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

[EWKehren at aol.com]

In the boating world there is a product out there that I think but do not  
know for sure made by Garmin that has two antennas separated by a small 
distance  2 meter) that gives you azimuth information.
Bert  Kehren
 
 
In a message dated 8/26/2010 9:58:33 A.M. Eastern Daylight Time,  
jimlux at earthlink.net writes:

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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