Navigate by the Stars—From Beneath the Waves
A periscope can be a fine stand-in for a sextant.
By Lieutenant Matthew G. Homeier, U.S. Navy
October 2021
Proceedings
Vol. 147/10/1,424
Submarines rely on electronic navigation systems to determine the ship’s position while operating submerged. Submarines typically use inertial navigation systems, such as ring laser gyroscopes (RLGs), in conjunction with inputs from an electronic speed log, fathometer-generated seafloor topography, and periodic GPS position updates when the boat comes to periscope depth to accurately determine the ship’s position.
But the military operates in a dynamic geopolitical environment, and the possibility that an adversary could render GPS inaccurate or inoperative is something submarine crews can prepare for. For example, in 2017, more than 20 cargo ships in the Black Sea received spoofed GPS positions that indicated they were more than 25 nautical miles inland on Russian soil.1
RLGs and other inertial navigation systems are independently reliable because they do not rely solely on GPS input. However, RLGs can lose accuracy because of changes in ocean bottom topography and errors in the electronic speed log, gyrocompass, and GPS fixes. Even though there are two independent RLG units for redundancy, underway failure of one would place the boat in a precarious position. And a shipboard casualty could potentially take out both—a fire, for example, could claim both units.
https://www.usni.org/magazines/proceedings/2021/october/navigate-stars-beneath-waves
