11

Moving Target IndicatorMoving Target IndicatorMoving Target IndicatorMoving Target Indicator

22

ObjectivesObjectives

• Know how Pulsed Doppler radar works and how it’s able to determine target velocity.

• Know how the Moving Target Indicator (MTI) determines target velocity.

• Be able to explain how pulse compression can provide a high-resolution radar return at the receiver.

33

Velocity Determination for Velocity Determination for Pulse RadarsPulse Radars

Velocity Determination for Velocity Determination for Pulse RadarsPulse Radars

• Basic Pulse Radar cannot determine velocity.

• However, simple radar modes enable us to calculate velocity.

- Moving Target Indicator (MTI).– Utilizes Phase Shift.– Pulse Doppler– Utilizes Frequency Shift.

44

Moving Target Indicator (MTI)Moving Target Indicator (MTI)Moving Target Indicator (MTI)Moving Target Indicator (MTI)

• Measures changes in Phase of the return signal.– Identifies targets in motion – ONLY!

• Phase Comparator.– Samples Transmit and Return signal.– Compares Phase.

• In phase = largest positive value.• Out of Phase = largest negative value.

Pulse 3

Pulse 4

Pulse 5

Next Pulse

Avg PhaseDifference

Subtracted

DisplayedSignal

Only moving Displayed

Pulse 1Pulse 1 phasetime

Pulse 2

Moving target: range changes by ¼ wavelength

λ/4

66

DisplayDisplay

• The display can be ‘A Scope’• Several sweeps are required to register

moving target on the screen• This produces ‘Butterfly’ effect on the

screen due to simultaneous display of several sweeps

77

DisplayDisplay

• Another option – PPIThen a delay line canceller is a must

• It delays the echo by a pulse time• Then the present echo will be compared

with previous echo to indicate moving targets

• The bipolar resultant will be converted to unipolar using a full wave rectifier.

88

Moving Target Indicator (MTI)Moving Target Indicator (MTI)Moving Target Indicator (MTI)Moving Target Indicator (MTI)• Delay line circuit saves previous phase

evaluation.• Cancellation circuit subtracts previous phase

from current phase.• Return from Stationary targets will have same

phase comparison and be cancelled out.• Return from Moving targets will have different

phase comparison and will be retained / displayed.

99

MTIMTI

Range

Output = return × phase

Transmitter

Receiver Phase comparator

Return strength

Phase difference

1010

The more common MTI radar The more common MTI radar employing a power amplifier is employing a power amplifier is

shown nextshown next

1111

Coherent MTI RADAR

Pulse mod

Duplexer Power amp

Mixer Stalo f1Mixer

IF amp

Phase detector

Coho fc

fc

fl + fcfl + fc ± fd

fc ± fd

Ref signal

fd

fl

1212

System DetailsSystem Details

• Power amplifier – Triode, Tetrode, Klystron, TWT or CF Amplifier

• The combination of power oscillator and power amplifier makes it to be called as

‘MASTER OSCILLATOR POWER AMPLIFIER ( MOPA)’

1313

Why master oscillator?Why master oscillator?

• Because a reference signal can not be generated by a continuously running oscillator

• Coherent reference signal can be obtained by readjusting the phase of the COHO at the beginning of each sweep

1414

Another type with Power Another type with Power Oscillator Transmitter is as Oscillator Transmitter is as

shown nextshown next

1515

Power Oscillator TransmitterPulse mod

Duplexer Power amp

Mixer Stalo f1Mixer

IF amp

Phase detector Coho fc

fc

fl + fcfl + fc ± fd

fc ± fd

Ref signal

fd

fl

1616

Delay Line CancellerDelay Line Canceller

• It is a time domain filter

• It operates at all ranges and does not require separate filter for each range resolution cell

1717

Delay Line Canceller

Rx Delay line canceller

Substractor circuit FWR

PPI

1TPRF

=

1818

Filter CharacteristicsFilter Characteristics

• This rejects DC components of the clutterThe video o/p V1 = K Sin ( 2πfdt – Φ0)PreviousV2= K Sin ( 2πfd(t-T) – Φ0)V = V1 - V2 =2K SinπfdTCos(2πfd(t-T/2) – Φ0)

1919

Filter ResponseFilter Response• The amplitude of the waveform is

2K SinπfdTvoltage

Time 1/T14/T12/T1 3/T1

+

0

_

2020

Blind SpeedBlind Speed

• The response is zero whenever• πfdT = nπOr 2VrT/λ = n

Vr = n λfp /2When the response is zero means, the target is stationary or moving with this speed. Hence, the radar can not detect such a moving target. So, this speed is “BLIND SPEED”.

