Range Doppler Response has a linear bias
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The range doppler response of matlab's phased array system toolbox seems to have a linear bias for doppler frequency. The system is a moving radar platform (at certain altitude) and a stationary ground target and ground clutter (modeled as phased.ConstantGammaClutter object). In my particular case, 0 velocity will have everything in correct frequency (0 Hz for ground clutter and target), however for 10 m/s velocity, the frequency is 88 Hz more than what it should be (for both ground clutter and ground target), for 20 m/s it is 88*2 Hz more and so on. I am not sure if I'm missing some parameters, my phased.RangeDopplerResponse object is defined as follows
myResponse = phased.RangeDopplerResponse('RangeMethod','Matched Filter', ...
'PropagationSpeed',c, 'DopplerOutput','speed',...
'SampleRate', fs,'OperatingFrequency',fc,...
'DopplerFFTLengthSource','Property', ...
'DopplerFFTLength',1024);
Also, the limit on doppler frequency is supposed to be -PRF/2 to PRF/2 but it is a bit beyond that. This is not of too much concern, but the bias on doppler frequency is an issue. Any help or clarification is greatly appreciated. thanks
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Answers (1)
Honglei Chen
on 25 Jul 2014
Hi Ramesh,
Is it possible for you to provide a little more details? Like what are the fs and fc? What are the position and velocity of the radar and the target? Otherwise it's hard to guess the issue.
3 Comments
Honglei Chen
on 28 Jul 2014
Edited: Honglei Chen
on 28 Jul 2014
I don't quite get why you say the correct Doppler frequency is 597 Hz. In fact I get neither 597 Hz nor 686 Hz. Based on what you have
rpos = [0;0;3000];
tpos = [8.5e3;0;0];
rvel = [50;0;0];
tvel = [0;0;0];
The relative speed I get is
rspeed = radialspeed(tpos,tvel,rpos,rvel)
About 47.15 m/s. So the corresponding Doppler is
c = 3e8;
Fc = 10e9;
fd = 2*tvel/(c/Fc)
This turns out to be about 3.14 kHz.
The following code shows a simple example I made based on your description. The PRF is made up but it should work for other values too. The figure I get correctly identifies the speed at about 47.15 m/s. Maybe you can take a look and see what I missed?
c = 3e8;
Fs = 10e6;
Fc = 10e9;
rpos = [0;0;3000];
tpos = [8.5e3;0;0];
rvel = [50;0;0];
tvel = [0;0;0];
dmax = 12e3;
prf = c/(2*dmax);
bw = 2e6;
myWav = phased.LinearFMWaveform('SampleRate',Fs,'PRF',prf,...
'SweepBandwidth',bw,'PulseWidth',0.1/prf);
myRadarPlat = phased.Platform('InitialPosition',rpos,'Velocity',rvel);
myTargetPlat = phased.Platform('InitialPosition',tpos,'Velocity',tvel);
myChannel = phased.FreeSpace('SampleRate',Fs,'PropagationSpeed',c,...
'OperatingFrequency',Fc,'TwoWayPropagation',true);
N = 64;
tstep = 1/prf;
for m = 1:64
[rpos,rvel] = step(myRadarPlat,tstep);
[tpos,tvel] = step(myTargetPlat,tstep);
x = step(myWav);
y(:,m) = step(myChannel,x,rpos,tpos,rvel,tvel);
end
myResponse = phased.RangeDopplerResponse(...
'RangeMethod','Matched Filter', ...
'PropagationSpeed',c, 'DopplerOutput','speed',...
'SampleRate', Fs,'OperatingFrequency',Fc,...
'DopplerFFTLengthSource','Property', ...
'DopplerFFTLength',1024);
plotResponse(myResponse,y,getMatchedFilter(myWav));
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