[time-nuts] Slightly OT - GPS-Based Accurate Direction Finding

jimlux jimlux at earthlink.net
Thu Aug 26 13:58:01 UTC 2010


Attila Kinali wrote:
> On Thu, 26 Aug 2010 11:04:01 +1000
> David Smith <david at smithfamily.net.au> wrote:
> 
>> The system seems to work by taking the raw satellite phase information 
>> from two separate GPS systems and crunching the data to come up with an 
>> azimuth figure.  Has anyone heard of a (Open Source?) program that could 
>> be used to do these calculations?
> 
> AFAIK there is none out there (at least i've never seen one).
> But it should be not too dificult write the software yourself.
> The bigger problem is to get GPS receivers that provide you
> with accurate phase information. Especially with a small baseline
> you either need to sync the clocks of the two receivers or have
> receivers with precise TCXO and take some additional samples to
> calculate the frequency difference/drift. The precision you get
> will mostly depend on the baseline and the number of samples you
> use. The longer you have time to measure, the smaller your error-band
> will be.
> 


If you get a bit "closer to the metal" you could use two GPS L1 samplers 
running off a common clock, and do the PN code acq and track, which 
would give you carrier phase.  If you do the nav solution, you know the 
"look angle" to the various SVs, which would tell you the phase 
differential vs azimuth.

I believe that there are open source codes out there to do the 
processing.  The data rate isn't all that high.. the GPS samplers are 1 
bit.  There's certainly lots of papers from grad students on this kind 
of thing.

If you aren't concerned about acquisition time, and you can get the 
ephemeris from somewhere else (so your initial guess for acquisition 
isn't too far off), acquiring the signal isn't difficult.



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