I guess I couldn't quite grok how IBIS would measure the Earth's rotation whilst being on Earth, but as I've now just learned (through various slaps of the forehead) a perfectly vertical spinning gyroscope will definitely tilt with time due to the Earth's rotation and this is measurable to high degrees of precision.
Not just randomly tilt - it will align itself with the poles of a spinning celestial body. It's used in applications that can't rely on correct magnetic variation, like surveying and aircraft inertial navigation systems.
