Hi,
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
why?
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.
Fabrizio
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