[jandrewrogers]

It depends on the quality of the IMU but yes. All US military systems use ultra-high precision IMUs and will only accept GPS corrections within the (classified) error margins of their inertial measurements. US military-grade IMUs lose precision very slowly, it is an area where they have a huge technology lead over everyone else so (ironically) they are less dependent on GPS than anyone else that might use GPS. Access to state-of-the-art IMU technology is very strictly controlled by the US.

GPS was created in part to allow the US to measure the world precisely enough in peacetime that they have an accurate model to feed their IMUs in wartime. It was never designed to be a robust navigation system even though everyone commonly uses it that way.