chapter 14
TRANSCRIPT
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Radar System Design
Chapter 14MTI and Pulsed Doppler Radar
14 - 1Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
MTI 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 range
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14 - 2Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulsed Radar
High-PRF: unambiguous Doppler frequency, highlyambiguous range
- solve TX-RX coupling problem of CW 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 coupling
- Improve noise-limited detection relativeto low-PRF waveform
- Minimize the number of introduced blindzones relative to low-PRF system.
14 - 3Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulsed Radar ParametersRange: range is obtained from transmit-to-receive
pulse delay T
- ,
Range Resolution: Pulse width must be shorterthan the propagation time from target 1 to target 2and back
Unambiguous range
, T: pulse repetition interval (PRI)
- There are ways to get around this by using astaggered PRI (Multi-PRF)
2R cT= R ct 2=1s 150m 1ns 15cm
2 R2 R1 ct= t 2 R2 R1 c=
R c2=
Runamb cT 2=
R ct2
-------=
Transmitpulse
Target 1Return Target 2Return
R1 ct1 2=
R2 ct2 2=
Transmitpulse
Combined returnedfrom target 1 and 2
T
Unambiguous Range (R < cT/2) Ambiguous Range (R > cT/2)Rct'2
--------=
R
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14 - 4Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulsed Doppler Power Spectrum
fd2Vc
---------- cos=
: angle between the platform velocityand theline of sight (LOS)
14 - 5Chapter 14: MTI and Pulsed Doppler Radar 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
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14 - 6Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulsed Doppler Radar
Range gate
1 2 3 4
switch sampling
Sampling
Analog
Digital
FFT(filter bank)
Doppler Information
Range Information
14 - 7Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Power Spectrum Density (Pulsed)
As the antenna scans, the beam dwell time is finite.
: interpulse period; : pulse periodTi p
N+1 = 5
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14 - 8Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Power Spectrum Density (Pulsed) 2
A five-burst waveform: the return from a scatter at aslant range .
- contains four pulse samples
- contains only one pulse sample
The shape of both the spectrum and ambiguity functionfor is important determine performance of MTIand pulsed Doppler radars.
RTRT RAMBRT RAMB+
N=4
N = 5
N+1=5
N+1=4
N+1=4
N+1=1
14 - 9Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Power Spectrum Density (Pulsed RF)
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14 - 10Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Radar 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 formost pulse radars.
In some cases, we need a better idea aboutbandwidth for estimation S/N ratio.
Recall that we previously developed a radarequation of the form
previously we consider this to be a single pulse.
Integration of pulses: Depending on scan rate &PRF, we may receive more than 1 pulse from atarget. We can use that our advantage
- : number of pulses forintegration during dwelling time.
- : beamwidth, : antenna scan rate
- : PRF
BIF 1 =
RmaxPtG22
4 3kTBFL So No min-----------------------------------------------------------------
1 4/
=
nB B s fP=
B s
fP
B s
An example: If , ,PRF=300Hz
s 5rpm= B 1.5=
s 5rpm round/per min. =5 360 1 60 = 30sec=
nB1.530
---------- 300 15 pulses= =
14 - 11Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulse Integration
Two techniques- Predetection Integration
- Postdetection Integration
Predetection Integration is coherent butsomewhat more difficult to implement thanpostdetection
Postdetection is incoherent but someimprovement in (So/No) can be obtained.
IFEnvelopedetection
VideoAmp.
Predetection Postdetection
coherent
integration
Coherent addition
signal Noise
Psignal nv 2n2
Noncoherent addition
Psignal nPn in a pulse
Predetection
Postdetection
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14 - 12Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulse Integration
Recall we were developing alternate expression for theradar system equation
- : single pulse S/N required forprespecification .
- :efficiency factor; n: # of pulses integrated
- Note that for a pulse radar is a peak power, we canalso express in terms of average power
, where : Pulse width, :PRI; : duty cycle.
