Chris Groves – How submarines work out where they are when dived

Simon: The Navigation side, it’s kind of upside down for I guess people on the street. You have valleys and mountains but they’re under the water, or am I being completely fooled by thinking that way?

I guess Navigation now is all done through satellite navigation and GPS on the surface …

Simon: But there’s still a mapping of the … what do call the bit under … the seabed I guess.

Yes, the bottom topography, or the pethrimetry.

Simon: Pethrimetry?    

Pethrimetry is the is more around the speed of sound in water and the way that sound travels through that water. From a Navigation perspective, on the surface it’s pretty much identical to being in a surface ship, to navigate. Once you get under the surface, then you have two states effectively. You’re either at periscope depth or your below periscope depth. We call it ‘deep’. So, when we say we’ve gone deep in a submarine, it means you’ve gone below periscope depth. And periscope depth is a depth at which you can raise a periscope above the sea surface and see around you, but it’s also the depth at which you can raise any of the masts on the submarine so you’re communications antenna or your radar or your snort induction or your diesel exhaust mast, and so all the time you’re at periscope depth, you can effectively use the same navigation aids as you would do on the surface, so you’ve got a GPS antenna for example on the top of one of your periscopes.

You’ve also got the ability to raise radar and operate radar, so you can use radar for navigation and collision avoidance. Once you get below periscope depth, you lost that ability to be able to fix your position using normal means, so tell where you are from GPS for example, because the antenna’s below the surface, and so what you’re really then relying upon is your ability to navigate without those fixing aids so as long as you know your position when you leave periscope depth, then you can, knowing the speed you’re doing and the course that you’re doing at any stage, you can work out where you should be through a system of what’s called ‘dead reckoning’. What a Navigator would call dead reckoning, ‘cos all you’re doing is you’re calculating ‘I’ve been doing that course and that speed for a particular time.

Therefore, I must be there’. Just speed times distance, and then you have to apply to that the current or tidal stream that you’re experiencing or expecting to experience and so that gives you an estimated position, so if you’ve done a particular course and speed for a length of time, you can then tell what the vector for the tidal stream would have been for that same time.

You apply that to your dead reckoning position and you get an estimated position of where you are. And then, as a Submariner, you know that there are some errors in that calculation. Perhaps your log was telling you that you were doing 10 knots but it might be slightly inaccurate. Your gyro was telling you were steering a particular speed but it might be slightly inaccurate, and so what you do is you put in a number of errors which include your gyro and your log and also your calculation of tidal stream which could be wrong, to make sure that you can define what we call a ‘pool of errors’ so it’s a geographic puddle if you like of water which you must sit in. And as long as you can drive that puddle of water, that pool of errors, around the ocean without it touching any navigational hazards then you will stay safe. Fundamentally that’s how you do it manually.

Backing that up nowadays is an Inertial Navigation System which is an electronic automatic system which knows a start position and through accelerometers and gyros can then work out where you are at any given time to an error and we generally have back-to-back systems so two systems working back to back which are effectively checking each other.

So, that’s the Inertial Navigation System. And then we have a bunch of techniques for fixing our position when we’re dived, so when we can’t put up our traditional fixing methods, GPS and that sort of thing, then what we’re starting to do is use … you talked about bottom … the seabed. Well, that seabed is mapped pretty accurately, and is reflected on Admiralty charts and you know what the depths are for the position you’re in and then you can then use the bottom contours and your Echo Sounder to then try and fix your position using bottom topography.