[USRP-users] problems with recording of baseband complex GPS samples

Fabrizio Turin fabrizio.turin at rina.org
Fri Oct 27 11:22:58 EDT 2017

I'm pretty much new to USRP. For a research project we are trying to record baseband in-phase and quadrature GPS samples with USRP device, here's our experimental set up:

- Motherboard: USRP N210
- Daughterboard: WBX (40 MHz)
- active GPS antenna Ublox ANN-MS powered through biasT (providing around 27 dB gain)
- flowchart to receive and save samples on file via GNU Radio
- GPS SDR in the host computer to process the data and get position information

Here's the configuration of the GNU Radio:

   USRP Source:
	- output type = complex int16
	- wire format = complex int8
	- clock rate = default
	- clock and time source = default
	- samp rate = 10 MHz

	- center freq = 1575420000 Hz
	- gain value = 31.5 dB
	- antenna= RX2
	- ch bandwidth = 0 Hz

   File Sink
	- input type = short
	- vec length =2

We operate minutes long acquisitions in clear sky conditions (rooftop of the building). At the moment we are able to record a signal, and process it up to the acquisition stage of the SDR (satellites ID in view are correctly extracted).
The problem is we are not able to track the code and extract the navigational message except for few seconds along the acquisition (not enough for a fix). I point out that the SDR processing part should be ok since we tested it against several example signals.
By looking more closely at the real (or imaginary) part of the acquired raw signal we expected it to be a representation of noise with values spanning through the range allowed using signed int16. But, in reality, samples seem to acquire just one among five values (symmetrically around 0). Any idea
Furthermore we are now using the USRP internal oscillator, I heard that it may be lacking the needed frequency accuracy for this application, do we really need an external oscillator with much higher frequency stability?
Thanks in advance.



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