Chapter 14 MTI And Pulsed Doppler Radar

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Radar System DesignChapter 14MTI and Pulsed Doppler RadarRadar System DesignMTI and Pulsed Doppler Radar Moving Target Indication (MTI) radar: A delay linecanceller filter to isolate moving targets fromnonmoving background- Ambiguous velocity- Unambiguous range Pulsed Doppler radar: Doppler data are extractedby the use of range gates and Doppler filters.- Unambiguous velocity- Unambiguous or ambiguous rangeChapter 14: MTI and Pulsed Doppler Radar14 - 1Dr. Sheng-Chou Lin

Radar System DesignPulsed Radar High-PRF: unambiguous Doppler frequency, highlyambiguous range- solve TX-RX coupling problem of CW system- Improve noise-limited detection relativeto low-PRF waveform- Minimize the number of introduced blindzones relative to low-PRF system.- Range blind during TX time periods Low-PRF: unambiguous range, highly Dopplerfrequency- circumvents the TX-RX coupling- Introduce Doppler blind zones (ground clutter) Medium-PRF: ambiguous Doppler frequency,ambiguous range- circumvents the TX-RX couplingChapter 14: MTI and Pulsed Doppler RadarDr. Sheng-Chou Lin14 - 2Radar System DesignPulsed Radar Parameters Range: range is obtained from transmit-to-receivepulse delay T2R cT R ct 2- 1 s 150m , 1ns 15cmTarget 1 Target 2ReturnReturn Transmitpulse Range Resolution: Pulse width must be shorterthan the propagation time from target 1 to target 2and backR 1 ct 1 2R 2 ct 2 22 R 2 –R 1 ct t 2 R 2 –R 1 c Combined returnedfrom target 1 and 2 R c 2 Unambiguous range R R unamb cT 2, T: pulse repetition interval (PRI)Transmitpulse- There are ways to get around this by using astaggered PRI (Multi-PRF)TR ct------2ct'R -------2Unambiguous Range (R cT/2)Chapter 14: MTI and Pulsed Doppler Radar14 - 3Ambiguous Range (R cT/2)Dr. Sheng-Chou Lin

Radar System DesignPulsed Doppler Power Spectrum2Vf d ---------c- cos : angle between the platform velocityandtheline of sight (LOS)Chapter 14: MTI and Pulsed Doppler Radar14 - 4Dr. Sheng-Chou LinRadar System DesignPulsed Radar Noncoherent Pulsed Radar- No reference signal Coherent Pulsed Radar- TX phase is reserved MTI Radar- detection of moving target bysuppressing fixed targetsChapter 14: MTI and Pulsed Doppler Radar14 - 5Dr. Sheng-Chou Lin

Radar System DesignPulsed Doppler RadarAnalog12 3 4Range gateswitch samplingRange InformationSamplingDigitalFFT(filter bank)Doppler InformationChapter 14: MTI and Pulsed Doppler Radar14 - 6Dr. Sheng-Chou LinRadar System DesignPower Spectrum Density (Pulsed) As the antenna scans, the beam dwell time is finite. T i : interpulse period; p : pulse periodN 1 5Chapter 14: MTI and Pulsed Doppler Radar14 - 7Dr. Sheng-Chou Lin

Radar System DesignPower Spectrum Density (Pulsed) 2 A five-burst waveform: the return from a scatter at aslant range R T .N 5R T R AMB contains four pulse samples- R T R AMB contains only one pulse sample- The shape of both the spectrum and ambiguity functionfor is important determine performance of MTIand pulsed Doppler radars.N 1 5N 1 4N 4N 1 4N 1 1Chapter 14: MTI and Pulsed Doppler Radar14 - 8Dr. Sheng-Chou LinRadar System DesignPower Spectrum Density (Pulsed RF)Chapter 14: MTI and Pulsed Doppler Radar14 - 9Dr. Sheng-Chou Lin

