Radar Systems- Unit-III : MTI and Pulse Doppler Radars

JNTUK - IV B.Tech I sem - Radar Systems Unit-3 MTI and Pulse Doppler Radars

1. RADAR SYSTEMS
B.TECH (IV YEAR –I SEM)
Prepared by:
Mr. P.Venkata Ratnam.,M.Tech.,(Ph.D)
Associate Professor
Department of Electronics and Communication Engineering
RAJAMAHENDRI INSTITUTE OF ENGINEERING & TECHNOLOGY
(Affiliated to JNTUK, Kakinada, Approved by AICTE -Accredited by NAAC )
Bhoopalapatnam, Rajamahendravaram, E.G.Dt, Andhra Pradesh

2. Unit -III
MTI andPulseDoppler Radar
➢Introduction
➢Principle of Operation
➢MTI Radar with Power Amplifier Transmitter
➢MTI Radar with Power Oscillator Transmitter
➢Delay Line Cancellers-Filter Characteristics
➢Blind Speeds

3. ➢Double Cancellations
➢Staggered PRFs
➢Range Gated Doppler Filters
➢MTI Radar Parameters
➢Limitations to MTI Performance
➢MTI vs. Pulse Doppler Radar

4. Introduction :
➢TheDopplerfrequencyshift[fd=2Vr/λ]produced
byamovingtargetmaybeusedinapulseradar.
➢Todeterminetherelativevelocityofatargetto
separatedesiredmovingtargetsfromundesired
stationaryobjects(clutter).
➢Thetarget'srelativevelocityismadefromthe
Dopplerfrequencyshift,
➢TheuseofDopplertoseparatesmallmoving
targetsinthepresenceoflargeclutterhasbeenof
greaterinterest.

5. ➢ThepulseradarthatutilizestheDoppler
frequencyshiftasameansofdiscriminating
movingtargetsfromfixedtargetsiscalleda
MTI(movingtargetindication)orapulseDoppler
radar.
➢ThephysicalprincipleofMTIandPulseDoppler
radarissame,butinpracticetherearedifferences
betweenthem.
➢TheMTIradarusuallyoperateswithambiguous
Dopplermeasurement(so-calledblindspeeds)
butwithunambiguousrangemeasurement.
➢ApulseDopplerradaroperateswithambiguous
rangemeasurementbutwithunambiguous
Dopplermeasurement.

6. Salient Features of MTI:
➢MTIisanecessityinhigh-qualityair-surveillance
radarsthatoperateinthepresenceofclutter.
➢Its design is more challenging than that of a simple
pulse radar or a simple CW radar.
➢A MTI capability adds to a radar's cost and complexity
and often system designers must accept compromises
they might not wish to.
➢The chief factor that made this possible was the
development of reliable, small, and inexpensive digital
processing hardware.

7. Principle of Operation:
➢TheMTIradarsensesthetargetmovementby
comparingthephaseshiftofthereceivedsignalwith
respecttotransmittingsignal.
➢Whenitisdesiredtoremovetheclutterdueto
stationarytargetsanMTIradarisemployed.
➢ThebasicprincipleofMTIradaristocompareasetof
receivedechoeswiththosereceivedduringthe
previoussweep.

8. ➢Movingtargetswillgivechangeofphaseandare
notcancelled.
➢Thusclutterduetostationarytargetsboth
manmadeandnaturalisremovedfromthedisplay
andthisallowseasierdetectionofmovingtargets.

9. Types of MTI Radars :
➢WecanclassifytheMTIRadarsintothe
followingtwotypesbasedonthetypeof
transmitterthathasbeenused.
1. MTI Radar with Power Amplifier Transmitter
2. MTI Radar with Power Oscillator Transmitter
➢Now, let us discuss about these two MTI Radars
one by one.

10. MTI Radar with Power Amplifier Transmitter :
➢MTIRadarusessingleAntennaforboth
transmissionandreceptionofsignalswiththehelp
ofDuplexer.
➢TheblockdiagramofMTIRadarwithpower
amplifiertransmitterisshowninthefollowing
figure.
➢ThefunctionofeachblockofMTIRadarwith
poweramplifiertransmitterismentionedbelow.

