Localized variation in the ocean´s transmission properties: Its drastic effect on a sonar display

A ship-positioning sonar system repeatedly experienced severe problems while operating in rough seas. The visual display for this system is a CRT screen with a bright spot that represents the position of an undersea beacon relative to a fixed point on the ship in the X-Y plane parallel to the sea surface. In operation, the bright spot is manipulated to the center of the screen by moving the ship in a compensating direction, the object being to keep the vessel stationary directly over the beacon. Difficulties in performance arose when the seas became very rough. The bright spot jumped erratically and sometimes took excursions off the screen entirely. This left the operator very uncertain of the ship´s position and caused a general loss of confidence in the system. At first the problem was thought to be a malfunction of the hardware that manifested itself when the ship pitched and rolled severely. However, a careful analysis of test data showed that the problem originated in the transmission medium itself. Because of the rough seas, aerated patches of water were introduced into the signal path between the beacon and the receiving hydrophones beneath the hull. The presence of this aerated water reduced the phase velocity and increased the attenuation of the beacon signal, accounting for the erratic behavior on the sonar display.