FrSky R9 rssi scale

ArduPlane Plane 4.2
1

I have a FrSky R9MM-OTA receiver which sends rssi on channel 16.
My config looks like this, rssi scale is 1000 to 2000us.

config

Radio shows 100dB, channel 16 is 2006us, MP shows rxrssi 99.

ch161

rxrssi1

If I put the radio into range test, radio shows 70dB, channel 16 is 1490us, MP shows rxrssi 45.

ch162

rxrssi2

Obviously ArduPlane uses a linear scale from 1000…2000us.
The R9MM seems to have a different scale, 70dB is 1490us.

Does anyone know the secret and how to fix this in ArduPlane ?

ArduPlane 4.2.1
Matek F405 Wing
Radiomaster TX16S
OpenTx 2.3.15
FrSky R9M
FrSky R9MM-OTA

2

I put the rssi telemetry value on channel 15 in the radio and changed the rssi channel to 15.
Now the radio and ArduPilot show the same values.
I asked FrSky which scale the R9MM uses but got no answer and probably never will get one.