How can the crew of a submarine know the exact place they are in the World when the submarine is submerged? How can they navigate accurately knowing the position they are? Similarly, how can subs avoid collision with seamounts and rocky outcrops?
I was in the Navy in the 1980s, when the first half-dozen GPS satellites were in orbit. But even in the pre-GPS days we still had a pretty good idea of our position, using the Ship’s Inertial Navigation System (SINS). SINS actually dates back to the 1950s or so - it uses three very precise gyroscopes with their spin axes oriented in the X, Y, and Z directions to determine the submarine’s acceleration and movement. It’s not as good as GPS, but it gave us a pretty good idea as to where we were at any given time.
The first GPS satellites started going up in the 1980s - there were 6 up when I was on my submarine and as long as three were “visible” we could get a fix from them. In practice that meant that we had to plan our times at periscope depth (PD) so we could get a fix - submarines now can do it as easily as you can get a GPS location on your phone. As to how it’s done - you just take the boat to PD, stick a mast with a GPS antenna out of the water a few feet, and get your fix. Easy-peasy!
My boat had a number of masts - two periscope, a radio mast, radar, the snorkel mast, and one or two others that were more specialized. We also had radio antennas on one of our periscopes so we could copy some radio traffic when we were making a periscope observation with it. Anyhow - getting one’s GPS location is fairly straightforward, but even if the boat can’t go to PD to put up a mast they still have SINS.
The other part of your question - how do we know where the shallow water and seamounts are - is pretty straightforward as well. Thanks to a handful of satellites we’ve had a fairly good map of the seafloor for over a decade now. All the satellites do is to map the level of the ocean’s surface very, very precisely - but that is a reflection of the subsurface topography. If you have a seamount, for example, it’s pretty massive and it increases the local gravitational pull ever so slightly. This, in turn, pulls a little more water over top of the seamount. So by mapping the ocean’s surface with enough precision we can have a very good idea as to where the seamounts and shallow waters are. In addition to that, the world’s navies have been mapping the seafloor using sonar and even by sounding (lowering lines to the bottom and measuring the amount of line paid out) for centuries. (Seafloor mapping - Wikipedia)
The final part of the puzzle is to make sure that the submarine’s position - including any accumulated possible SINS errors - never comes close to an underwater hazard. If, for example, you’ve been running deep for, say, 24 hours then you’ll have a SINS position. We’ve been using SINS long enough to know how much the position might be off by for every hour it’s running, so you take that possible error, factor in the ship’s speed and the length of time since the last satellite fix, and that gives you the maximum possible error in your position. Use that distance to make a box around your submarine, and make sure not to let any seamounts come inside the box - you ought to be OK. U.S. Bathymetric and Fishing Maps
Every now and again a submarine will still be surprised (15 Years Ago, a U.S. Navy Submarine Ran Into a Mountain) - but it happens less and less frequently now than in the past.
