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Re: [Discuss-gnuradio] GPS with DBSRX, Almost There

From: Robert McGwier
Subject: Re: [Discuss-gnuradio] GPS with DBSRX, Almost There
Date: Wed, 07 Mar 2007 08:48:50 -0500
User-agent: Thunderbird (Windows/20070221)

You have not read or internalized the specifications for the oscillator on the USRP which is intimately involved in this system. It is 50 ppm accuracy which is bad enough, but look at the can. It is begging to have thermal variances. Start up the usrp and your process and investigate Newton's Law of Cooling (blow on the oscillator) and watch everything dance! I stopped working on GPS until I could come up with a replacement. In my professional applications for the USRP, I am replacing the oscillator with an external stabilized injection.


Peter Monta wrote:
Gregory W Heckler wrote:

The real problem lies in the fact that the carrier tracking loop (a 3rd order PLL) of my software receiver cannot achieve phase lock. The phase jitter looks high, and the LO frequency drifts so much it dominates over the Doppler derived from satellite motion.

Yes, it looks like my dbs_rx is wandering 10 Hz or so over timescales
of a few seconds.  A second-order loop with bandwidth of about 20 Hz
seems to track all this out, but at the expense of noise---it would be
nice to use a smaller bandwidth for a stationary receiver.

Below are plots of 4 seconds of an actual dbs_rx recording (one point
per millisecond).  The first plot is unwrapped phase (y-axis in radians)
and the second is the demodulated data after the PLL (also attached as an
octave/matlab file).

So things look reasonably okay for strong L1 signals, but there may
be limits to how far the dbs_rx can be pushed for weak signals, if
cycle slips are at all important.  Maybe the strong PRNs can aid the
weak ones, since the LO jitter is common to all.

If anyone would like any GPS IF data I would be happy to email it to your personal email address (indicate how many seconds of data you would like). Thanks!

I could take a look if you like---two seconds perhaps?

Peter Monta


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