- ; : Energy per pulse
RmaxPtG22nEi n
4 3kTBFL So No min-----------------------------------------------------------------
1 4/
=
So No min PFAEi n
Pt
Pavg Pt T Ptfp= = TTPt Pavg fp= E Pavg fp=
RmaxPavG22nEi n
4 3kT B FL So No minfp-------------------------------------------------------------------------------
1 4/
=
EG22nEi n4 3kT B FL So No min
------------------------------------------------------------------------1 4/
=
We can express
- for ideal predetection ;
- : effective #pulses integrated
Example: , ,
find . If 1000
pulses are integrated (postdetectionsquare law)
S N min So No min nEi n =Ei n 1=
n 1 2/ Ei n 1 Ii n nEi n=
PFA 10 12= PD 0.9=So No min 15.8dB
S N min So No min nEi n =15.8 10 130 log= 5.34 dB=
RmaxPtG22
4 3kTBFL S N min-----------------------------------------------------------
1 4/=
PtG22nEi n4 3kTBFL So No min
-----------------------------------------------------------------1 4/
=
14 - 13Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Noncoherent Pulsed Radar
Noncoherent Pulsed Radar- No reference signal used by the receiver is phase
coherent to the output phase of the transmitter.
- A free-running pulsed transmitter
- Automatic Frequency Control (AFC) Local OSC ismade to track the transmitter frequency
- IF signal is bandpass filtered and amplified by IFamplifier
- Square law (noncoherent) detection noncoherentintegrated signal processor CFAR
frequency coherent
Bandpass Filtered
Problems encountered in detectingsmall-RCS in expected backgroundclutter environments
- WB Filter high noise
- Radar designer is left with only a fewtechniques to minimize theperformance limits imposed by returnfrom background clutter.
- constrain parameters: operatingfrequency, maximum permittedantenna dimension dimensions...
to TX frequency
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14 - 14Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Coherent Pulsed Radar
The phase of TX waveform is preserved is a reference signalthe receiver for signal demodulation
The use of STRALO and COHO reference signals to store thephase of the later signal processing identifies the radar.
No filter bank
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 returns
- Relative motion between desiredtarget and its background
Some techniques through whichnoncoherent radar can be used toaccomplish Doppler shift-aideddetection of targets
14 - 15Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulsed 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 targetDoppler filter
- A relative narrow bandwidthclutter is rejected.
- A broad passband (unknownDoppler shift)
- Post-Doppler processing stageNoncoherent 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.
Noncoherentintegration
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14 - 16Chapter 14: MTI and Pulsed Doppler Radar 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)
- 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 filter
14 - 17Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Range gate and filter
output of phase detector (I or Q) isprovided as input to N sample-and-hold circuits
Each sample-and-hold circuitreceives a sample gate alumped constant (active) bandpassfilter
: lower corner frequency. :
higher corner frequency. : PRF
Noncoherent MTI- A-scope range videopresentations contain 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 target
fL fHfR
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14 - 18Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Noncoherent MTI
Noncoherent MTI- A-scope range video presentations containbutterflies at the slant range of the moving target.
- Each butterfly is created by the fluctuatingamplitude of the sum of the return from boththe background and the target
A-scope
14 - 19Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulse 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 airboneradar
Two approaches
- Analog filtering
- Digital filtering
Analog filtering
- processing at IF
- Output of IF amplifier is gated
- gated output is then filtered by a bank ofcontiguous narrowband (NB) filters
- Automatic detection and signal-sorting.
select so that overlap occurs at -3dB
3dB
BW
fIF
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14 - 20Chapter 14: MTI and Pulsed Doppler Radar Dr. Sheng-Chou Lin
Radar System Design
Pulse Doppler Radar (Digital)
selection of digital approach for most modern pulsedDoppler radar designs.
Spectral leakage- uniformly weighted set of N samples
- Other Doppler shift High sidelobes in the samerange cell
- aperture weighting is used to minimize the spectralleakage.
Sin nx xsin
Digital filtering (FFT)- In-phase (I) and quadrature (Q) video
- Analog-to-digital conversion (A/D)
- 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.
- 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 matched-filter.