Chapter 14 MTI and Pulsed Doppler Radar - …

1 Radar System Design Chapter 14. MTI and Pulsed Doppler Radar Radar System Design MTI and Pulsed Doppler Radar Moving Target Indication (MTI) Radar : A delay line canceller filter to isolate moving targets from nonmoving background - Ambiguous velocity - Unambiguous range Pulsed Doppler Radar : Doppler data are extracted by the use of range gates and Doppler filters. - Unambiguous velocity - Unambiguous or ambiguous range Chapter 14: MTI and Pulsed Doppler Radar 14 - 1 Dr. Sheng-Chou Lin Radar System Design Pulsed Radar High-PRF: unambiguous Doppler frequency, highly - Improve noise-limited detection relative ambiguous range to low-PRF waveform - solve TX-RX coupling problem of CW system - Minimize the number of introduced blind - Range blind during TX time periods zones relative to low-PRF system. Low-PRF: unambiguous range, highly Doppler frequency - circumvents the TX-RX coupling - Introduce Doppler blind zones (ground clutter).

2 Medium-PRF: ambiguous Doppler frequency, ambiguous range - circumvents the TX-RX coupling Chapter 14: MTI and Pulsed Doppler Radar 14 - 2 Dr. Sheng-Chou Lin Radar System Design Pulsed Radar Parameters Range: range is obtained from transmit-to-receive pulse delay T. 2R = cT R = ct 2 . Target 1 Target 2. Return Return - 1 s 150m , 1ns 15cm Transmit pulse Range Resolution: Pulse width must be shorter than the propagation time from target 1 to target 2 R 1 = ct 1 . 2. and back R 2 = ct 2 . 2. 2 . R 2 R 1 = ct t = 2 . R 2 R 1 c . Combined returned R = c . 2 from target 1 and 2. Unambiguous range R.. R unamb = cT . Transmit 2, T: pulse repetition interval (PRI) pulse - There are ways to get around this by using a staggered PRI (Multi-PRF). T. R = ct ------- Unambiguous Range (R < cT/2). ct'. R = -------- 2 2 Ambiguous Range (R > cT/2). Chapter 14: MTI and Pulsed Doppler Radar 14 - 3 Dr. Sheng-Chou Lin Radar System Design Pulsed Doppler Power Spectrum 2V.

3 F d = ---------c- cos .. : angle between the platform velocityand the line of sight (LOS). Chapter 14: MTI and Pulsed Doppler Radar 14 - 4 Dr. Sheng-Chou Lin Radar System Design Pulsed Radar Noncoherent Pulsed Radar - No reference signal Coherent Pulsed Radar - TX phase is reserved MTI Radar - detection of moving target by suppressing fixed targets Chapter 14: MTI and Pulsed Doppler Radar 14 - 5 Dr. Sheng-Chou Lin Radar System Design Pulsed Doppler Radar Analog 1 2 3 4. Range gate switch sampling Range Information Sampling Digital FFT. (filter bank). Doppler Information Chapter 14: MTI and Pulsed Doppler Radar 14 - 6 Dr. Sheng-Chou Lin Radar System Design Power Spectrum Density ( Pulsed ). As the antenna scans, the beam dwell time is finite. T i : interpulse period; . p : pulse period N+1 = 5. Chapter 14: MTI and Pulsed Doppler Radar 14 - 7 Dr. Sheng-Chou Lin Radar System Design Power Spectrum Density ( Pulsed ) 2.

4 A five-burst waveform: the return from a scatter at a N = 5. slant range R T . - R T R AMB contains four pulse samples - R T + R AMB contains only one pulse sample The shape of both the spectrum and ambiguity function for is important determine performance of MTI. and Pulsed Doppler radars. N+1=5. N+1=4 N=4. N+1=4. N+1=1. Chapter 14: MTI and Pulsed Doppler Radar 14 - 8 Dr. Sheng-Chou Lin Radar System Design Power Spectrum Density ( Pulsed RF). Chapter 14: MTI and Pulsed Doppler Radar 14 - 9 Dr. Sheng-Chou Lin Radar System Design Radar Equation for Pulsed Radar Until now, we have not said a great deal about filtering of the return signal except to say that matched filtering is desirable and B IF = 1 for most pulse radars. In some cases, we need a better idea about bandwidth for estimation S/N ratio. B . s Recall that we previously developed a Radar equation of the form 1/4. P t G 2 2 . R max = ---------------------------------------- ------------------------ - 4 3 kTBFL So No min.

5 Previously we consider this to be a single pulse. An example: If s = 5rpm . , B = , 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 - nB = . B .. s f P : number of pulses for = 5 360 . 1 60 = 30 sec . integration during dwelling time. nB = .. B : beamwidth, s : antenna scan rate ---------- 300 = 15 pulses - 30 . - f P : PRF. Chapter 14: MTI and Pulsed Doppler Radar 14 - 10 Dr. Sheng-Chou Lin Radar System Design Pulse Integration Two techniques - Predetection Integration Envelope Video IF detection Amp. - Postdetection Integration Predetection Integration is coherent but Predetection Postdetection somewhat more difficult to implement than postdetection Postdetection is incoherent but some improvement in (So/No) can be obtained. Postdetection Predetection Coherent addition Noncoherent addition signal Noise coherent integration P signal nv.

