I built a 1-axes gimbal to stabilize a GPS antenna inside an aircraft. Purpose is to have the antenna always looking straight up in the sky, even when flying curves. In curves the aircraft rotates along its roll axes and the antenna points much more to the horizon instead of upwards, thus loosing satellites.
The gimbal works fine during and some seconds after rolling the aircraft (e.g. 30 degrees), and keeps the antenna pointing upwards. But after about 10 seconds (aircraft still in 30 deg. roll) it slowly starts to rotate and aligns the antenna in parallel to the aircraft again (following the g-vector of the aircraft flying the curve?). After approx. 20 seconds this alignment is completed and the antenna points in "aircraft up" direction instead of "sky up" direction. Vice versa rolling the aircraft back to horizontal when finishing the curve, the gimbal first stabilizes and stays pointed "sky sideways", then slowly rolls back to "sky up" parallel to the aircraft.
The gimbal is built with an SimpleBGC 32-bit Extended board on FW 2.63b0 (I know this is old, but building dates back..., and I am digging out the project again) and an iPower GM2804 with encoder, encoder enabled.

Does anyone have an idea how to make the gimbal stay roll-stable in reference to the horizon, not starting to roll back after 10-20 seconds? Did I miss some setting? It should stay stable for 1-2 minutes, that is how long turns usually take.

Any ideas appreciated, and thanks in advance
Thomas