When it detects a rotation with the aircraft assumed stationary, that rotation can only be the earth's rotation. The IRU knows which way the rotation is relative to the IRU (after all, it's an IRU, that's what it does) and it knows which way it's mounted to the airplane. Therefore, it's known which way the rotation is relative to the airplane. No magnetic or GPS motion info is needed.
For example, if it's on the equator and it's rolling right, it's pointed North.
If it's pitching down, it's pointed East.
Etc.
If it's not on the equator, then there are rotations in multiple axes at once, and the components are figured out with trig.
(This isn't about Boeing equipment, just genetic on what information is available and what can be inferred)
Last edited by Vessbot; 3rd May 2018 at 02:38.