In comments and links therein below the question How does Curiosity know how to point and move it's high gain antenna in real time? there is mention of a gyrocompass.
I am aware that gyros can be used to measure changes in attitude, i.e. rotations, and that conventional compasses can be used to measure attitude with respect to an external magnetic field.
But what is a gyrocompass, by what principle does it function, and how might one be used in the context of a planetary rover?