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

If you want to make sure that your collected data include GPS signals, get
rid of the BPSK modulation by squaring the complex signal, see
http://jmfriedt.free.fr/trans_num_l3_1_eng.pdf
slides 37-38.

I tried to do my best to describe the demodulation process in
http://jmfriedt.free.fr/sdr_gps_eng.pdf

JM

> 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
> <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
>



--
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe,
25000 Besancon, France

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