On my large rover, compass calibration is impossible when the GPS puck and the PX4 are close to the vehicle. If I unplug them and move them a few meters away, everything is fine. In trying to determine where to put the GPS puck and/or Pixhawk (i.e. the compass or compasses), I wanted to know where around the vehicle the magnetic field is distorted by motors, batteries, steel tube framing, etc., I found this app Sensor Kinetics: https://itunes.apple.com/us/app/sensor-kinetics/id579040333?mt=8 It displays on my iPhone the magnetic fields in a real-time chart. Very cool. So it became a no-brainer to measure the mag fields around my rover and determine where (seems like 4-5’ away, vertically) the mag fields are not distorted and hence a safe place to mount the magnetic compass.
@ktrussell you should check it out!
Hopefully this lesson will be useful to someone else. Without that app, after all, it’s quite difficult to see the magnetic fields…
Happy tinkering,
Chris