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

at least your fc in the Matlab script is incorrect: removing the
carrier frequency has been taken care of by the RF frontend (R820T2).
You are only left with the offset between the GPS carrier and the frontend
carrier + Doppler shift. Doppler is in the +/5 kHz range, but offset can
be up to +/-100 kHz for the cheap DVB-T dongles, so squaring the signal
(also done in one possible implementation of the Costas loop) will give
you a rough estimate of the offset to which the Doppler is added
by sweeping fc with typically 100 to 500 steps.

JM

> Thanks for the useful info but I am stuck after trying all the
> possibilities which I could possibly understand from the internet. Here is
> what I am doing:
>
> I am taking signal of 1ms from RTL-SDR using GNURadio Companion at
> Samp_rate 2.048M, and the flow graph is attached. after receiving the data
> (I and Q complex samples) the shape of the signal is shown in the figure
> attached.
>
> Since it is the complex IQ data, so for demodulation I used complex
> exponential function (exp(j*2*pi*fc*ts)). And after that I am doing the
> acquisition using 'Parallel Code Phase Search Acquisition' algorithm. But I
> am not getting the required result, instead I am getting some random peaks
> of correlation (very low amplitude) for every satellite PRN's. I am also
> attaching the MATLAB code for acquisition and example plot for one PRN.
>
> Even after reading so many articles online, I couldn't find the solution
> for that. I am not getting that what mistake am I doing in this whole
> process of acquisition?
>
> One problem that according to my understanding is that, may be the signal I
> am getting is very weak, or it's just Noise. because I have tried all the
> possible solutions, but couldn't find the good correlation peak for any
> satellite.
>
> And I am also getting two type of complex data from RTL-SDR, when I just
> plug-in my dongle, I am getting a complex signal of random amplitude, but
> when I take the data after some time then I am getting the data of constant
> amplitude. I don't know which of them is correct, so I tried processing
> both of them but results are same.
>
> Any help would be highly appreciated.
>
> Thank you
> Priyanka
>
>
>
> On Thu, Apr 19, 2018 at 8:14 AM, Jean-Michel FRIEDT <
> jean-michel.friedt@femto-st.fr> wrote:
>
>> I am collecting from the RTL-SDR dongle has amplitude in range
>>> of 0.04(peak to peak). Can we be sure of having GPS signal it and is such
>>> low strength of signal okay for GPS acquisition or not?
>>>
>>
>> that's the beauty of spectrum spreading and pulse compression: despite
>> the signal being below thermal noise level, pulse compression by 1024-fold
>> (1.024 Mb/s and 1 ms code repetition rate) will increase signal to noise
>> ratio by 10 bits. So even a single-bit A/D converter (comparator) is enough
>> to collect GPS data. You cannot see GPS unless you compress the pulse
>> (either
>> by squaring the signal to get rid of BPSK, or correlating with the local
>> copy of the Doppler shifted PRN).
>>
>> JM
>>
>>
>> --
>> JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Ep
>> <https://maps.google.com/?q=SeOR,+26+rue+de+l'Ep&entry=gmail&source=g>itaphe,
>> 25000 Besancon, France
>>
>> ----------------------------------------------------------------
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>>
>>
>



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

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