Abstract: An oscillator generates a fixed frequency ultrasonic output signal. After wer amplification this signal drives a transmitting transducer which projects an ultrasonic beam into water. Reflected signals are received by a receiving transducer and AGC preamplified. A chopper multiplies the received signal with the transmitted signal and generates an output signal if a target exists. To determine whether the target is a swimmer, this output signal is band pass filtered and amplified and then fed both to an automatic alarm and to a set of headphones. A human operator listening to the headphone output classifies the target and determines exact bearing.

Abstract: A line spectrum detector to distinguish motor vehicles acoustically in the resence of background noise. The detector senses periodic signals, such as the firing rates of internal combustion engines, by using a swept phase-lock loop spectrum analyzer and passes the sensed signals to a logic network which measures the frequency stability of the sensed signal to distinguish motor vehicles from aircraft vehicles and the like.