Just to add to this, submarines and ICBMs navigate by inertial systems. The good ones (incredibly expensive) are extremely precise. The problem with INS though

Just to add to this, submarines and ICBMs navigate by inertial systems. The good ones (incredibly expensive) are extremely precise. The problem with INS though is that every minuscule error compounds over time... since there is no external reference. The longer you navigate by INS, the less certain you can be. With GPS, the system is constantly getting new reference signals from the satellites (which periodically get their own from ground stations).