Re: RTK Base and its accuracy

[Gary E. Miller]

Yo Florian!

On Tue, 9 Aug 2022 17:06:28 +0200
Florian Kiera <florian.kiera@logicway.de> wrote:

> we recently started building an own RTK base for test purposes and
> seem to have some issues with it. We want to test a simple RTK
> base-RTK rover scenario.

Yeah, that's tricky.

> About the setup: As hardware we use a raspberry pi 3b+ that is
> connected to a ublox-M8P via USB.

Oh.  You need a 9-series, or equivalent, to get good results.  The
single frequency 8's are marginal for it.

> The antenna and the other
> components are all installed ontop of the building. The raspberry has
> the latest version of Raspberry Pi OS (Bullseye). I installed the
> latest gpsd version (3.24.1), RTKLIB
> (https://github.com/tomojitakasu/RTKLIB.git) and ntripcaster
> (https://github.com/roice/ntripcaster).

Maybe someday gpsd should be able to do that with no additional daemons.

> To get the base started I ran a few ubxtool commands:
> 
> ubxtool -e RTCM3 -P 20.30 -f /dev/ttyACM0
> ubxtool -d NMEA -P 20.30 -f /dev/ttyACM0
> ubxtool -p MODEL,2 -P 20.30 -f /dev/ttyACM0
> ubxtool -e PPS -P 20.30 -f /dev/ttyACM0
> ubxtool -e SURVEYIN3,100,2000 -P 20.30 -f /dev/ttyACM0

> It takes around 7 1/2 hours to reach the accuracy of 2000 (20cm
> right?). So that seems right compared to the mentions in the ubxtool
> file.

That is be pushing that part a bit too hard.

> However the position is far from right.

I would expect nothing better than 2 meters EP.

> root@rtk-base ~ # ubxtool -p NAV-SVIN -P 20.30 -f /dev/ttyACM0
> UBX-NAV-SVIN:
>    version 0 reserved1[0 0 0] iTOW 208863000 dur 27251
>    meanX 371818636 meanY 75095561 meanZ 511048862
>    meanXHP 33 meanYHP 1 meanZHP -42 reserved2 0 meanAcc 2000
>    obs 27247 valid 1 active 0
> 
> Using https://www.oc.nps.edu/oc2902w/coord/llhxyz.htm or any other
> ECEF to LLH translator the values are far off from where we actually
> are.
> 
> Results from the website:
> Latitude  : 53.41551   deg N
> Longitude : 11.41833   deg E

Which are not precise enough for what you are doing.  Just look at what cgps
is telling you.  That is much more "precIse" than some random wiebdite.

> "Actual" Position:
> 
> Latitude: 53.59931 11.41833

Determined how?

Runa 24 hour scatterplort with gpsprof, that will tell you what to expect
in "accuracy"/

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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