2121

Limitations of MTILimitations of MTI

• Blind speeds are the limitations of the MTI.

• The target will not be visible to the radar though it is advancing towards it. This is very dangerous in most of the cases

2222

How to overcome?How to overcome?

• If the blind speed is to be greater than maximum radial velocity expected, then λfpis to be large.

• Long wavelength and high PRF are preferable.

• Then with more than one PRF, operate more than one λ.

2323

Possible SolutionsPossible Solutions

• Long range MTI radars operate in L, S or higher bands

• Operate with blind speeds and ambiguous doppler speed for the sake of accurate range estimation.

• The practical answer is: Keep the first blind speed out of the expected range of doppler frequency.

2424

Practical SolutionPractical Solution

• The standard technique is to operate at multiple frequencies

• This is known as ‘Staggered PRF – MTI’.

Double CancellationDouble Cancellation

• It will have better clutter rejection null

2525

Delay line canceller

T=1/fp

-

+

-

+T=1/fp

o/p

Delay line canceller

Σ Σ

2626

ResponseResponse• The response= 4 Sin2 (fdT)

0

0

Amplitude

f 1/T1 4/T12/T1 3/T1

f 1/T2 4/T22/T2 3/T2

Amplitude response Rectified

waveform

2727

DiscussionDiscussion

• Zero response occurs only when the blind speeds of both radars coincide.

• The disadvantage is that region of low sensitivity might appear.

• The closer the ratio T1 : T2 is unity, the lower the value of the first blind speed.

• But first null in the vicinity of fd = 1/T1becomes deeper. This is the trade off.

2828

Pulse DopplerPulse DopplerPulse DopplerPulse Doppler• Attributes of pulse radar / technology of CW

radar.• “Mixer” added to Pulse Radar.• Sample of transmitted and received signal

are compared at mixer.– Mixer output is Doppler shift (velocity).– Doppler sorted into velocity categories.– Categories identified by color in display.

• Standard Weather Radar.– More rain / higher wind – higher Doppler.

2929

Hurricane Hugo Hurricane Hugo -- Photo Photo ImageImage

Hurricane Hugo Hurricane Hugo -- Photo Photo ImageImage

3030

Hurricane Hugo Hurricane Hugo –– Pulse Pulse DopplerDoppler

Hurricane Hugo Hurricane Hugo –– Pulse Pulse DopplerDoppler

3131

Pulse Doppler RadarPulse Doppler RadarPulse Doppler RadarPulse Doppler Radar

• Also used for weapon Fire Control systems

• Pulse Radar – ONE Antenna ONLY!• High PRF

– Many pulses / high frequency / large BW – Large volume of range and range rate info– High degree of accuracy

• Duty Cycle > 10%

3232

Pulse Doppler SystemPulse Doppler SystemTransmitter

Receiver

Doppler filter Display

Antenna

ft

fr

∆f

3333

General DefinitionGeneral Definition

• A radar that increases its PRF high enough to avoid problems of blind speeds is called a ‘pulse doppler radar’.

• But it may be acceptable to operate at a slightly lower PRF and accept both range and doppler ambiguities. Then it is ‘medium PRF – PDR’.

3434

Ambiguities possibleAmbiguities possible

• The MTI with no range ambiguities and many doppler ambiguities

• The high PRF – Pulse Doppler Radar with just the opposite : many range ambiguities and no doppler ambiguities.

• The medium PRF – Pulse Doppler Radar with some of each

3535

ComboCombo

• The equipment different between the two are no longer significant enough to distinguish one from the other.

• Pulse doppler radar generally receives much more clutter than an MTI radar.

• Pulse doppler radar requires much greater improvement factor than does an MTI radar of comparable performance.

3636

Logical conclusionsLogical conclusions

• The antenna sidelobe clutter is large in ahigh PRF pulse doppler radar since thereare many range ambiguous pulsessimultaneously illuminating the clutter.

• With a duty cycle of 50%, the antennasidelobes are simultaneously illuminatinghalf the total clutter within the radar’scoverage.

3737

DisadvantageDisadvantage

• The large sidelobe clutter viewed by apulse doppler radar is a reason why itrequires a high improvement factor thanan AMTI radar of equivalent performance.

3838

SpecificsSpecifics

• To detect aircraft targets within thesidelobe clutter region, a bank ofnarrowband doppler filters with adaptivethresholds can be used. So antenna musthave exceptionally low sidelobes.

3939

Specific AdvantageSpecific Advantage

• The presence of a clutter free region is animportant advantage of a high PRF pulsedoppler radar especially if detecting highspeed closing targets at long range isrequired.

