Magnetometer Calibration: example of poor calibration

Sometimes the magnetometer calibration won't be so bad that it triggers a warning in preflight, or it might only trigger a warning if the plane is pointed in a particular direction, but the discrepancy will be enough to cause poor performance, with poor roll, heading, altitude, and airspeed control.

If the calibration is poor, the indicated magnetic field magnitude ("sensor:magnetic:magnetic_total") will vary from the earth's local magnetic field value  ("sensor:magnetic:magnetic_earth_total"),  as the plane's orientation changes in flight or when you rotate the plane on each axis on the ground (away from magnetic influences like an automobile).  

Here's flight data from a poorly calibrated magnetometer:





Here's what flight data from a well calibrated magnetometer:

The magnitude doesn't quite match earth's field in this example (it would be better if it did), but the most important thing is that it's constant as the plane rotates.

(The blip at the beginning was probably due to the plane getting close to automobile, catapault launcher, or some other ferrous object during ground handling.)

To resolve poor magnetometer calibration, see this procedure: : Magnetometer Recalibration