Abstract: An acoustic distance measuring device using a predetermined frequency shift keyed bit pattern as the acoustic signal transmitted and received. A phase lock loop detector is provided. Variations in lag from the time required for the loop to lock are eliminated and the remainder of the FSK bit sequence is detected and accumulated in response to changes in the phase detector output. Preferably the FSK bit sequence is generated via radio frequency transmitter from a first location and reproduced and transmitted acoustically from a second station back to the first station.