12. ➢PulseModulator−Itproducesapulse
modulatedsignalanditisappliedtoPower
Amplifier.
➢PowerAmplifier−Itamplifiesthepowerlevels
ofthepulsemodulatedsignal.
LocalOscillator−Itproducesasignalhaving
stablefrequencyfl.Hence,itisalsocalledstable
LocalOscillator.
TheoutputofLocalOscillatorisappliedtoboth
Mixer-IandMixer-II.

13. ➢CoherentOscillator−Itproducesasignalhaving
anIntermediateFrequency,fc.Thissignalisused
asthereferencesignal.
➢TheoutputofCoherentOscillatorisappliedto
bothMixer-IandPhaseDetector.
➢Mixer-I−Mixercanproduceeithersumor
differenceofthefrequenciesthatareappliedtoit.
➢Thesignalshavingfrequenciesofflandfcare
appliedtoMixer-I.

14. ➢Here,theMixer-Iisusedforproducingtheoutput,
whichishavingthefrequencyfl+fc.
➢Duplexer−Itisamicrowaveswitch,which
connectstheAntennatoeitherthetransmitter
sectionorthereceiversectionbasedonthe
requirement.
➢Antenna transmitsthesignalhaving
frequencyfl+fcwhentheduplexerconnects
theAntennatopoweramplifier.
➢Similarly,Antennareceivesthesignalhaving
frequencyoffl+fc±fdwhentheduplexer
connectstheAntennatoMixer-II.

15. Mixer-II−Mixercanproduceeithersumor
differenceofthefrequenciesthatareappliedtoit.
Here, The signals having frequenciesfl + fc±fdand
fl are applied to Mixer-II.
Here, the Mixer-II is used for producing the output,
which is having the frequencyfc±fd
IF Amplifier− IF amplifier amplifies the Intermediate
Frequency (IF) signal.

16. ➢The IF amplifier shown in the figure amplifies the
signal having frequencyfc±fd.
➢This amplified signal is applied as an input to Phase
detector.
➢PhaseDetector−Itisusedtoproducetheoutput
signalhavingfrequencyfdfromtheappliedtwoinput
signals.
➢Whicharehavingthefrequenciesoffc±fdandfc.
➢The output of phase detector can be connected to
Delay line canceller.

17. MTI Radar with Power Oscillator Transmitter :
➢A block diagram of MTI radar using a power
oscillator is shown in Fig.

18. ➢Aportionofthetransmittedsignalmixedwiththe
StalooutputtoproduceanIFbeatsignalwhose
phaseisdirectlyrelatedtothephaseofthe
transmitter.
➢ThisIFpulseisappliedtothecoherent(Coho)and
causethephaseoftheCohoCWoscillationto
“lock”instepwiththephaseoftheIFreference
pulse.
➢ThephaseoftheCohoisthenrelatedtothephase
ofthetransmittedpulseandmaybeusedasthe
referencesignalforechoesreceivedfromthe
particulartransmittedpulse.

19. “BUTTERFLY” Effect in MTI Radar :
➢Movingtargetsmaybedistinguishedfrom
stationarytargetsbyobservingthevideooutputon
anA-scope(amplitudevs.range).

20. ➢AsinglesweeponanA-scopemightappearasin
Fig.(a).
➢Thissweepshowsseveralfixedtargetsandtwo
movingtargetsindicatedbythetwoarrows.
➢Onthebasisofasinglesweep,movingtargets
cannotbedistinguishedfromfixedtargets.
➢Successive A scope sweeps (pulse-repetition
intervals) are shown in Fig. (b) to (e).

21. ➢Echoesfromfixedtargetsremainconstant
throughoutbutechoesfrommovingtargetsvary
inamplitudefromsweeptosweepatarate
correspondingtotheDopplerfrequency.
➢ThesuperpositionofthesuccessiveA-scope
sweepsisshowninFig.(f).
➢Themovingtargetsproduce,withtime,abutterfly
effectontheA-scope.

