How a Submarine Knows Where It Is Without GPS

Deep underwater, a submarine has no GPS, no landmarks, no view of the sky, yet it always knows exactly where it is.

With no signal reaching that deep, how does a submarine track its own position across thousands of miles of ocean?

It navigates by feeling its own motion. Inside sits an inertial navigation system, built from spinning gyroscopes and accelerometers. Gyroscopes sense every turn the sub makes, while accelerometers sense every push, every speed-up and slow-down. Starting from a known position, a computer adds up all that motion, moment by moment, to work out how far and which way the sub has traveled. No outside signal is needed, because the system only measures itself. It is dead reckoning done by machine.

Here is the flaw: tiny errors never erase, they add up. Every hour, the guessed position drifts a little further from the truth. So even the best system must occasionally sneak toward the surface for a fix and reset itself.

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