SONAR: Full form of SONAR is 'Sound Navigation and Ranging'. This technique is used by ships to detect the presence of submarines, icebergs, sunken ships etc. in the sea. SONAR is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. Sonar can also be used for determining the depth of the sea.

Determination of the depth of a sea using SONAR Let the time interval between transmission and reception of ultrasound signal be t and the speed of sound through sea-water be v. The total distance, 2d travelled by the ultrasound is 2d = v × t