Sonic detection and tracking system

Two submerged rotatable sets of transmitting and receiving transducers are longitudinally spaced along the side of a river to establish intersecting lateral sonar beams obliquely oriented at opposing 45 DEG angles relative to the primary river current. Each beam can be slewed in opposite directions through 45 DEG to parallel orientation with the current. The outputs of the transducer arrays are passed to respective doppler sonar detection units which produce alarm output signals when the normalized doppler frequency shift within at least one of a plurality of narrow frequency bands in each unit exceeds a predetermined level. The transmitting frequency of each array is varied with rotation of the array. If only one beam detects a target, a control circuit causes the other beam to slew until it acquires the target. With detection by both beams, the position of the target can be computed from the bearing angles of the arrays.