[time-nuts] GPS position averaging

Azelio Boriani azelio.boriani at gmail.com
Mon Dec 15 15:52:43 UTC 2014


The Ublox M8N can send out raw measurements with messages TRK-MEAS and
TRK-SFRBX for observation and navigation data: you can postprocess
them to improve the accuracy.
<http://www.rtklib.com/rtklib_support.htm>

On Mon, Dec 15, 2014 at 12:43 PM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 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>
> wrote:
>>
>> 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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