Radar System DesignRadar Equation for Pulsed Radar Until now, we have not said a great deal aboutfiltering of the return signal except to say thatmatched filtering is desirable and B IF 1 formost pulse radars. In some cases, we need a better idea aboutbandwidth for estimation S/N ratio. B· s Recall that we previously developed a radarequation of the formP t G 2 2 R max -------------3 kTBFL 4 So No min1/4·, B 1.5 ,s 5rpm previously we consider this to be a single pulse. An example: If PRF 300Hz Integration of pulses: Depending on scan rate &·PRF, we may receive more than 1 pulse from a s 5rpm round/per min. target. We can use that our advantage- 5 360 1 60 30 sec ·nB B s f P : number of pulses forintegration during dwelling time.1.5 nB ---------- 300 15 pulses 30 · B : beamwidth, s : antenna scan rate- f P : PRF-Chapter 14: MTI and Pulsed Doppler RadarDr. Sheng-Chou Lin14 - 10Radar System DesignPulse Integration Two techniques- Predetection IntegrationIFEnvelopedetectionVideoAmp.- Postdetection Integration Predetection Integration is coherent butsomewhat more difficult to implement thanpostdetectionPredetection Postdetection is incoherent but someimprovement in (So/No) can be erent additionNoncoherent additionNoisesignalcoherentintegration2P signal nv 2 nP signal n P n in a pulse Chapter 14: MTI and Pulsed Doppler Radar14 - 11Dr. Sheng-Chou Lin

Radar System DesignPulse Integration Recall we were developing alternate expression for the We can expressradar system equation S N So No nE i n min minR max-P t G 2 2 nE i n -------------3 4 kTBFL So No min1/4E i n 1 ;n –1 / 2 E i n 1- I i n nE i n : effective #- for ideal predetection So No : single pulse S/N required forminprespecification P FA .pulses integrated Example: P FA 10 –12 ,P D 0.9 ,E i n :efficiency factor; n: # of pulses integratedfind So No 15.8dB . If 1000- Note that for a pulse radar P t is a peak power, we canmin-also express in terms of average powerP avg P t T P t f p , where : Pulse width, T :PRI; T : duty cycle.-P t P avg f p ; E P avg fp: Energy per pulseP av G 2 2 nE i nR max ---------------------------3 4 kT B FL So No fpminE G 2 2 nEi n ---------------------3 kT 4 B FL So No minChapter 14: MTI and Pulsed Doppler Radar1/4pulses are integrated (postdetectionsquare law) S N So No nE i n min min 15.8 –10 log 130 –5.34 dB1/4P t G 2 2 R max -------3 kTBFL 4 S N min1/4P t G 2 2 nE i n -------------3 kTBFL 4 So No min1/4Dr. Sheng-Chou Lin14 - 12Radar System DesignNoncoherent Pulsed Radar Noncoherent Pulsed Radar- No reference signal used by the receiver is phasecoherent to the output phase of the transmitter. Problems encountered in detectingsmall-RCS in expected backgroundclutter environments high noise- A free-running pulsed transmitter- WB Filter- Automatic Frequency Control (AFC) Local OSC ismade to track the transmitter frequency- Radar designer is left with only a fewtechniques to minimize theperformance limits imposed by returnfrom background clutter.- IF signal is bandpass filtered and amplified by IFamplifier- Square law (noncoherent) detection noncoherentintegrated signal processor CFAR- constrain parameters: operatingfrequency, maximum permittedantenna dimension dimensions.Bandpass Filteredfrequency coherentto TX frequencyChapter 14: MTI and Pulsed Doppler Radar14 - 13Dr. Sheng-Chou Lin

