Re: Shirley...
Submarines use expensive inertial navigation systems, because GPS is no use to them unless they're at antenna depth with an aerial up.
I don't know if they put that sort of kit on surface ships or not. Or if it's still kept on planes for a backup. That Korean flight 007 that was shot down by the Soviets strayed into Russian airspace because of an INS failure (or at least not setting it up properly).
Biting the hand that feeds IT
