Underwater Acoustic Telemetry

An underwater telemeter system has been developed for the White Fish Authority which can provide the fishing trawler skipper with information concerning various features of the trawl he is towing, e.g., mouth opening, ambient conditions, etc. The system employs time-division multiplexing to provide four channels with the information being transmitted by modulating the PRF of pulses of 40 kHz carrier. The measurements made in the prototype system are 1) water temperature at the sea bed, 2) strain in the cables on the trawl, 3) height of the trawl opening, and 4) width of the trawl opening. The equipment described is: 1) A cylindrical shaped transmitter floated in the center of the trawl head-line, housing a thermistor for sensing temperature, and a pulsed echo sounder operating at 200 kHz for measuring the height of the trawl opening. 2) Two transponder units mounted on the wing ends of the trawl, measuring the width of the opening and carrying sensors for the strain in the towing cable. 3) Either a ship mounted or towed receiver followed by signal decoding circuitry and display.