For important things like ICBMs, you don't want to trust easily spoofable navigation like GPS. Instead, you double integrate over the acceleration data obtained

For important things like ICBMs, you don't want to trust easily spoofable navigation like GPS. Instead, you double integrate over the acceleration data obtained from very accurate accelerometers and gyros. since these sensors measure acceleration, and gravity is acceleration, any change in gravity over the missile's path will cause it to think it had moved differently than it did. Knowing the precise earth gravity then you can account for these changes. Changes in the magnetic field will affect compass readings, which help you decide what direction the missile is facing. Knowing the (fixed over time) change will help account for measurement error in the compass. I'm not sure how much that helps though, since the compass' earth field is constantly changing anyways due to the sun and magnetisation of the missile itself. The actual surface shape has an effect on the gravity experienced above the ground, but is also important to make sure you know what height you're aiming for, and that there's nothing in the way.