GPS Signals recorded IQ samples file

GNU Radio
1

Hi!

After googling a lot and searching in this lists archive, I couldn’t
find
any recording of IQ samples from GPS signals.

I’m trying to record one myself, with no luck so far because (I suspect)
of
a malfunctioning active antenna.

Would anybody with the right equipment be so kind to record some minutes
of
4 MHz around 1.57542 GHz and share the recorded iq samples file?

Many thanks in advance!

Francisco.

2

You may try this GPS signal simulator to generate IQ samples for
testing.

–
Wang Kang
Blog: http://scateu.me
Fingerprint: 011F 0492 97D6 5D75 8AC4 6458 D43F 3CE2 3353 B7BD

3

GPS data records and processing scripts for GNU/Octave are available at
http://jmfriedt.free.fr/efts_archive.tar.gz. The tutorial I presented on
this
topic during the EFTS seminar is summarized at
http://jmfriedt.free.fr/sdr_gps_eng.pdf

JM

Would anybody with the right equipment be so kind to record some minutes of
Discuss-gnuradio Info Page
–
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l’Epitaphe,
25000 Besancon, France

4

Hi Francisco,

are you sure you did not see GPS? The problem is that GPS is often below
the thermal noise floor and only detectable by the virtue of processing
gain.
I’d try and take a whole lot of samples (like: 2s worth of samples), and
calculate the autocorrelation[1]. You should see peaks at multiples of 1
ms, because that’s the spreading code’s period.

Best regards,
Marcus

[1] Warning, a 8-million-points autocorrelation might take some CPU
power. You might want to apply a bit of FFT magic.

5

Wang: Sounds good! Thanks! Did you try it?

Jean: many many thanks for sharing all the scripts! Your paper has
caught
my attention. I will print it and read it.

Marcus: I was not expecting to “see” the signal because I already knew
it
was under noise floor. My conclusion of “signal absence” was after my
GPS
receiver was not able to decode it when I replayed it with an USRP.
However, due to my little knowledge of GPS, it did not occur to me any
other way of searching, so I thank you for your suggestion. I nearly
killed
my pc with a naive approach. :stuck_out_tongue: I need a wiser one.

Sylvain: interesting. I will try that too. Thanks!

2015-08-21 9:07 GMT-03:00 Sylvain M. [email protected]:

6

On Fri, 21 Aug 2015, Francisco A. wrote:

Wang: Sounds good! Thanks! Did you try it?

Yeah. It works like a charm, in both HackRF and BladeRF.

7

Hi,

are you sure you did not see GPS? The problem is that GPS is often below the
thermal noise floor and only detectable by the virtue of processing gain.
I’d try and take a whole lot of samples (like: 2s worth of samples), and
calculate the autocorrelation[1]. You should see peaks at multiples of 1 ms,
because that’s the spreading code’s period.

You can also square the signal, then decimate it a bit and FFT it and
you’ll see peaks at whatever doppler the sat currently has. (so you’ll
see different peaks for different sats).

Cheers,

Sylvain
8

On Fri, Aug 21, 2015 at 8:41 PM, Francisco A. <
[email protected]> wrote:

Marcus: I was not expecting to “see” the signal because I already knew it
was under noise floor. My conclusion of “signal absence” was after my GPS
receiver was not able to decode it when I replayed it with an USRP.

​If you replayed the capture file over the USRP for reception by a
commercial GPS receiver, you’ll have to carefully adjust the digital
amplitude sent to the USRP and USRP PA gain setting in order to 1)
transmit
enough signal power, as the captured data typically has very low
numerical
values, and 2) avoid overloading the front end of the GPS receiver that
is
designed to work with low-level signals.

I’ve done this successfully in the past in building GPS interference
mitigation applications.