SparkFun RTK Facet – internal geoid model used for MSL height output

GPS/GNSS
1

Hello,

We are using a SparkFun RTK Facet (ZED-F9x) with RTK corrections via NTRIP.

Horizontal accuracy is excellent and stable (RTK Fixed). However, we observe a
systematic vertical offset when comparing Facet-derived heights to official
national benchmarks.

This suggests that the Facet applies an internal geoid model when outputting
MSL (orthometric) height. We are trying to understand exactly what vertical
reference is being used.

Our questions are:

  1. What geoid model (or vertical reference) is used internally by the Facet
    to compute MSL height output (e.g. NMEA GGA)?
    Is it a global model, a simplified / coarse grid, or something else?

  2. Is there any way to obtain this exact internal geoid model (or its parameters)
    in order to post-correct already collected data?

  3. Can the internal geoid application be disabled, or can the receiver be
    configured to output raw ellipsoidal height, or both ellipsoidal and MSL
    heights, for use with GIS field collection apps?

  4. Is it possible to apply a local national geoid model (e.g. Israel’s ILUM/ILLUM)
    on the Facet / ZED-F9x, or is geoid handling fixed and non-configurable?

Clear documentation of the vertical reference behavior is critical for
engineering and GIS workflows that must align with official national vertical
datums.

Thank you.

2

Hi @Lior_Hammer ,

I believe @clive1 has already answered this in reply to one of your other posts: “EGM96 with limited resolution”. Appendix B of the ZED-F9P Integration Manual or the u-blox portal may help. You may need to open a support case directly with u-blox if you need more detail.

The RTK Facet firmware uses a very simple model to convert Geodetic coordinates to ECEF. But that is only provided to help set initial Fixed Base coordinates in ECEF format. It is completely separate to the altitude above mean sea level and geoid separation reported in (e.g.) the NMEA GGA position message.

The ZED-F9P firmware does support a custom (user-defined) geodetic datum. This is defined using configuration settings: CFG-NAVSPG-USE_USRDAT, CFG-NAVSPG-USRDAT_MAJA, etc.. For more detail, you can search for “ellipsoid” in the ZED-F9P interface description. But the RTK Firmware does not support those settings directly. You would probably need a custom build of the RTK Firmware where those settings could be defined, stored and loaded as needed. Or you could try configuring the ZED-F9P directly using u-center, and save the configuration to ZED-F9P RAM, Battery-Backed RAM and/or Flash. So long as you do not overwrite those settings with a “factory reset”, they should persist between power cycles.

I hope this helps,
Paul

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