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.

2 Low-PRF: unambiguous range, highly Doppler frequency - circumvents the TX-RX coupling - Introduce Doppler blind zones (ground clutter). 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.

3 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. 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.

4 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. 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.

5 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.

6 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.

7 Postdetection Predetection Coherent addition Noncoherent addition signal Noise coherent integration P signal nv . 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.

8 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 . 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.

9 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.

10 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.