Or, 3-Dimensional Acoustic Positioning System.
3DAPS relies on the transmission and reception of sound waves to determine the position of a body in a medium. It is particularly useful in underwater situations, such as naval testing, because radio waves (and hence, radar) do not propagate well through water.
At least four sound emitters are placed orthogonally to each other around the area containing the body to be located. Inside the body is a microphone, hydrophone, or other reception device, and lots of other electronics such as an amplifier and high- and low-pass filters. Both the receiver and the emitters are hooked up to a timing device and a computer. By measuring the time (in nanoseconds, probably) between emission and reception of the signals from each emitter, fired at unique frequencies and in sequence, the exact position of the body can be determined.
The data collected results in a distance (via d = v * t) from each emitter - in actuality, a possible radius inside which the body is located. Four emitters are used because the intersection of four spheres is a point - the exact point where the body is located. It follows that using more than four emitters would serve to increase the accuracy of the measurement.