I am working with a synthetic aperture radar system with fmcw signal, which transmits and receives signals continuously. The received signals are dechirped and their type is double (not complex). I want to separate the received signal of each pulse and prepare it for the range and cross-range compression.
In some instances, I've seen that the Hilbert transform is implemented on the signals to generate analytical complex signal, but I don't know its main reason and in many cases, it doesn't work appropriately!
I attached part of the received and transmitted signals.
I appreciate your comments in advance.
I am not an expert in SAR, but my guess is that you will want to use a fully complex FFT to extract range data. Hilbert transforms are very easy to implement in the frequency domain, if you need them. Furthermore, using IQ demodulation of the radar return tones leaves you with phase information on the return signal which, if I remember correctly, can be important in SAR. Prepreocessing of FMCW signals will always involve some form of frequency extraction either by Fourier or wavelet transforms.
Someone else who is expert in the area of SAR will surely have a better answer.
Cheers!
Many thanks George Slade
I have implemented the Hilbert transform but still, I'm not sure about the results. I think the generated phase of the Hilbert transform is analytical and is not actual. I guess my system's demodulator is real because I have only one value for each bin.
Thank you so much again. You helped and guided me a lot today.
The question may be in another form
I mean to ask about what are the process performed after receiving the raw data
You have to measure three categories of the parameters of the signal
1- Navigation parameters as velocity of the aircraft, and the instantaneous location of the aircraft (x,y,z)
2- Antenna parameters as antenna tilting angle , antenna dimensions
3- Signal parameters as carrier frequency, pulse width.....etc
After measuring these parameters you have to regenerate the raw data according to these parameters
So the problem is not only in receiving the signal but also in measuring these parameters
Dear Mohamed E Hanafy thank you for your comment.
Actually, your mentioned processing steps do not matter in this question. However, Some of them are auxiliary data useful for phase error compensation and other parameters are used in the main SAR processing section.
My problem is so basic: I have FMCW signals with real values. I want to be sure what are the required preprocessing (or signal preparing) steps, between acquiring the raw signal and generating 2D raw data?
Sincerely,
Samples can conventiently be held in a 2D array of samples within each FM sweep (rows) vs samples from successive sweeps (columns). You will first want to focus the array by making phase offsets on samples as a function of their range and location within the synthetic array. Then a 2D FFT process of data will yield the cross-range vs range map. The FFT of the slow-time samples from successive sweeps gives the Doppler shift of a point which is a function of its cross-range location. The FFT of the fast-time samples within any given sweep gives the beat frequency which is a function of its range. This will get you a basic image/map.
Thank you, dear Clive Alabaster ,
Based on my chosen sampling frequency, which is 44100 Hz and the sweep duration, which is 2ms, the samples of the received dechirped signal should be 882, but as you can see from my attached figure, the number of samples are 2*882 for each pulse. I think it's because they are in the frequency domain.
Should I hold all the 2*882 samples? or only the first part is needed?
Thanks again.
Sincerely,
I think you've made a mistake here. I guess you mean 441kHz sampling frequency since this gives 882 samples over 2ms. Are you taking complex data (I and Q channels) and so obtaining a double sided spectrum (both positive and negative frequency domains). You're graph suggests just positive frequencies. I don't know why you are obtaining twice the number of samples as expected. Sorry I can't be any more helpful. Best regards, Clive
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