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

[Robert McGwier]

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.

Bob



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?
>
> Cheers,
> Peter Monta
>
>
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