[time-nuts] GPS position averaging

Hal Murray hmurray at megapathdsl.net
Mon Dec 15 04:48:58 EST 2014

olepr01 at gmail.com said:
> My question is twofold; 1) is this for some reason a bad idea? And 2) How do
> I average the numbers? I can not put my finger on it but it "feels wrong" to
> average lat, long and height independently. 

It seems like an interesting idea to me.

It would be interesting to run a test: setup a pair of identical GPSDOs 
running off the same antenna but using different locations and see if you can 
see any difference in the PPS or 10 MHz output.

You can do a filtering pass to dump anything with fewer than 4 satellites.  
The survey code has a mode where it says "bad geometry".  I don't know the 
fine print on how that works.

I can get my elevation off a topo map.  I'm not sure how to translate that to 
GPS coordinates.  Something like that might be another opportunity for 

The refclock part of ntpd filters a batch of samples by discarding outliers.  
Time is only one dimension, so the recipe is pretty simple: sort, compute the 
average, discard either the top or bottom, whichever is farther from the 
average.  I'm not sure how to do something similar in 2 or 3 dimensions.

It might be interesting to make histograms of HDOP and friends.  (before and 
after any filtering you can come up with?)  That may cover the bad-geometry 

These are my opinions.  I hate spam.

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