sonobuoy

A sonobuoy is a buoy equipped with sonar sensors and a radio transmitter. It is used to detect underwater sounds and transmit them. In antisubmarine warfare, sonobuoys help fill the gaps in sonar coverage left by ships and submarines, allowing patrolling aircraft to track submarines by receiving their signals.12

Sonar, which stands for sound navigation ranging, is a technique that uses acoustic means to detect and determine the distance and direction of underwater objects. Sonar systems can be active, emitting sound waves and analyzing reflections, or passive, listening for sounds produced by targets like submarines.3