Re: RTK Base and its accuracy

[Florian Kiera]

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.

> > > > 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.
> > Here is probably more info needed.
> Yup.
>
> > 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. 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. 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.
> > 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.
> > The rover
> > currently is inside the building
> M8P do not work well inside buildings.
>
> > but close to one of our gps
> > repeaters which get their values from the roof as well.
> When you get a GPS signal off a repeater, you are getting the position
> of the receiver, not the position is the antenna inside the building.
>
> > > 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. 
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. 
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.

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

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

> > The rover itself reports
> > a 2D error of 1m with the RTCM3 messages.
> Those "errors" are a joke.  Ignore them except as "googness" indicators.
Will do.
> > Is this to be expected?
> That is way better than I expect.
>
> > > 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 am very sure that gpsprof works very well, and it the right tool
> to check your "accuracy".
>
> > 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.

Regards
Florian

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