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Monday, March 23, 2015

How military submarines find their way around

It occurred to me that a person finding this blog after a text search might have noticed that a fair number of the posts are about submarines.  Random readers may not be familiar with military subs, and they might wonder how subs move around the ocean without banging into stuff, or how they know which way they are going, etc, etc.  Sometimes submarines *DO* bang into stuff, but that's the exception, not a normal daily event.

So in that spirit, I thought I would share what little I know about underwater navigation.  Confession time:  I worked in propulsion, not navigation, so I will have to find this information before I can post it.  I have a general idea, but I will use the internet for more clarity before exposing my ignorance!

First and foremost, military submarines do NOT have windows.  Windows are strictly for rescue and civilian research submarines.  During a battle, or depth charging, a window would certainly shatter, and then you would have a hole in the ship letting in water.  In any event, visibility through ocean water is only tens of feet, so the field of vision isn't very large.  This is why military subs don't have windows.

So how do you see out?  You don't!  The only time you can see out of a submarine is when it is near the surface, at periscope depth, with the periscope extended.  Below that minimum depth, where a modern submarine rarely operates, nobody can see a thing outside of the ship... so it makes navigating a bit challenging.  Technology has come to the rescue, however.

Submarines use a technique called "inertial navigation" to determine where they are.  This is a pretty cool technique that doesn't require any outside measurements, other than setting the initial coordinates and ship's heading into a computer.  Since inertial navigation doesn't rely on measuring outside sources for determining the ship's position, it cannot be spoofed or jammed.

How does it work?  First you need to know your exact location and bearing, which is provided by GPS.  This data is entered into a navigation computer.  The computer is connected to several very sensitive gyroscopes and accelerometers that produce a signal whenever the ship moves or changes direction.  These signals are averaged and interpreted by the navigation computer, which continously updates the ship's location.

Over time, errors inevitably creep into the inertial navigation system, because even very tiny errors of measurement from the gyros and accelerometers are compounded over time.  Therefore it is necessary for a submarine to get near the surface from time to time, and put up an antenna to get correct GPS coordinates, then input these into the navigation computer.

The mechanical portion of an Inertial Guidance System (gyroscopes and accelerometers) looks like this:

Pretty impressive!  Also it beats the heck out of surfacing and using one of these, like they on submarines back in WWII.

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