sonar

Method of locating underwater objects by the reflection of ultrasonic waves. The time taken for an acoustic beam to travel to the object and back to the source enables the distance to be found since the velocity of sound in water is known. Sonar devices, or echo sounders, were developed in 1920, and are the commonest means of underwater navigation.

The process is similar to that used in radar. During World War I and after, the Allies developed and perfected an apparatus for detecting the presence of enemy U-boats beneath the sea surface by the use of ultrasonic echoes. In the US it was called sonar. In Britain it was first named ASDIC (from anti-submarine - or Allied submarine - detection investigation committee), but in 1963 the name was changed to sonar, to accord with Nato practice.