22. Delay Line Cancellers-Filter Characteristics :
➢Delaylinecancellerisafilter,whicheliminates
theDCcomponentsofechosignalsreceivedfrom
stationarytargets.
➢Thismeans,itallowstheACcomponentsofecho
signalsreceivedfromnon-stationarytargets,i.e.,
movingtargets.
➢Delaylinecancellerscanbeclassifiedintothe
followingtwotypesbasedonthenumberofdelay
linesthatarepresentinit.
1. Single Delay Line Canceller
2. Double Delay Line Canceller

23. Single Delay Line Canceller :
➢ThesimpleMTIdelay-linecancellershowninFig.
isanexampleofatime-domainfilter.
➢Thecombinationofadelaylineandasubtractoris
knownasDelaylinecanceller.
➢TheblockdiagramofMTIreceiverwithsingle
Delaylinecancellerisshowninthefigurebelow.

24. ➢Themathematical equationof the received echo
signal after the Doppler effect as −
➢TheoutputofDelaylinecanceller,by
replacingtbyt−TPinEquation1

25. ➢Then thesubtractor outputby subtracting
Equation 2 from Equation 1.

26. ➢TheoutputofsubtractorisappliedasinputtoFull
WaveRectifier.
➢Therefore,theoutputofFullWaveRectifierlooks
likeasshowninthefollowingfigure.
➢Itisnothingbutthefrequencyresponseofthe
singledelaylinecanceller.

27. ➢FromEquation3,wecanobservethatthe
frequencyresponseofthesingledelayline
cancellerbecomeszero,whenπfdTPisequal
tointegermultiplesofπ.
➢This means,πfdTPis equal to nπ,it can be written
as

28. Blind Speeds :
➢SingleDelaylinecancellereliminatestheDC
componentsofechosignalsreceivedfrom
stationarytargets,whennπisequaltozero.
➢Inadditiontothat,italsoeliminatestheAC
componentsofechosignalsreceivedfromnon-
stationarytargets,
➢When the Doppler frequency fdis equal to
integer(other than zero) multiples of pulse
repetition frequency fP

29. ➢So,therelativevelocitiesforwhichthefrequency
responseofthesingledelaylinecancellerbecomes
zeroarecalledblindspeeds.
➢Mathematically,wecanwritetheexpressionfor
blindspeedvnas

30. Double Delay Line Canceller :
➢Asingledelaylinecancellerconsistsofadelayline
andasubtractor.
➢Iftwosuchdelaylinecancellersarecascaded
together,thenthatcombinationiscalledDouble
delaylinecanceller.
➢TheblockdiagramofDoubledelaylinecancelleris
showninthefollowingfigure.

31. Wewillgetthefollowingmathematicalrelation
fromfirstdelaylinecanceller.
Theoutputofthefirstdelaylinecancelleris
appliedasaninputtotheseconddelayline
canceller.
Hence,q(t)willbetheinputoftheseconddelay
linecanceller.


32. ➢Letr(t)be the output of the second delay line
canceller.
➢Now, Replacetby t−TP , we get,
➢Substitute in r(t) equation, we get,

33. ➢Theadvantageofdoubledelaylinecancelleristhat
itrejectstheclutterbroadly.
➢Theoutputoftwodelaylinecancellers,whichare
cascaded,willbeequaltothesquareoftheoutput
ofsingledelaylinecanceller.
➢So,themagnitudeofoutputofdoubledelayline
canceller,whichispresentatMTIRadarreceiver
willbeequalto4A
2
(sin[πfdTP])
2

34. Multiple or Staggered PRFs :
➢TheuseofmorethanonePRFoffersadditional
flexibilityinthedesignofMTIDopplerfilters.
➢Itnotonlyreducestheeffectoftheblindspeeds,
butitalsoallowsasharperlow-frequencycutoffin
thefrequencyresponse.
➢Theblindspeedsoftwoindependentradars
operatingatthesamefrequencywillbedifferentif
theirPRF’saredifferent.

