"Nodding" indication when GPS position active

[N21HV]

I have not installed my HDX setup in the aircraft but I have most of the components hooked up on the panel so that I can bench-test and program the unit as much as possible. The dual ADAHRS, EMS, GPS receiver, AP Servos are all wired up and detected by the head unit. When I don't have the GPS reception, the attitude indication is rock solid and there is no drift indication in any axis.
When GPS position is obtained, the attitude indication becomes unstable. Especially notable is an occasional "nose diving" indication that, without any motion to the ADAHRS, the attitude indication "dips" as if the aircraft is nosing over. Sometimes, this is accompanied by "CROSSCHECK ATTITUDE" annunciation.
This only happens when GPS position is acquired and SVT symbology is shown.
What's going on here?
Thanks.
Hiroo

 

[KRviator]

AIUI, Dynon - and most other Experimental manufacturers - use GPS position & movement to validate their ADAHRS calculations. When the GPS is initialising you will probably find it changes your position slightly, multipath effects, etc, but without any corresponding change in the ADAHRS sensor data, the system doesn't like it as it is also looking for the change in airspeed and altitude that would have got you to where the GPS suddenly 'says' you are.

It is relatively normal and you won't see it during taxi or in flight when the ADAHRS is also showing movement.

 

[N21HV]

Thank you. I was curious since I have not seen this behaviors on two other non-Dynon EFIS system I currently have in my aircraft which also has GPS and ADAHRS setup.
I will monitor it to see if it happens in aircraft operation.

 

[Dynon]

We use GPS as an ADAHRS crosscheck/assist only if airspeed is not working/valid and there is movement per the GPS. We also have thresholds on GPS fix quality, but there are cases, when things are coming and going, such as when you're in a hangar or otherwise have spotty reception, that can lead to what you're seeing. When you're outside with a clear view of the sky, you shouldn't see any attitude changes because the GPS fix will be much better and the position won't "walk".

 

[N21HV]

Make sense. The air ports are still covered while I bench test the setup and while I was conducting the test on my outside deck, the location is shaded by houses and trees making the GPS reception less reliable than on the aircraft.
Thank you for the explanation.