aeronautics &

Astronautics, 2006. [Sections 2.6 & 2.7]Astronautics, 2006. [Sections 2.6 & 2.7]2)2) David Adamy, David Adamy, EW101 EW101 -- A First Course in Electronic WarfareA First Course in Electronic Warfare, Artech House, 2000. , Artech House, 2000.

[Chapters 3,4 & 6][Chapters 3,4 & 6]3)3) George W. Stimson, George W. Stimson, Introduction to Airborne RadarIntroduction to Airborne Radar, Second Edition, SciTch , Second Edition, SciTch

Publishing, 1998.Publishing, 1998.4)4) Principles of Radar SystemsPrinciples of Radar Systems, student laboratory manual, 38542, student laboratory manual, 38542--00, Lab00, Lab--Volt (Quebec) Volt (Quebec)

Ltd, 2006.Ltd, 2006.5)5) Mark A. Hicks, "Clip art licensed from the Clip Art Gallery on Mark A. Hicks, "Clip art licensed from the Clip Art Gallery on

DiscoverySchool.com"DiscoverySchool.com"