Abstract: A sonar, lidar, or radar system generates an alarm signal if a child enters a swimming pool when the system is enabled, and includes multiple safeguards against sounding false alarms due to wind-activated waves in the pool or self interference arising from multi-path propagation of the sonar signals. An acoustic or electromagnetic receiver having a narrow bandwidth is employed to demodulate a composite signal spectrum produced by a target object such as a child and signals generated by wind-activated waves. It also performs an envelope amplitude monitoring function to suppress false alarms that might be generated by low amplitude envelopes of the type created by self interference. In a first embodiment, an alarm signal is generated only if the demodulating function and the envelope amplitude monitoring functions both indicate a legitimate alarm.