[ntp:hackers] Drift of position with NTP/GPS.

Reg Clemens reg at dwf.com
Wed Oct 8 18:06:37 UTC 2008


This is really a question to Poul-Henning Kamp, since I think he knows more 
about
GPS than the rest of us, but I will post it to the hackers list, since it is 
probably of
general interest.

The ONCORE driver writes certain of the messages from the receiver to an incore
file so other programs can produce statistics, generate plots, etc.

Now, some 10 years ago, when we were first putting the ONCORE driver together, 
I did a several day run to determine my position, averaging the positions I 
got over
that time, and have used that position ever since.

I recently drug out that graphics program (I was recompiling the graphics 
library
it depends on and wanted to see that it still worked) and have let it run for 
two days.

I find that my position has changed. The changes are 2-3m in each of the three
directions, viz:

	dLat = -3.202m
	dLon =  3.255m
	dHt  =  2.217m

The antenna is in the same place as it has always been, so this has to be 
something
generic to GPS.  Anyone have any thoughts?
-- 
                                        Reg.Clemens
                                        reg at dwf.com




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