ICBM's use inertial and (in the case of Trident) star mappers. The reason they don't use GPS is that you would (hopefully) only start throwing ICBMs around if

ICBM's use inertial and (in the case of Trident) star mappers. The reason they don't use GPS is that you would (hopefully) only start throwing ICBMs around if you are in "nucular combat toe to toe with the Rooskies" - and the way you know you are in a proper war is that all your GPS satellites have just been destroyed by the other side.