4040

Eclipsing LossEclipsing Loss

• Since the pulse doppler radar can notreceive when it is transmitting the highduty cycle can result in a loss if the echosignal arrives when a pulse is beingradiated and the receiver is turned off.This is called eclipsing loss.

4141

• The degree of eclipsing varies as thetarget range changes with time soeclipsing can cause periodic holes in thecoverage.

• A rapidly approaching target will notremain eclipsed for long so that detectionswill occur at a slightly shorter range wheneclipsing is present.

4242

• A reduction of the duty cycle and anincrease in the no. of range gates willreduce the effect of eclipsing.

4343

Medium PRF Medium PRF -- PDRPDR

• It has both range and doppler ambiguities.• It results in less clutter being seen by the

antenna sidelobes than the high PRFradar since there are fewer pulse viewingambiguous range cells.

• There is no clutter free region.

4444

High PRF High PRF -- PDRPDR

• No ambiguities in doppler frequency, noblind speeds but many range ambiguities

• Range ambiguities can be resolved bytransmitting three redundant waveformseach at a different PRF

• Transmitter leakage and altitude return areremoved by filtering

4545

• Main beam clutter is removed by a tunablefilter.

• High closing speed aircrafts are detectedat long range in the clutter free region.

• There is poor detection of low radial speedtargets that are masked in the frequencydomain by short range sidelobe clutterfolded over in range.

4646

• Often only a single range gate is employedbut with a large doppler filter bank.

• For comparable performance, a muchlarger improvement factor is required thanlower PRF systems since the high PRFresults in more clutter being viewed by theantenna sidelobes.

4747

• The antenna sidelobes must be quite lowin order to minimize the sidelobe clutter.

• Range accuracy and the ability to resolvemultiple targets in range are poorer thanother radars.

4848

Medium PRF Medium PRF -- PDRPDR

• Has both range and doppler ambiguities• There is no clutter free region as in the

high PRF system, so detection of highspeed targets will not be as good as withthe high PRF system.

4949

• Fewer range ambiguities mean that therewill be less clutter seen in the antennasidelobes so that targets with low relativespeeds will be detected at long range thanwith a high PRF system.

5050

• The trade off of detectability of high speedtargets for better detection of lower speedtargets often makes the medium PRFsystem preferred over the high PRFsystem for airborne fighter/interceptorradar applications, if only a single systemis used.

5151

• The altitude return can be eliminated by rangegating

• More range gates are required than in the high –PRF system, but the number of doppler filters ateach range gate is less.

• Seven or eight different PRFs must be used toinsure that a target will have the proper dopplerfrequency to be detected on atleast three PRFsin order to resolve range ambiguities.

5252

• For comparable range performance thelarge number of redundant waveformsmeans that the transmitter must be larger.

• Better range accuracy an range resolutionare available than with high PRF systems.

• The antenna must have also low sidelobesto reduce the sidelobe clutter.

5353

LOW PRF LOW PRF –– Airborne MTIAirborne MTI

• No range ambiguities but many dopplerambiguities ( blind speeds)

• Requires TACCAR and DPCA to removeeffects of platform motion

• TACCAR – Time Averaged Compensationfor Clutter Doppler Shift

• DPCA – Displaced Phase Center Antenna

5454

• It operates clutter free at long range where noclutter is seen due to curvature of earth.

• Sidelobe clutter is usually not as important as itis in pulse doppler systems.

• Best employed at UHF or perhaps L band.• Increase blind speeds and the lower

effectiveness of platform motion compensationprevent its use at high microwave frequencies.

5555

• The lower RF of AMTI radar results inwider antenna beamwidth than a higherfrequency ( S band) pulse doppler radarwhose mission is wide area airsurveillance.

• Because there are no range ambiguities tobe resolved, redundant waveforms withmultiple PRFs are not needed.

5656

• For comparable performance the requiredproduct of average power and antenna apertureis less than that for pulse doppler radars.

• Usually simper than pulse doppler radar.• Cost is generally much less than pulse doppler

radar of comparable performance.• AMTI can not be used in fighter/interceptor X

band radars for look down detection of targets inclutter.

5757

ComparisonComparison• PDR• Fr is estimated

accurately• High PRF - not to

have blind speeds• But time around

echoes do exist• Power amplifier with

high duty cycle• Range gates are used

• MTI• Range is estimated

accurately• Low PRF – no time

around echoes• Blind speeds do exist• Delay line cancellers

are used.

5858

Salient FeaturesSalient Features

• When the PRF is so high that the numberof range ambiguities is too large to beeasily resolved the performance of thePDR approaches that of CW radar.

