DTM sentence and Russian datums in US phones (yes, that's clickbait!)

[Greg Troxel]

Over at GPSTest (and Android program for telling you what your GNSS chip
is doing), people report that some phones have strange elevations, and
digging in there appear to be be two notable oddities:

  geoid models that are wrong to a degree even more surprising than
  u-blox's reporting of -33m when it should be -29m.

  a DTM statement that says positions are in PZ-90, the grame used for
  GLONASS.

At least DTM has been observed in both Samsung phones and a Google Pixel
5, and perhaps both have the Qualcomm 765 (5G, and dual-frequency GPS).

an example is:

  $GNDTM,P90,,0.000021,S,0.000002,E,0.989,W84*57,,,,,,,,,,,,

explained, sort of, at:

  https://www.trimble.com/OEM_ReceiverHelp/V4.44/en/NMEA-0183messages_GPDTM.html

full thread at

  https://github.com/barbeau/gpstest/issues/503


I wonder if anyone has seen a DTM sentence, and if so what it contained.
Certainly, I'd expect that if one put a Garmin into NAD27, you'd have
the transform.  But today, that seems unusual (unless navigating in a
country with a local datum that isn't more or less ITRFF2008), using
local maps.  Even in the US, which has a national datum, people navigate
with their phones in WGS84.

What's funny is that PZ90.11 is so close to ITRF2008 that unless you are
playing at the 10 cm level or better it can be treated ~= ITF2008 !=
WGS84(G1762).

I do not believe that the frame being odd and the bad geoid model are
logically related, but they may both be in the same chip out there.

Has anybody seen this?



A related question is  if anywone knows if recievers that do
multiconstellation solutions transform among the system's  frames, or
just assume all of them are ~== ITF2008.  My working hypothesis is that
one can't really detect any errors from assuming equivalence in a
navigation solution and that once you do RTK you are in the reference
stations frame anyway.  Therefore this multiconstellation frame question
is of academic interest only.   Any actal info?

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