[USRP-users] How to use N210 to detect and collect weak satellite signals

Marcus Müller marcus at hostalia.de
Wed Sep 4 05:12:01 EDT 2013


Hi Ruoyu,

I'm not really surprised you don't see GPS - it's a spread spectrum system, and power levels (let's say a GPS satellite transmits 1kW == 60dbm; free space loss should be around 182dB for a 21e6m distant satellite on 1GHz...) can be indistinguishable from noise; not seeing this in an FFT plot is totally ok, even with your high end setup.
I've got no knowledge on Iridium, however. This should be a TDMA/FDMA system, and might be (due to low power, a quick google turned up 400W per satellite, low duty cycle due to no active terminals where you are) undetectable by looking at an FFT plot, too.

So basically: With your high-end setup you're aiming to find things that are to weak for the unequipped eye(antenna); you will therefore most probably never see anything on your FFT plot. Employ your RF comm knowledge and specialized sensing techniques (correlation for GPS etc), and you should get better results.

Greetings,
Marcus

On 09/04/2013 09:40 AM, Li Ruoyu wrote:
> Hi, All
> I am working on a project which requires to receive signals from LEO satellites like orbcomm and iridium.
> I tried to use USRP N210 to collect the data, but I found the signal may be too weak to be observed.
> Stuffs I used:
> 1. USRP N210, DBSRX2 800-2400MHz
> 2. /Minicircuit/ cable amplifier, provides 37 dB gain at 15V/0.68A supply input. To ensure signal is enough enhanced, I used two of them in recent experiments.
> 3. Horn antenna with frequency range from 0.8 - 18 GHz. beamwidth 60 degree.
> After connected all things and warming up, I run "uhd_fft" to check if signals can be seen in frequency domain. I expected a peak around the center frequency. However, it is just noise. But if I turned center frequency to GSM band, it showed signal clearly. And then I turned it to 1.57542 GHz, which is the L1 band of GPS, it also shows nothing but noise.
> So, I am wondering if it is because the signal it too weak to be detected in that way.
> Have anyone ever done weak signal detection and collection before with USRP ? not only satellite, any weak signals is fine.
> If you have done similar project before, could you please tell me how you know signal is there if it cannot be seen by FFT. Any other function can help ?
> Many thanks ahead.
> Best regards,
> Ruoyu
>
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