System: 550 Hex, APM2.6 Flight Controller (supposedly with no onboard compass), 3DR GPS, latest Mission Planner, Hexacopter 3.3.1 firmware.
Problem: After calibrating the accelerometers, the compass, the radio, arming then dis-arming the motors, I switched Mission Planner to show flight details. The HUD showed the front of the 'copter was facing North. Roll and pitch changes displayed properly, but when I checked yaw the compass moved in the opposite direction of rotation. I pointed the front of the 'copter to the West, yet the HUD compass and the GPS show it heading East.
This also happened with this same hex when I used the same system set-up - but with a APM2.5 Flight Controller (which DOES have an on-board compass), the same Mission Planner, but with Hexacopter 2.3.1 firmware.
A. I could see this happening IF the unit which houses the compass was mounted upside down - but that can't be the case because neither the APM2.5 Flight Controller (the one with the compass) nor the GPS unit (apparently also with a compass) are definitely NOT mounted upside down.
B. The fact that this happens with two different Flight Controllers - and with two different versions of firmware - Tells me it is not a Flight Controller issue nor a firmware issue.
C. Magnetic interference - I've considered this but cannot conceive how it could produce the compass and GPS errors I am seeing.
Anyone have any ideas here?