With SLBMs, GPS (and the predecessors to GPS e.g. NAVSAT) are/were used to determine where exactly they were being launched from. The ballistic missile submari

With SLBMs, GPS (and the predecessors to GPS e.g. NAVSAT) are/were used to determine where exactly they were being launched from. The ballistic missile submarine would determine its precise location before launch using these satellite geolocating systems and use that position to calculate the flight path which inertial guidance in the missile would follow. This was useful because inertial navigation in the submarines was imperfect, but having a precise location fix for the submarine was important if you wanted to shoot at things precisely. Of course with ICBMs launched from fixed silos, the origin point was precisely known from the onset.