inaccuracy of compass

[davidrt]

Hello, we use the MP2x28 on a static carrier with an optic payload carried by an aerostat. i have an error with the compass heading.
I take several measurements comparing different targets at different distances and try to learn how the error in accuracy is, i  see that the error is mostly 9 degrees to the right but gets up to 1 degree to the left, the target's degrees between them is around 20 degrees. What can be done about this? 
Below attached an image describing of the measurements my customer did to asses the error:

[Evan Schellenberg]

What does the compass heading (1208) and current yaw (1060) show when pointing to a given direction? These can be viewed in sensor monitor.

The current yaw is the direction that the autopilot believes it is facing. It comes from the Kalman filter yaw estimate and is corrected towards compass heading slowly.

If yaw and compass heading match (within a few degrees), but it is not the actual heading of the vehicle then the compass calibration is off with a bias and should be re-calibrated. There may have been metal items added to the airframe or moved since it was calibrated.

If the compass and yaw don't match within a few degrees then the compass calibration is even worse and should be re-calibrated for sure as an error like this will affect navigation.

I wouldn't recommend it except for testing but you can use the declination field (92) to apply an offset to the compass. This field is designed to hold the local declination so magnetic heading can be converted to true heading. This value is just applied to the magnetic heading of the compass. So if you increase it or decrease it by 5 degrees for example, you can move the heading the compass reads 5 degrees either way. It can be negative or positive.

The real answer is to re-calibrate the compass so it is accurate though.

Most likely the compass error will change at different headings, so there is not one fixed offset that will work, so trying to navigate with an incorrect compass declination could result in worse compass headings as you turn, so don't use an improper declination as a solution, compass calibration is the way to go.