Relay transponder
This relay transponder "pings" continuously. Its signals are bounced off an acoustic net of transponders placed nearby on the seafloor. The time lag of each return determines where on the bottom ROPOS is at any given time. The tracking is integrated with differential GPS, the ship's heading, and wind speed and direction. All told, this navigation system enables ROPOS pilots to know where their charge is to within 20 feet.

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