Geoid models in GPS receivers

[Greg Troxel]

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).

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

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.

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.

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!

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?