How to make sure, external compass is used (AC 3.6.8)

How can I make sure with Cube 2.1 (black), that the external magnetometer of the GPS puck is used as primary compass?

By default, in Mission Planner (1.3.66) I see all three compasses activated, for #1 “externally mounted” is checked and it is set as “Primary” compass. But in the status panel (yaw value) and on the map display (current heading, red line) I only see changes, when I turn the Cube itself. It also makes no difference, when I change the primary compass to Compass 2 or 3 or when I deactivate one or the other compasses alternately. Yaw / heading always change only when the Cube is turned.

From the mx/my/mz, mx2/my2/mz2 and mx3/my3/mz3 values I can see that mx/my/mz belong to the external magnetometer while the others change, when I move the Cube.

After playing around with these values and power cycling the Cube several times, the behavior is slightly different: Now, when turning the external compass (GPS puck), the yaw/heading turns very slowly (with a large lag) and I get complains about compass variance. When I turn the Cube, the yaw/heading still changes rapidly and I also get a compass variance error message. So now both compasses are fused somehow, but the internal one of the Cube still seems to have priority although in the configuration, Compass 1 is still set as primary compass.

You are seeing the gyroscopes causing that.

Thanks Philip - but why does the yaw / heading only move very slowly? Do the gyros have so much priority over the magnetometer?

The question for me still is, how I can test (in order to make sure) that from the magnetometers - at least from the part that they contribute to the fusion, the external magneto has priority over the internal ones (which are much more influenced by currents floating around for the propulsion)?

The gyroscope is much better than the magnetometer