[PSUBS-MAILIST] Underwater Navigation

[Alan via Personal_Submersibles]

I was looking at GPS receivers for my sub, mainly for estimating my
surface speed & determining my range based on my power consumption.
But it occurred to me that you could make a similar system to GPS for
underwater navigation.
Have 4 buoys placed around your intended dive location that have GPS
receivers & ultrasonic transmitters. With a bit of electronics the submarine 
could locate itself with trilateration ( as in GPS) in relation to the 4 bouys.
The 4 buoys in turn are locating their position off satellites & relaying this
information via the ultrasonic transmitters to the sub. This way the pilot
could use charts to plot where they were & where they had been. 
It wouldn't matter too much if the buoys were blown about or if they moved 
in the current as they would update their position.
It may be helpful for grid searching.
If you wanted to get more serious you could put small motors on the buoys
that automatically held them in position as Minnkota are now doing with
their radio controlled motor.
Or is this already done?
Alan