... floor) and bounce backup to the listener. Essentially, the listener is listening for and timing the echo. The listener knows how fast the sound travels through the water andso can quickly and easily ... two-way ranging. In the case of the diagram at left, the submarine sendsout a unique and recognizable sound (the “ping”) and measures the timeit takes to reach something (in the diagram, the sea ... fast light travels, for all intents andpurposes, we see the flash the instant it occurs. This is, conceptually, howGPS works. The difference is that GPS measures radio-wave transit timerather...