Radar System DesignCoherent Pulsed Radar The phase of TX waveform is preserved is a reference signal the receiver for signal demodulation The use of STRALO and COHO reference signals to store thephase of the later signal processing identifies the radar. Relative complexity between coherentand noncoherent systems- If it were not for performance,noncoherent configuration would beused extensively used in search radarapplications. Advantage of coherent detection:exploitation of different Doppler shift toisolate desired target responses fromlarge dominating (in amplitude)background returnsNo filter bank- Relative motion between desiredtarget and its background Some techniques through whichnoncoherent radar can be used toaccomplish Doppler shift-aideddetection of targetsChapter 14: MTI and Pulsed Doppler RadarDr. Sheng-Chou Lin14 - 14Radar System DesignPulsed Coherent MTI The detection of moving targets are improved by suppression offixed targets. This is expanded to incorporate Doppler processing as onepossible form of MTI implementation is deified as one uses simple band reject to reject the returnfrom fixed targets Enhanced detection of the moving target Doppler filter- A relative narrow bandwidthclutter is rejected.- A broad passband (unknownDoppler shift)- Post-Doppler processing stage Noncoherent integration- Rejection notches in thepassband should be placed infrequency about the responsethat are to be rejected andshould be as wide as required toachieve the desired cluttercancellation.Chapter 14: MTI and Pulsed Doppler RadarNoncoherentintegration14 - 15Dr. Sheng-Chou Lin

Radar System DesignMTI filter (Delay line canceller)Two techniques are available for realizing MTI filter Delay line canceller- Digital filter (Multipulse canceller)- A real-time delay is equal to the PRI- Digital implementations can provide the desiredpassband with the flexibility of passbandprogrammability The preferred choice Range gate and filterChapter 14: MTI and Pulsed Doppler RadarDr. Sheng-Chou Lin14 - 16Radar System DesignRange gate and filter output of phase detector (I or Q) isprovided as input to N sample-andhold circuits Each sample-and-hold circuitreceives a sample gate alumped constant (active) bandpassfilter f L : lower corner frequency. f H :higher corner frequency. f R : PRF Noncoherent MTI- A-scope range videopresentationscontain“butterflies”at the slant range of the movingtarget.- Each butterfly is created by thefluctuating amplitude of the sumof the return from both thebackground and the targetChapter 14: MTI and Pulsed Doppler Radar14 - 17Dr. Sheng-Chou Lin

Radar System DesignNoncoherent MTI Noncoherent MTI- A-scope range video presentations target.- Each butterfly is created by the fluctuatingamplitude of the sum of the return from boththe background and the targetA-scopeChapter 14: MTI and Pulsed Doppler RadarDr. Sheng-Chou Lin14 - 18Radar System DesignPulse Doppler Radar (Analog)A pulsed Doppler radar exploits Dopplershift to obtain velocity information from apulsed waveform- High-PRF pulsed Doppler airbone radar- Medium-PRF pulsed Doppler airboneradarTwo approaches- Analog filtering- Digital filteringAnalog filtering- processing at IFselect so that overlap occurs at -3dBBW- Output of IF amplifier is gated- gated output is then filtered by a bank ofcontiguous narrowband (NB) filters3dB- Automatic detection and signal-sorting.f IFChapter 14: MTI and Pulsed Doppler Radar14 - 19Dr. Sheng-Chou Lin

Radar System DesignPulse Doppler Radar (Digital)- In-phase (I) and quadrature (Q) video selection of digital approach for most modern pulsedDoppler radar designs.- Analog-to-digital conversion (A/D) Spectral leakage Digital filtering (FFT)- N-point fast Fourier transform (FFT)- N contiguous filter passbands (spanfrom zero to PRF)- The number of range cells (m) andDoppler cells (N) increases theamount of hardware growsdramatically. Sin nx sin x- Other Doppler shift High sidelobes in the same- uniformly weighted set of N samplesrange cell- aperture weighting is used to minimize the spectralleakage.- Modern digital technology leads tohardware efficient realizations N-point FFT which produces a finiteimpulse response filter is the desiredmatched-filter for a finite sequence(step scan algorithms) NB filter (infinite impulse response) inthe analog approach can be only anapproximation of the desired matchedfilter.Chapter 14: MTI and Pulsed Doppler Radar14 - 20Dr. Sheng-Chou Lin

Chapter 14: MTI and Pulsed Doppler Radar 14 - 20 Dr. Sheng-Chou Lin Radar System Design Pulse Doppler Radar (Digital) selection of digital approach for most modern pulsed Doppler radar designs. Spectral leakage - uniformly weighted set of N samples - Other Doppler shift High sidelobes in the same

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