[time-nuts] GPS 1 PPS Averaging ?

John Ackermann N8UR jra at febo.com
Sun Mar 13 15:45:55 EST 2005

Brooke Clarke wrote:

> Hi John:
> But the data does not include the variation we get because of the 
> sawtooth error in the Motorola GPS receivers.   I'm using an old 8 
> channel UT+ timing receiver with the asymmetrical sawtooth error.  
> Maybe if I get one of the newer M12+Timing receivers and correct the 
> sawtooth either in software or hardware then I could approach the 
> numbers shown on the NIST GPS archive web page.  The ideal would be an 
> Ashtech Z12 setup for carrier phase time transfer.
> Based on Tom's post suggesting looking at one day prompted me to look 
> at what averaging is needed for one day readings, hence the subject post.
> I'm going to try 5,000 second averages to try and reduce the GPS 1 PPS 
> variation.  The SR620 can only average in a 1, 2, 5 * En sequence up 
> to 5E6 readings.  It would be nice to be able to do 86400 second 
> averages.

I think any averaging over 1000 seconds will pretty much squash the 
sawtooth, even in the old receivers.  My current plot is not old enough 
to have really good data, but right now I'm showing AVAR of 7.2x10e-13 
for tau=3600 seconds (but that's with only 21 samples).

I normally do my averaging in software rather than the counter, although 
the setup for the current experiment is actually averaging an average -- 
I'm doing 100 sample averages in the counter and logging that to the 
data file, then reducing that to one hour averages by averaging 36 
readings from the data file.  But since each counter average is based on 
the same number of samples, I don't think I'm breaking any statistical 
laws by doing it this way.


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