US Navy ships (and most commercial ships) have multiple backup systems. One is the shipboard inertial navigation system (SINS), which is entirely self contained once you tell it your current location. It is accurate enough to find Tokyo after leaving San Francisco if you lost GPS, LORAN and all other forms of electronic navigation. I recall reading that the current generation would be off by no more than a mile on this voyage. Since the 1940's there is the DRT (Ded Reckoning Tracer) [No, "ded" is not misspelled - it is a shortening of "deductive"]. This device was on US Navy ships during WW II; like SINS it needs a starting location, but it then monitors ship's motion to project the course on a chart. It is still carried as backup. The "ded reckoning" part is because it cannot account for currents, so errors will accumulate, and it needs to be recalibrated using either landmarks or celestial navigation. It also needs to be recalibrated when you go off the edge of the current map. There are also classified VLF systems, used mostly by submarines. The point is any military service needs multiple backups. In spite of sophisticated telephone and data systems on modern ships, they all still have point-to-point sound powered phones and voice tubes.