35. ➢Insteadofusingtwoseparateradars,thesame
resultcanbeobtainedwithoneradarwhichtime-
sharesitsPRFbetweentwoormoredifferent
values.
➢ThePRFmightbeswitchedeveryotherscanor
everytimetheantennaisscannedahalf
beamwidth,ortheperiodmightbealternatedon
everyotherpulse.
➢Whentheswitchingispulsetopulse,itisknown
asastaggeredPRF

36. ➢SupposethefirstPRFisF1showninboldlineand
thespeedofsecondPRFisF2shownindotted
lines.
➢Ifweobservedthefigure,itisclearthatat
particularpositionwhen2f1=3f2,boththePRFs
havethesameblindspeed.

37. Range gated Doppler Filters :
➢Inordertoseparatemovingtargetsfromstationary
clutter,thedelaylinecancellerhasbeenwidely
usedinMTIradar.
➢Quantizingthetimeintosmallintervalcan
eliminatethelossofrangeinformationand
collapsingloss.
➢Thisprocessisknownastherangegatingwhere
widthdependsonrangeaccuracydesired.
➢Afterquantizingtheradarreturninterval,the
outputfromeachgateisappliedtonarrowband
filter.

38. ➢MTIradarusingmultiplerangegatesfollowedby
clutterrejectionfilterisshowninFig.
➢Theoutputofthephasedetectorissampled
sequentiallybytherangegates.

39. ➢Eachrangegateopensinsequencejustlongenough
tosamplethevoltageofthevideowaveform.
➢Therangegateactsasaswitchoragatewhichopens
andclosesatthepropertime.
➢Therangegatesareactivatedonceeachpulse-
repetitioninterval.
➢Anechofromamovingtargetproducesaseriesof
pulseswhichvaryinamplitudeaccordingtothe
Dopplerfrequency.

40. ➢Theoutputoftherangegatesisstretchedina
circuitcalledtheboxcargenerator,orsample-
and-holdcircuit.
➢Thepurposeistofilteringanddetectionprocess
andeliminatingharmonicsofthePRF.
➢Theclutterrejectionfilterisabandpassfilter
whosebandwidthdependsupontheextentofthe
expectedclutterspectrum.
➢ThefilteredoutputfromtheDopplerfilteris
furtherfedtoafullwavelineardetectorwhich
convertthebipolarvideo.

41. ➢Theoutputoftheintegratorisappliedtoa
threshold-detectioncircuit.
➢Onlythosesignalswhichcrossthethresholdare
reportedastargets.
➢Followingthethresholddetector,theoutputsfrom
eachoftherangechannelsmustbeproperly
combinedfordisplayonthePPIorA-scope.
➢Thefrequency-responsecharacteristicoftherange
gatedMTIappearsasinFig.

42. MTI Performance:
➢MTIimprovementfactor:Thesignal-to-clutterratioat
theoutputoftheMTIsystemdividedbythesignal-to-
clutterratioattheinput.
➢Subcluttervisibility:ItisdefinedastheabilityofMTI
radartodetectthemovingtarget,ifthetargetis
superimposedovertheclutters.
➢Cluttervisibilityfactor:Thesignal-to-clutterratio,after
cancellationorDopplerfilteringthatprovidesstated
probabilitiesofdetectionandfalsealarm.
➢Theimprovementfactor(I):Isequaltothesubclutter
visibility(SCV)timesthecluttervisibilityfactor(VOC).

43. Limitations to MTI Performance :
➢Equipment instabilities
➢Internal fluctuation of clutter
➢Antenna scanning modulation

44. Pulse Doppler Radar Vs MTI :
➢APulseradarthatextractstheDopplerfrequency
shiftforthepurposeofdetectingmovingtargetsin
thepresenceofclutteriseitheraMTIRadarora
PulseDopplerRadar.
➢MTIusuallyreferstoaRadarinwhichthePRFis
chosenlowenoughtoavoidambiguitiesinrange.
➢ThepulseDopplerradar,ontheotherhand,hasa
highPRFthatavoidsblindspeeds.

47. Thank you