Support true MSL altitude for VectorNav VN-300 in vectornav driver

PX4 Autopilot
1

Hello everyone,

I’m currently using the VectorNav VN-300 and taking a closer look at the vectornav driver (src/drivers/ins/vectornav/VectorNav.cpp). I noticed that the driver assigns the GNSS ellipsoid altitude to both the MSL and ellipsoid altitude fields in the sensor_gps topic:

sensor_gps.altitude_msl_m = positionGpsLla.c[2];
sensor_gps.altitude_ellipsoid_m = sensor_gps.altitude_msl_m;

Because positionGpsLla.c[2] represents the WGS84 ellipsoid altitude, the MSL altitude ends up being inaccurate. I’m guessing this was a standard workaround for older firmware versions that lacked a dedicated MSL output.

Since newer firmware versions (v1.1.0.1+) now provide the true Mean Sea Level (MSL) altitude, I’d like to propose an update to the driver. The modifications would be relatively straightforward:

  • Configuration: Turn on the GnssAltMSL bit (Bit Offset 18) in the binary output config register during initialization.

  • Parser: Update VnUartPacket to extract the GnssAltMSL measurement.

  • Mapping: Route the new GnssAltMSL value directly to sensor_gps.altitude_msl_m, and leave positionGpsLla.c[2] strictly for sensor_gps.altitude_ellipsoid_m.

Does this approach sound good to the maintainers? I’d love to hear your thoughts or any potential edge cases I should keep in mind!