• But PDR advantage is that detectionperformance is not limited by transmitterleakage.

5959

Limitation to MTILimitation to MTI

• MTI improvement Factor(S to C Ratio) o/p--------------------------------_

(S to C Ratio) i/p

• This is averaged over radial velocities of all targets

6060

Contd…Contd…

• Subclutter Visibility: To detect the target within the clutter of same power with specified detection and false alarm probability

6161

Contd…Contd…

• Clutter Visibility Factor : (S/C) after doppler filtering

• Clutter Attenuation: At the clutter canceller, Ratio of i/p to o/p,

6262

Contd..Contd..

Canceler voltage amplification for fixed target

• Cancellation Ratio-----------------------Gain for a single pulse passing through unprocessed

channel of canceler

6363

Doppler Filter BankDoppler Filter Bank

• It is a set of contiguous filters for detecting targets as shown .

• The advantages are quoted next

6464

AdvantagesAdvantages

• 1. Multiple Moving Targets can beseparated from one another in a filter bank

• When the clutter and target echo signalappear in different doppler shifts, theclutter echo need not interfere with thedetection of the desired moving target.

6565

Contd..Contd..

• A measure of target’s radial velocity canbe obtained. The ambiguity in themeasurement can be resolved by achange in PRF.

6666

Contd…Contd…

• The narrowband doppler filters excludemore noise than do the MTI delay linecancelers described previously andprovide coherent integration.

6767

LimitationLimitation

• The price paid is greater complexity,difficulty in achieving filters with lowenough side lobes to reduce clutter andthe need for a significant number of pulsesto produce desirable filter characteristics.

6868

TheThe basicbasic methodmethod forfor achievingachieving aa dopplerdopplerfilterfilter bankbank isis toto employemploy thethe transversaltransversalfilterfilter.. TheThe timetime delaydelay betweenbetween eacheach taptap ofofthethe transversaltransversal fitlerfitler isis TT == 11/f/fpp..

6969

• The amplitude weights at each tap are allthe same, the first nulls of each filter are atthe peaks of the adjacent filters.

• The remainder of the frequency domain iscovered by similar filters but withambiguity and aliasing.

7070

• Lower sidelobes are needed if largevalues of the improvement factor are to beobtained with a doppler filter bank.

• However, the main response is widened,the peak gain is reduced but the straddlingloss at filter crossover is less.

7171

• The filter bank requires more pulses forgood performance and it requires a largersignal to noise ratio if the true radialvelocity is to be extracted when two ormore prfs are employed.

7272

Limitation to MTI PerformanceLimitation to MTI Performance

• The reasons are :• 1. Antenna Scanning Modulation• 2. Internal fluctuation of clutter• 3. Equipment instabilities• 4. Limiting.

7373

EffectsEffects

• The limitations cause the clutter spectrumto widen. More clutter energy is passedthrough the doppler filters which lowersthe improvement factor.

7474

JSTARJSTAR

JSTAR –

Desert Storm MTI

7575

Question 1Question 1How butterfly effect is produced on the screen in the case of MTI radar?

When MTI radar signal is processed, the indicationrepresents only the moving target. This is given to aPlan Position Indicator. The screen will have somepersistence. This display remains for some time.During this time, if the echo for the next pulse isprocessed and is displayed, it will show a slightchange from the previous display. Like this, afterseveral pulses, the screen consists of several slightlychanged displays which appears like a butter fly.

7676

Question 2Question 2In the case of PPI, what is the need of converting bipolar signal to unipolar signal?

A PPI displays range versus azimuth. Therange signal input must be only positive. Butthe receiver output is going to be bidirectional.Because the phase of the present time echomay be sometimes more and less other timeswhen compared to previous time echo. So,using a FWR, this bipolar signal is converted toa unipolar signal.

7777

Question 3Question 3What happens if the duty cycle goes below 10% for pulse Doppler radar?

If the duty cycle is below 10%, theamount of power contained within thatwill be very small. So, far off target willgive rise to very little echo at the receiverwhich may not be detected.

7878

Question 4Question 4How multiple moving targets are separated from one another by using Doppler filter bank?

A doppler filter bank is a frequency selectivefilter bank . Each filter selects only a narrowrange of frequencies which correspond tonarrow range of velocities. As the targets arelikely to be moving at different velocities, theechoes from different targets enter intodifferent receivers due to these filter banks.

7979

Question 5Question 5

Why first blind speed should be kept out of the expected range of Doppler freq?

The blind speed can not be avoided in theMTI system. To avoid this blind speed,the delay line canceller is designed so thatit is definitely higher than the maximumvelocity of the expected targets in aspecific location.