How Do Submarines Know Where They Are?

The problem

The stealthiest vehicle on earth has a navigation problem

A submarine’s greatest asset is silence. But navigation has traditionally meant broadcasting — GPS requires receiving signals from satellites, sonar requires pinging the seafloor. The moment you use either in a way that can be detected, you announce yourself. Near the surface, a submarine can thread a thin antenna above the waterline and receive GPS just fine. But radio waves are absorbed by seawater almost immediately; drop the antenna a few centimeters below the surface and the signal vanishes.

Sonar offers a quieter alternative in principle: a submarine can map the seafloor contour and compare it against archived charts, like terrain-matching used in cruise missiles. It works. But actively pinging sonar pulses when you are trying to remain hidden defeats the purpose. What is needed is a way to know exactly where you are using only instruments already on board — no signal in, no signal out.

How they solve it

Light in a loop, crystal under tension

The answer is inertial navigation: a system that tracks movement by measuring forces and rotations inside the hull, with no external reference at all. Two small instruments divide the job between them.

The first is a fiber optic gyroscope. The device winds hundreds of meters of optical fiber into a tight coil and launches two beams of light around it simultaneously, one travelling clockwise and one counter-clockwise. When the submarine is still, both beams cover the same distance and return in phase — they match perfectly. When the submarine rotates, the coil moves beneath the beams in transit: one beam now has to travel a slightly longer path than the other before it arrives back at the detector. The two beams return slightly out of phase, and a computer reads that mismatch to calculate exactly how much the vessel has rotated. No moving parts, no gimbals, no mechanical wear — just the geometry of light.

The second instrument is an accelerometer built around a vibrating quartz beam. A tiny weight is suspended from a thin crystal, which is driven to vibrate at its natural resonant frequency — like a guitar string tuned to a specific note and kept ringing. When the submarine accelerates, the inertia of the suspended weight pulls or pushes against the crystal, placing it under tension or compression. That stress changes the crystal’s resonant frequency: the beam tightens and its pitch rises, or it slackens and its pitch falls.

By measuring that frequency shift, the computer calculates acceleration. Integrate the acceleration once to get velocity; integrate again to get displacement from the last known position. The submarine tracks its own journey through the ocean with the same mathematical tools used in every smartphone’s step counter — only at an oceanic scale and with sub-meter precision.

Takeaway

The quick version

• GPS is useless below the waterline — radio waves are absorbed by seawater within centimeters of the surface.
• A fiber optic gyroscope tracks orientation using the phase difference between two light beams sent in opposite directions through a long coil — no moving parts.
• A vibrating quartz accelerometer detects acceleration through a shift in resonant frequency: acceleration changes crystal tension the way tightening a string changes its pitch.

“Just like tightening a guitar string raises its pitch, that changes the frequency at which the quartz vibrates.”— Derek Muller, Veritasium

Two small instruments, no transmitted signals, no external reference — and a submarine can track its position across the entire ocean floor. The precision is in the physics, not the infrastructure.