A portable, towed sonar system has been developed which is suited for a variety of tasks, such as the study of fish schools, bottom features, and reverberation. The towed platform consists of an active transducer mounted on an underwater wing. The depressing action of the wing permits towing at depths greater than 60 m at speeds up to 15 kn. The sonar beam can be pointed in a variety of directions by rotating the spherical transducer housing with respect to a conical fairing. A four-channel receiver permits signals from separate transducer quadrants to be recorded independently. Normally channel-to-channel phase information would be lost because of tape skew, but special signal-conditioning electronics are included to overcome this. A sensor package on the towed platform monitors the pitch, roll, turning rate, depth, speed, and water temperature. The attitude information is used to determine the exact orientation of the beam.

A new signal-processing technique, spectrum clipping, has been investigated. The spectrum clipper is a passive two-port nonlinear circuit that employs parametric excitation of vibrational modes in a magnetoelastic crystal. Spectrum clipping can be considered as a means of recovering periodic and nearly periodic signals from noise. signal-to-noise ratio (SNR) improvements of up to 30 dB are predicted, the largest improvement occuring for simple sinusoidal signals. Preliminary theoretical and experimental results suggest that spectrum clipping may conceivably provide a low-cost method of interference reduction for domestic communications systems.