Ill refer you to this excellent article re the AI sensors.
http://www.subsim.com/radioroom/showthread.php?t=104377
It was written for SH3 however the concepts are very much the same.
Quote:
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They can calculate your postion by using the time it takes for the sound waves to return.
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Time gives them range.
Early sonars could not determine position other than "we pointed it this way and got an echo so it must be this way"