---
title: 'How Do Submarines Know Where They Are?'
source: 'https://youtube.com/watch?v=_1G8nrmBKeY'
video_id: '_1G8nrmBKeY'
date: 2026-08-27
duration_sec: 91
channel: 'Veritasium'
---

# How Do Submarines Know Where They Are?

> Source: [How Do Submarines Know Where They Are?](https://youtube.com/watch?v=_1G8nrmBKeY)

## Summary

This video explains how submarines navigate underwater without GPS, using a combination of inertial navigation systems and sonar. It details the principles behind fiber optic gyroscopes and accelerometers, which work together to track a submarine's position.

### Key Points

- **GPS Limitations Underwater** [00:02] — GPS relies on radio waves that are easily absorbed by seawater, so an antenna must be above the surface to work. Even a few centimeters below the surface, GPS fails.
- **Sonar as an Alternative** [00:16] — Submarines can use sonar to map the seafloor and compare it to historical maps for navigation, but this is noisy and compromises stealth.
- **Fiber Optic Gyroscope** [00:28] — This device sends two beams of light in opposite directions. When the submarine rotates, the distance each beam travels changes, causing a phase mismatch that indicates rotation.
- **Accelerometer Design** [00:54] — A common accelerometer design uses a tiny weight attached to a thin beam of crystalline quartz. The weight's inertia under acceleration changes the beam's tension, altering its vibration frequency.
- **Computing Position** [01:19] — The computer calculates acceleration from the frequency changes and integrates it to determine velocity and position, tracking how far the submarine has moved.

### Conclusion

Submarines navigate by combining inertial sensors—gyroscopes for orientation and accelerometers for movement—to maintain accurate positioning without external signals, preserving stealth.

## Transcript

surface they can just stick out an antenna above the water to use GPS, but if the antenna is even a couple of centimeters below the surface, the GPS will stop working because it uses radio waves that are easily absorbed by
seawater. So, as an alternative, a submarine can actually use its sonar to map out the seafloor and then it can compare those maps to maps that were [music] made in the past. But, if you're trying to stay hidden in your submarine,
blasting loud sonar waves probably isn't a good idea. So, most submarines rely on something quieter. To work out which way the sub is pointing, they use something called a fiber optic gyroscope. What it does is it sends two beams of light
directions. [music] And then when the submarine turns, the rotation is going to change the distance each beam has to travel, meaning they're going to return slightly out of phase. And that tiny [music] mismatch can be used to work out
how much the submarine is rotating. And to work out how far a submarine has moved, [music] they use an acceler- -ometer. So, a common design uses a tiny weight that is attached to this thin beam of crystalline quartz, which is
[music] frequency. It's kind of like a guitar string. Then, when the submarine accelerates, the weight's inertia pulls or pushes on the beam, which puts it under tension or compression. And just like tightening a guitar string raises
its [music] pitch, that changes the frequency at which the quartz vibrates. >> the computer can work out the acceleration and from that the position acceleration and from that the position and how far the submarine has moved.