What is the height refference for GPS?

[jack_john (jack)]

Hello guys
I’m a little bit confused about GPS height refference.

from the screenshot the
RED is the height data from chinese RTK gps
BLUE is height data from Baro
GREEN is the height data from the M9N gps

the Blue is clearly that the refference from the ground
but why the red and the blue is look likes it does not refference from the same thing

from my understanding the GPS height is the height relative to WGS84, world model. , but not for the case.
I try connecting the RTK gps directly to the GPS software, ie. Satrack, it shows that the height is very different from the height from shows in the log file

…this log is tested on ground, and the RTK gnss get RTK fixed not all the time

[Yuri_Rage]

GPS altitude is HAE (height above ellipsoid), which should be very close to a true MSL altitude.

The most reasonable explanation here is that your fixed base has an incorrect altitude (like zero) for its location.

What happens if you turn off RTCM3 corrections?

[jack_john (jack)]

thanks Yuri
I learn writing the lua script from your channel. very good channel indeed ,

I use fix rtk base ntrip data that provide by good credit provider, which I think altitude should be set correctly.

tomorrow, I will try disable rtcm and see the result, and will post the screenshot of the height form Satrack and from the Missionplanner.

[PMTAug]

As far as I know, you can refer to some parameters such as GPS_DRV_OPTION and EK3_OGN_HGT_MASK.

[Juergen-Fahlbusch]

@jack_john

…this log is tested on ground, and the RTK gnss get RTK fixed not all the time

In general the height information (Z axis) of the GNSS systems is the most inaccurate value, especially near ground.

[jack_john (jack)]

Here is the RTK Fixed WGS84 height

Seem likes the GPS alt in Arduplane log is the Relative alt to the Orgn.alt

Does anyone know, What is the Orgn.alt, and where it comes from?

[jack_john (jack)]

Found the answer
The GPS height in the log file is derive from height above msl., from the xxGGA message or PVT message of UBX protocol.

In contrast, the height from Satrack is the height above WGS84 which equal hmsl+Geoid seperation height.