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Re: [Discuss-gnuradio] Fwd: RTL-SDR for GPS/Galileo signal reception


From: Martin K
Subject: Re: [Discuss-gnuradio] Fwd: RTL-SDR for GPS/Galileo signal reception
Date: Thu, 19 Apr 2018 11:20:14 -0400

Priyanka,

I believe the easiest way to find GPS satellites* is to perform an FFT on the baseband as well as FFT of each of the 32 PRN sequences (in reality you don't need to look at all 32 of them) 
Perform a cross correlation with something like (pseudocode follows): 
for binshift = -n:n
{
 a= circshift(fft(input_baseband),n)
 b= conj(fft(PRNcodes)) 
 fftdata = xcorr(a , b)
 correlation_spectrum = ifft(fftdata)
 [look for peaks..]
}

As you and others allude to, there is frequency drift. So a simple xcorr will not result in a productive search. Shift over some frequency range F, by doing a circular shift before the xcorr of a number of FFT bins. As a reminder, 1 fft bin is Fs/fftlen in frequency width.

If you are unsure of what your baseband is actually tuned to, you can tune your radio to around 1625 MHz and see Iridium bursts well above the noise floor. This is assuming your input filters would pass 1625 MHz.

-
Martin K
(* aside from JM's idea of squaring the BPSK. I didn't think of that!)


On Tue, Apr 17, 2018 at 3:33 PM, PRIYANKA PRIYADARSHINI <address@hidden> wrote:

Hello All,

I want to acquire first the GPS L1 signal to get a basic idea of how acquisition works and then move to Galileo E1 signal, but I am facing some problems.

1. I am using RTL-SDR Dongle, with Rafael R820T Tuner, and I want to acquire GPS L1 signal with that.

2. I am using GNURadio to receive the data. In GNURadio, in the RTL-SDR Block, I am giving my sample rate of 2.048 Msps.

3. Incoming L1 signal has frequency of 1575.42 MHz and I am using GPS active antenna with 40 db gain.

4. In RTL-SDR dongle, it will be down-converted to IF (that is unknown to me), through local oscillator. and after the down-conversion, it will be passed to ADC.

5. In some articles, I have studied that RTL-SDR dongle that I am using has IF= (3.57 MHz or 4.57 MHz). (http://superkuh.com/rtlsdr.html)  

6. Now my question is that if it is true, then the signal going into the ADC has frequency of (3.57 MHz or 4.57 MHz) , and my sample rate is 2.048 Msps, that is below the frequency of the incoming signal, then it is not following the Nyquist Criterion. Also, according to this, the incoming signal must be at (3.57 MHz-2.048 MHz= 1.652 MHz). Is it Right? (screenshot attached for reference)

7.The samples coming out of RTL-SDR are complex (I & Q). In one articles I have studied that the I-part are the C/A codes and the Q-part are P-Codes and these samples are at Baseband(zero-frequency).  If I just want to acquire C/A code, then shall I need to just take I-part of the output samples coming out of the RTL-SDR? Or what should be my data on which I need to do the further DSP?

These are my problems till now. I have studied dozens of web-pages and articles, but couldn't find the fruitful answer. I shall be very thankful for any help.

Thanks a lot for your time.

Regards,
Priyanka 



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Martin K.

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