I wonder if I could have a bit of clarification regarding the Pixhawk 2.1 and compasses.
So if I understand correctly, the PX2 has an onboard compass - at least one. Are there one or two compasses onboard?
If I plug in the Here GPS (which has a compass within it) - does this become Compass 1?
There’s a bit of crossover with the mission planner forum however I think it makes more sense here. In MP (compass calibration) there is an option for PX4/Pixhawk. So I have selected this. I have then ticked compass 1 as in use and external. Compass 2 as in use and internal and left compass 3 out of use. When running through the calibration I still get very high offsets - am I setting up the compasses incorrectly? I have chosen Compass 1 as primary.
I am 95% sure that compass 1 is the external one (very hard to find documentation but read it on the forum somewhere).
I ended up getting VERY high offsets on compass 2, but reasonable ones on compass 3 and so disabled number 2 and seems to work ok. Still have not managed to diagnose exactly why this is.
So config for me is 1 - external 2 - disabled - 3 internal enabled
Pixhawk2 has 3 internal compasses. When you check the compass ID in full parameter list, the ones start with 7 is external and one start with 2 is internal (or the other way round if i could not remember exactly), you can see that compass ID for first compass changes when you plug in the external compass, which means that the first compass changed from internal to external.
Based on my understanding, due to different positions of three compass, there are subject to different electronic interference, therefore the large compass variance message is common. When encountering this, just need to disable the compass with large variance, so that the compass orientation estimation will still perform nicely.
I did some test flying tonight. Compass 1 once plugged in did become the external Here GPS compass. Similar to you @mrwooster I had very high offsets on compass 2 so disabled it.
After the live calibration it gave an accurate heading. I did have some issues with a compass various error during one flight, according to mission planer, so I’m just about to have a look at logs and try to see what happened. I’m not sure if mission planner was accurate in it’s reporting as telemetry was poor.