then (as explained by other posters) that autopilot has two basic modes of operation: either it maintains a selected heading or track (both referenced to the magnetic north),
Point of order/correction that may or may not be of relevance to the greater debate..the 777 autopilot track/heading function can be referenced (by pilot selection) to either magnetic north
or True north.
FWIW looking further back in the thread I see some comments regarding a 14 degree/hour track change and..
Inertial navigation failure, causing a drift in the platform?
...Don't forget (?) the Flight Management System uses a
mix of both ADIRU and GPS data to perform nav functions. In the real world you'll see a small amount of ADIRU "drift" on every sector and the FMC position will be anchored pretty much on the GPS position.