Re: RTK Base and its accuracy

[Gary E. Miller]

Yo Florian!

On Tue, 16 Aug 2022 09:25:48 +0200
Florian Kiera <florian.kiera@logicway.de> wrote:

> Hello Gary!
> 
> Am 11.08.22 um 21:42 schrieb Gary E. Miller:
> > Yo Florian!
> >
> > On Wed, 10 Aug 2022 10:49:24 +0200
> > Florian Kiera <florian.kiera@logicway.de> wrote:
> >  
> >>> 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.  
> > Then you missed my point.  They can all be correct, and different.  
> 
> Yet they calculated Latitude somehow "wrongly". (wrong by multiple 
> kilometers) I understood that it can have some deviations. Still I
> would expect them to be in a meter scale and not kilometer. I would
> like to stick with the fact that cgps does it right and I am happy to
> use it.

There are USGS grid scales that differ by up to 3 meters.  My
expectations are not as optimistic as yours.  If gpsd, by way of cgps,
works for you, great.  cgps does not cumpte anything, it just passes on
what gpsd tells it.

> 
> >  
> >>>> Latitude: 53.59931 11.41833  
> >>> Determined how?  
> >> With the position of the rover.  
> > I don't think you understood my question.  How do you know the
> > "accurate" position of the rover?  
> Using a map. It can either be OpenStreetMaps or Google Maps. Both end
> up in the same spot (roughly where our "repeater source antenna" is 
> positioned) with the Lat/Long I provided. Also the base has nearly
> the same coordinates as the base-antenna is close to the
> rover-repeater antenna.

There you go.  That is what "repeaters" do, they repeat what the repeater
sees.  Not good for precision work.


> >> Considering the base was not that wrong after all we can go back to
> >> the RTK base-rover scenario.  
> > How wrong?  How do you know how wrong?  
> cgps gave the right position and therefore hardly can produce
> gibberish RTMC3.

Ah, lost me.  cgps has no effect on the RTCM3 from your base.

RTCM3 is gibberish by design...

> A right position is determined in my opinion by
> taking the Lat/Long to a map and actually end up in the position
> where I actually stand at.

Google maps are not known for their accuracy.  Go find a benchmrk.
And the accuracy varies minute by minute, so you need to run gpsprof
for many hours to determine your real performance.

> In this scenario I expect Lat/Long values
> which when used on a map end up close to where the antennas (base
> antenna and gps repeater antenna for the rover) are positioned on the
> roof.

We already agree your repeater is messing up your measurements, and yet
you still use it?

> >> 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.  
> > Which can work, but there are easier wasys to get the same retuls.  
> Is it documented? I would always prefer easier ways when they produce 
> the same outcome. We do want to use a NTRIP caster tho.

Not docuemnted by gpsd, but it should be in the u-blox manuals.  Just
read the raw RTCM3 from the base, and nectcat (nc) it to the rover.
That is what most do for short range stuff.

> >>> The latitude  
> >> on the rover was accurate but the longitude was a bit off (~20m).  
> > Once again, you do not use "accurate" propertly.  I'm guessing your
> > repeater antenna is 20 m away?  
> 
> I understand that the position of the repeater is "repeated". So the 
> rover should think it is at the position of the gps repeater antenna. 

Sort of.  There are other effects in play.  Too many to correct for.

> The antenna is like 4m above and 5m in a horizontal direction away.
> The offset probably comes from the fact that the rover antenna
> obtains values not only from the repeater but from outside the
> building as well.

Yeah, that will royally confuse your rover.  I have played with
feeding 2 antennas into one GPS,  The results are not pretty.

> The offset actually goes in the direction of the
> windows. (shortest way inside the building) The rover thinks it is
> 20m outside the building in that direction while it *should* think
> its on the roof of the building.

Then maybe you need to rethink "should" considering your data does not
match your expectations.

> We are going to test the rover outside the building soon and
> hopefully receive what we expected all along.

Hopefully.

> >> I
> >> used the latitude and longitude that gpsd gave out in google maps
> >> which gave me the visual offset on the map.  
> > Not exactly a high precision method.  
> 
> If the Lat/Long isnt precise/accurate enough to be used on a map what 
> are they for then?

Depends on what you need.  If you are +/- 3 meters off 10% of the time,
then that is perfectly good o navigate a ship or fly an airplane.  It is
not good enough to driave a car in a lane.

You need to quantify what you actually need.  Then  run tests, like with
gpsprof, to see what you are actually getting.  Very few initially
expect how bad a GPS position can be 2% of the time.

> Simply imagining where I am? This is not about a 
> single meter.

Getting under a meter with 8-series will be very hard, and certainly not
with a repeater invovled.  Your own data tells you this.

> >> I will add the output.  
> > Looking forward to it.  Be sure to run gpsprof for at least 12
> > hours.  
> 
> I will run gpsprof today. I already added the output of the "simple", 
> probably wrong, gpsprof run to my last email tho.

Sorry, if it was there, I missed it.  Dunno what you mean by "simple".
Unless it runs modulo 12 hours, the data will be misleading.

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

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