There is an obvious case where you can't rely on GPS on Earth. Pershing-2 missiles had radar correlation guidance back in the 80's. An obvious consequence of

There is an obvious case where you can't rely on GPS on Earth. Pershing-2 missiles had radar correlation guidance back in the 80's. An obvious consequence of Google maps imagery and open source is that a capable college student can make an optical terminal guidance unit out of a mobile phone.