Re: Geoid models in GPS receivers

[Gary E. Miller]

Yo Greg!

On Sat, 21 Nov 2020 21:02:50 -0500
Greg Troxel <gdt@lexort.com> wrote:

> I am continuing to measure things and evaluate an F9P.   It's working
> remarkably well with MaCORS, and I am feeling like I'm getting
> repeatability at the small numbers of cm level (just not sure exactly
> how many yet).

My NTRIP is 1km away and I have trouble getting more than a few meters.
The long term wander is better than the 8-series, the big win is the
much reduced short term fuzz in the fix.

> MaCORS is in "NAD83(2011) epoch 2010.0" which is I think normal for US
> reference networks.

Not to be confused with WGS84.

> Because the receiver doesn't know that, it's processing solutions as
> if they were in WGS84(G1762), and taking the HAE and applying a geoid
> correction to give me something, which ought to be "EGM2008 height" as
> proj calls it.

A fine mess.

> Looking at geographiclib and NGA's Fortran code for EGM2008 (thanks
> Javier if you are listening here), I found the geoid separation for
> 42.5 -71.5, which is the canonical round number for "halfway from
> Boston to Worcester", more or less, and I got -28.772 m which I
> believe.  EGM96 is only 10 cm different.

Out here on the West coast, the error goes up to 2 meters.  10x that
in Hawaii.

> On an android phone, I'm seeing a geoid offset in internal NMEA (via
> GPSTest) as -33.0m.  I'm also seeing -33.2ish from Vespucci using the
> F9P over wifi, getting high-precision NMEA.  These values are pretty
> stable over 10s of km; there is no geoid slope of 4m in 20km around
> here!

No, there is a geoid slope, but your phone does not have the 165MB data
base to do it right.  Get it here:

https://sourceforge.net/projects/geographiclib/files/geoids-distrib/

> I'm still digging into this and have more investigating to do, but my
> question is:  Does anybody have any experience with the geoid models
> in u-blox receivers as  exposed over NMEA?  Do they do the right
> thing? Any clues?

Not enough ROM to do it right.  If you really care, take the ECEF
position and compute the geoid offline.  If you want a really accurate
geoid, go here:
        https://geographiclib.sourceforge.io

gpsd uses those two to compute a much cut down database that approximates
well.

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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