I encounter with this problem frequently.
When I did some research, I found that Link. Briefly, In the EKF solution, compass heading disagrees with the heading estimate from other inertial sensors . I am using ERLE-BRAIN GPS Module and compasses . Internal & external compasses are well calibrated ( OFFSET values look good )
My question is here.
Is the externally mounted param "None" true? I also checked Default Param file [ Link ] . But I have some suspicion about that issue.
My Setup Photo here.
Here axis shematics