How a submarine keeps track of motion
An inertial navigation system (INS) estimates where a submarine is by measuring how it moves. Accelerometers detect changes in speed; gyroscopes track turns and orientation. The system combines those measurements with a known starting position, then continually calculates the craft’s changing position and direction—a process called dead reckoning.
Many modern gyroscopes use lasers. In a ring-laser gyro, two beams travel in opposite directions around a closed path. When the submarine turns, the rotation slightly changes the beams’ relationship, letting the instrument measure the turn without needing an exposed compass or an outside signal. The lasers do not need to be aimed at satellites, and they are not spinning atoms.
Why the estimate slowly drifts
The INS measures motion rather than directly checking the submarine’s location. Tiny sensor errors accumulate: a small error in direction can make the system interpret forward motion as partly sideways, and a slight speed error adds up over distance. The longer the submarine navigates without a position check, the less exact its estimate can become.
That makes a reliable starting fix important. When conditions and mission needs allow, the submarine can surface or raise an antenna near the surface to receive GPS and reset its position estimate. It can also compare its movement with information from other sensors, such as sonar measurements of the seafloor. Each check helps limit accumulated error while allowing the submarine to remain underwater between updates.
A tradeoff built into stealth
GPS is useful because it can provide a precise external reference, but receiving it requires access to satellite signals that do not penetrate deep water. The INS solves the opposite problem: it works continuously and quietly underwater, but its answer gradually becomes less certain. Submarine navigation is therefore a balance between staying hidden and periodically gathering enough information to keep the internal estimate trustworthy.
