Comment by ddlsmurf

true, sensor fusion of accel/gyro/mm only gets you so far, though of the 3, the mm is the only one that doesn't require integration, so while they are terrible at finding magnetic north in a given environment, the environment's big chunks of metal tend to not move, so with gravity, is the only external referential. That's why to do this stably you need something more, lidar/cameras+slam, the IR trick here, gnss (ideally with rtk/ppk/ppp).