6 2. P signal n P n . in a pulse . n 2. Chapter 14: MTI and Pulsed Doppler Radar 14 - 11 Dr. Sheng-Chou Lin Radar System Design Pulse Integration Recall we were developing alternate expression for the We can express Radar system equation S N min = So . No . min . nE i . n . 1/4. P t G 2 2 nE i n - for ideal predetectionE i . n = 1 ;. R max = ---------------------------------------- ------------------------ 4 kTBFL 3 So No . - n 1 / 2 E i . n 1. - I i . n = nE i . min n : effective #. - So No min : single pulse S/N required for pulses integrated prespecification P FA . Example: P FA = 10 12 ,P D = , - E i . n :efficiency factor; n: # of pulses integrated - Note that for a pulse Radar P t is a peak power, we can find . So . No min . If 1000. also express in terms of average power pulses are integrated (postdetection P avg = P t T = P t f p , where : Pulse width, T : square law). PRI; T : duty cycle. - P t = P avg . f p ; E = P avg.

7 Fp : Energy per pulse . S N . min = . So . No . min . nE i . n .. 1/4 = 10 log . 130 = dB. P av G 2 2 nE i n R max = ---------------------------------------- -------------------------------------- - 4 kT 3 B FL So No min f p P t G 2 2 1/4. 1/4 R max = ---------------------------------------- ------------------ - E G 2 2 nEi n 4 3 kTBFL S N min = ---------------------------------------- -------------------------------- 1/4. 4 3 kT B FL So No min P t G 2 2 nE i n = ---------------------------------------- ------------------------ - 4 3 kTBFL So No min Chapter 14: MTI and Pulsed Doppler Radar 14 - 12 Dr. Sheng-Chou Lin Radar System Design Noncoherent Pulsed Radar Noncoherent Pulsed Radar Problems encountered in detecting small-RCS in expected background - No reference signal used by the receiver is phase coherent to the output phase of the transmitter. clutter environments - A free-running Pulsed transmitter - WB Filter high noise - Automatic Frequency Control (AFC) Local OSC is - Radar designer is left with only a few made to track the transmitter frequency techniques to minimize the performance limits imposed by return - IF signal is bandpass filtered and amplified by IF from background clutter.

8 Amplifier - constrain parameters: operating - Square law (noncoherent) detection noncoherent frequency, maximum permitted integrated signal processor CFAR antenna dimension Bandpass Filtered frequency coherent to TX frequency Chapter 14: MTI and Pulsed Doppler Radar 14 - 13 Dr. Sheng-Chou Lin Radar System Design Coherent 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 the phase of the later signal processing identifies the Radar . Relative complexity between coherent and noncoherent systems - If it were not for performance, noncoherent configuration would be used extensively used in search Radar applications. Advantage of coherent detection: No filter bank exploitation of different Doppler shift to isolate desired target responses from large dominating (in amplitude). background returns - Relative motion between desired target and its background Some techniques through which noncoherent Radar can be used to accomplish Doppler shift-aided detection of targets Chapter 14: MTI and Pulsed Doppler Radar 14 - 14 Dr.

9 Sheng-Chou Lin Radar System Design Pulsed Coherent MTI. The detection of moving targets are improved by suppression of fixed targets. This is expanded to incorporate Doppler processing as one possible form of MTI implementation is deified as one uses simple band reject to reject the return from fixed targets Enhanced detection of the moving target Doppler filter - A relative narrow bandwidth clutter is rejected. - A broad passband (unknown Doppler shift). - Post- Doppler processing stage Noncoherent integration - Rejection notches in the Noncoherent passband should be placed in integration frequency about the response that are to be rejected and should be as wide as required to achieve the desired clutter cancellation. Chapter 14: MTI and Pulsed Doppler Radar 14 - 15 Dr. Sheng-Chou Lin Radar System Design MTI filter (Delay line canceller). Two techniques are available for realizing MTI filter Delay line canceller - Digital filter (Multipulse canceller).

10 - A real-time delay is equal to the PRI. - Digital implementations can provide the desired passband with the flexibility of passband programmability The preferred choice Range gate and filter Chapter 14: MTI and Pulsed Doppler Radar 14 - 16 Dr. Sheng-Chou Lin Radar System Design Range gate and filter output of phase detector (I or Q) is provided as input to N sample-and- hold circuits Each sample-and-hold circuit receives a sample gate a lumped constant (active) bandpass filter f L : lower corner frequency. f H : higher corner frequency. f R : PRF. Noncoherent MTI. - A-scope range video p r es en t ationsc ontain but ter fli es . at the slant range of the moving target. - Each butterfly is created by the fluctuating amplitude of the sum of the return from both the background and the target Chapter 14: MTI and Pulsed Doppler Radar 14 - 17 Dr. Sheng-Chou Lin Radar System Design Noncoherent MTI. Noncoherent MTI. - A-scope range video presentations contain butterflies att hesl ant rangeoft hemo v i ng target.