Aircraft Appears Underground on Ardupilot + FlightGear

ArduPlane Plane 4.5
,
1

I’m using a combination of Ardupilot (JSBSim) and FlightGear. The model utilized is Rascal110-JSBSim.
Upon launch, the aircraft appears underground with an altitude of -585.5.
What could this be attributed to?
One assumption is that different coordinate systems are being used - Ardupilot indicates altitude above ground, while FlightGear, upon receiving this data, positions it relative to sea level.

Perhaps someone has encountered a similar issue before; it would be interesting to delve into this further.

UPD:
Once turned on the Ardupilot, the coordinates are synchronized and the plane ends up underground again. I still can’t say for sure why this is happening, why the different coordinate systems(?).

2

can you tell me exactly what version of JSBSim, FlightGear and ArduPilot you are using?

3

FlightGear version: 2020.3.13
Ardupilot from the main branch
JSBSim Version: compiled from FlightGear 2020.4.0 (Automatically installed when you run apt-get install flightgear)

For communication between Ardupilot & FlightGear, I have added the following string to /aircraft/Rascal/Rascal.xml

<output name="localhost" type="FLIGHTGEAR" port="5503" protocol="udp" rate="30"/>

Launch:
FlightGear: ./Tools/autotest/fg_plane_view.sh
Ardupilot: sim_vehicle.py -v ArduPlane -f jsbsim:Rascal --console --map

Case: As soon as Ardupilot is loaded and the aircraft reaches the take-off point (FlightGear was launched earlier), the coordinates are synchronized, revealing that the aircraft’s altitude is -585 feet in FlightGear.

4

It looks like my height is currently relative to ground level. Can I change the coordinate system relative to sea level?
Ardupilot Altitude AGL = 0, FlightGear AGL = -585 (airport)