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

[Sakthivel Velumani]

Hi Priyanka,

Answering to your question (6), you don't really need to worry about the sampling frequency of ADC as those are internal things and taken care of according to the bandwidth specified by the user (host PC). The sample rate (bandwidth) that you mention in GRC is the rate at which the 'USB Control' transfers samples to the host PC (refer attached image). Coming to your question about Nyquist sampling rate - The input of ADC is at IF range and not at baseband. So a sampling rate of twice the bandwidth of IF signal is enough (instead of twice the maximum frequency). You can find more details via internet search with terms like 'bandpass sampling' or 'undersampling'. The theories in courses don't talk about this as they assume that input to ADC is always baseband but in practice, for high carrier frequencies, undersampling make things easier.

For the question (7), I don't know much about GPS signals but I can say that you need to consider both I and Q samples as the samples from RTL-SDR are raw samples and hence you need the whole complex samples to demodulate (if it is not BPSK modulated) and to perform other signal processing like synchronization.

Hope this helps.

Regards,

Sakthivel

On Tue, Apr 17, 2018 at 9:33 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 

_______________________________________________
Discuss-gnuradio mailing list
address@hidden
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

rtl_sdr_block.jpg
Description: JPEG image