# How do submarines know where they are?

Source: https://www.youtube.com/watch?v=_1G8nrmBKeY
Recap page: https://rapidrecap.app/video/_1G8nrmBKeY
Generated: 2026-10-01T18:04:01.312+00:00

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## The Gist

Submarines navigate deep underwater without GPS by using fiber optic gyroscopes to track rotation and vibrating quartz accelerometers to measure distance traveled.

## Quick Overview

Submarines rely on dead reckoning through specialized internal sensors rather than external radio waves when submerged. Because GPS signals fail in seawater and active sonar ruins stealth, crews use fiber optic gyroscopes and quartz accelerometers to calculate position accurately over long distances.

**Key Points:**
- Radio waves used by GPS are easily absorbed by seawater, causing signals to fail even a few centimeters below the surface.
- Submarines can use active sonar to map the seafloor, but this creates loud noises that compromise their stealth.
- Fiber optic gyroscopes track the submarine's rotation by sending two beams of light in opposite directions through a coiled fiber optic cable.
- When the submarine turns, rotation changes the travel distance of each light beam, making them return slightly out of phase.
- Accelerometers measure how far the submarine has moved by tracking changes in a tiny weight attached to a vibrating quartz beam.
- The quartz beam is kept constantly vibrating at its natural frequency, similar to a guitar string.
- Inertia from acceleration pulls or pushes on the quartz beam, altering its tension and changing its vibration frequency.
- A computer measures the tiny frequency shift in the quartz beam to calculate acceleration, overall distance, and current position.

![Screenshot at 1:00: Explanation of how accelerometers use vibrating quartz beams to measure submarine movement.](https://ss.rapidrecap.app/screens/_1G8nrmBKeY/00-01-00.jpg)

**Context:** Operating a massive military vessel completely submerged poses extreme navigation challenges because traditional satellite systems cannot penetrate water. Submarines must maintain strict stealth while crossing thousands of miles of ocean, requiring sophisticated internal navigation instruments that do not rely on external contact.

## Detailed Analysis

Submarines solve the extreme challenge of underwater navigation by utilizing sophisticated internal systems that avoid surface detection. While GPS works near the surface, radio waves fail immediately in seawater, and active sonar destroys tactical concealment by broadcasting loud noises. To remain hidden and precise, modern vessels employ inertial navigation systems centered on two core technologies. First, a fiber optic gyroscope shines dual light beams through a coiled cable to measure rotation through phase shifts. Second, a quartz accelerometer detects movement by measuring tension and frequency shifts on a constantly vibrating quartz beam affected by the vessel's inertia.

### The Problem with Surface Navigation

- Submarines can use standard GPS antennas only when operating directly at the water surface.
- Radio waves fail completely once the antenna drops even a few centimeters beneath the waves due to seawater absorption.
- Active sonar provides seafloor mapping capabilities by comparing past charts, but broadcasting loud noise destroys tactical stealth.

![Screenshot at 0:06: Discussion of GPS limitations and why radio waves fail in seawater.](https://ss.rapidrecap.app/screens/_1G8nrmBKeY/00-00-06.jpg)

### Tracking Rotation with Fiber Optic Gyroscopes

- Submarines utilize quiet internal sensors like fiber optic gyroscopes to determine heading and orientation.
- The system circulates two beams of light in opposite directions through a coiled fiber optic cable.
- Submarine turns alter the travel distance of each light beam, creating a measurable phase mismatch upon return.

![Screenshot at 0:43: Illustration of light beams returning out of phase during a turn.](https://ss.rapidrecap.app/screens/_1G8nrmBKeY/00-00-43.jpg)

### Measuring Distance with Vibrating Quartz Accelerometers

- Calculating distance traveled relies on a specialized accelerometer containing a tiny attached weight on a crystalline quartz beam.
- The quartz beam maintains a constant natural frequency vibration analogous to a plucked guitar string.
- Submarine acceleration generates inertia that pulls or pushes the weight, altering beam tension and shifting its vibration frequency for computer calculation.

![Screenshot at 1:15: Demonstration of how tension changes the vibration frequency of the quartz beam.](https://ss.rapidrecap.app/screens/_1G8nrmBKeY/00-01-15.jpg)