Re: Virtual Reference Stations (Was Re: gpsprof: higher resolution)

[Greg Troxel]

John Ackermann N8UR <jra@febo.com> writes:

> I got a login to the Ohio DOT virtual reference station network, which
> uses the Trimble VRS system.  It appears the only unique requirement
> is that the rover needs to send an NMEA GGA sentence to provide an
> estimated location used to construct the virtual reference station
> location.

MassDOT is like this too, but it is leica.  rtklib on Emlid (I have an
old reach) can send the sentence just fine.  I have not traced it enough
to write a spec.

My impression is that just sticking an nmea sentence in the tcp stream
is sufficient and that it's not really all that tricky.


Also, with MassDOT there are ntrip mount points for each reference, and
then different ones for various VRS flavors.

> Other than that, the traffic flow is all outbound from the
> network to the rover.  (I don't know if it needs just a single GGA
> sentence at login, or at some interval.)

I suspect it keeps going until it gets a new one, and maybe even just
stays with the first one unless it's really wrong.

VRS is tricky business; there are multiple schemes.  I'm not entirely
clear on them, but this article seems to cover slightly more than I
understand:

http://hitarget.ir/en/concept-and-comparison-of-vrs-max-imax-fkp-nearest-in-gps-networks/

This talks about sending the GGA:

https://www.use-snip.com/kb/knowledge-base/subtle-issues-with-using-ntrip-client-nmea-183-strings/

it seems to be:

 in the http setup for ntrip, wait until  you get the OK line.
 then squirt a GGA sentence with a line terminator

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