Re: RTK Base and its accuracy

[Florian Kiera]

Hey Gary and Greg,

Am 09.08.22 um 19:20 schrieb 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.
>     
    Noted, thank you!
> > 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.
>     
    That would be quite helpful.
> > 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.
>     
    That pretty much seems to be the issue already... the internet
    translators seem to have some issues with calculating the latitude
    correctly. cgps worked fine and gave the expected position.
> > "Actual" Position:
> >
> > Latitude: 53.59931 11.41833
> >       
> Determined how?
>     
    With the position of the rover. Here is probably more info needed.
    Considering the base was not that wrong after all we can go back to
    the RTK base-rover scenario. I start the survey-in as described in
    the first mail and than start str2str from the RTKLib to push the
    RTCM3 messages to the running ntripcaster. The rover than connects
    to the ntrip caster with gpsd and runs another ublox-M8P. The rover
    currently is inside the building but close to one of our gps
    repeaters which get their values from the roof as well. The latitude
    on the rover was accurate but the longitude was a bit off (~20m).  I
    used the latitude and longitude that gpsd gave out in google maps
    which gave me the visual offset on the map. The rover itself reports
    a 2D error of 1m with the RTCM3 messages. Is this to be expected? 
> Runa 24 hour scatterplort with gpsprof, that will tell you what to expect
> in "accuracy"/
>     
I am not sure if gpsprof works as intented. I will add the output. 

Regards
Florian
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Florian Kiera               Tel:    (0385) 201 000 58
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