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accuracy in NMEA, F9P data point, UBX library and GUI

From: Greg Troxel
Subject: accuracy in NMEA, F9P data point, UBX library and GUI
Date: Mon, 08 Nov 2021 20:37:00 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (berkeley-unix)

I have just been dealing with making my ardusimple F9P work again after
a firmware upgrade and getting u-center (via wine) to configure NMEA
output on the UART going to the wifi interface.   That has led me to
look at the NMEA messages it is sending to see if there is some kind of
accuracy, so that I could parse it for Vespucci to get an error circle,
even if it's useful only as status rather than real accuracy.

Looking at the standard NMEA messages, I see
  - counts of SVs and CNR
  - HDOP and other DOPs
  - mode, including 2d/3d, DGNSS, RTK/fixed and RTK/float
but I do not see any EPE, URA, UERE and so on.

I gather there is some proprietary UBX NMEA message "UBX,00" with some
kind of horizontal accuracy, but I'm not super inclined to go there for
a Vespucci patch.  I also don't want to add UBX parsing.

Am I correct that with standard NMEA, there is no way to get an expected
error (other than making it up from CNR, #sats, HDOP)?  This seems to be
how it is, but it strikes me as an odd thing to leave out.

I am contempating generating a faux accuracy:

  10m * HDOP (non-differential)
  5m * HDOP (differential)
  1m (RTK/float)
  0.1m (RTK/fixed)

I am relaively confident in the RTK values, and I suspect the answer
about the other ones is to go observe a known mark a bunch and see how
it is, but I am curious if anyone thinks that the above is off more than
3 dB.

random datapoint:
  I know others have not been able to get good results with the F9P.
  Just now I have seen both 4 and 5 for fixed/float, matching the RTK
  status blinkenlight on the board.  Using an ardusimple L1/L2 antenna,
  it is actually operating in RTK/fixed, inside on the top floor, via
  network RTK from MassDOT (MSM4 IMAX), with reference data on L1/L2 x
  {GPS, GLONASS, Galileo, BeiDou}, and holding position to better than
  10 cm.  (Yes, I know it's ridiculous to try to do RTK indoors and the
  position is probably not entirely right, but with 4 constellations
  there are an impressive number of satellites.)

Also, when looking, I came across this code, which seems perhaps useful.
So far I have only skimmed the README.

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