Acronym for sound navigation and ranging. A method of detecting and locating underwater objects that operates on a similar principle to radar but the transmitted pulse is a burst of sound energy, usually ultrasonic, rather than radiofrequency waves.

A sonar system that is used to measure the depth of the sea bed by projecting the pulse vertically downwards is termed an echo-sounder. Any sonar system that detects submarines is termed asdic. One type of asdic employs echo-ranging, in which the location of the target is deduced from the time difference between the two echoes from simultaneously transmitted sonic and ultrasonic pulses.

The specially designed electro-acoustic transducer used to transmit the pulse in a sonar system is known as an underwater sound projector; the return echo is detected with a hydrophone.