[time-nuts] GPS position averaging

Tim Shoppa tshoppa at gmail.com
Mon Dec 15 06:43:01 EST 2014

I would recommend determining your terrain's elevation using a topo map and
estimating or measuring antennas height above ground. And then excluding
survey results with wacko altitudes before averaging.

While we often set the elevation angle mask high for timing purposes, for
survey purposes especially altitude it will likely be best to do survey
with no or very low (10 degree default?) elevation mask. The Z3801A manual
tells how to set elevation mask for at least some of your units.

Tim N3QE

On Mon, Dec 15, 2014 at 4:03 AM, Ole Petter Ronningen <olepr01 at gmail.com>
> Hello, all
> I have a couple of GPSDOs (KS, z3805a) hooked up to a fixed outdoor antenna
> through an HP 58516 splitter. I notice that when I powercycle the GPSDOs
> (which I generally try to avoid), the position they come up with differs
> from time to time in at least the last three digits (and height in
> particular varies with up to 30m).
> I would like to get a better idea of what my actualt antenna location is,
> so that I can manually set the position in the GPSDOs. For that purpose I
> set up a Ublox M8N with logging in U-center, and I've collected about 250K
> readings over several days. For each reading I also log number of SV's
> used, as well as HDOP, VDOP. PDOP, thinking I would filter the list and end
> up with a subset of really good fixes.The idea is then to average the "best
> readings" and use that as my position.
> 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.
> Any hints are greatly appreciated.
> Thank you!
> Ole P
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