[00:02] 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
[00:16] 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,
[00:28] 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
[00:41] 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
[00:54] 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
[01:06] [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
[01:19] 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.