[wongarsu]

It depends on what you are doing. ICBMs predate GPS, so for anything flying you can fall back to the old way of comparing terrain height to known maps. This is reliable and a widely-implemented technique (at least in weaponry).

If that's not an option you can use an IMU, but because of errors adding up over time IMUs aren't all that great if you can't calibrate from time to time. Satellites do it by looking at stars, if you're a car you might look at the streets and compare them to maps.

The only things that really have fundamental problems without GPS are ships and anything that flies over water. In any other application GPS is used because it's cheap to implement and reliable, not because it's the only way to do it.

[pps43]

ICBMs don't compare terrain height, they use a combination of gyroscopes and telescopes instead. Cruise missiles can follow the terrain.