Sonar, an acronym for Sound Navigation and Ranging, is a remote sensing device used for detecting and locating sound waves under water. It has many uses. Naval uses include detection of submarines, and underwater navigation. Civilian uses include water depth measurement, locating fish schools and mapping the ocean floor. It is also used for locating objects such as pipe lines or ship wrecks lying on the ocean floor, for measuring water current profiles and determining characteristics of ocean bottom sediments.
There are two types of sonar; active (echo-location) and passive. An active sonar sends out a signal in a narrow beam which propagates at a speed of about 1500 m/s into water. If there is an object in the path of the beam it reflects a part of the sound energy back to the sonar receiver which detects the echo. By measuring the time taken by the echo to return, the distance of the target can be determined.
A passive sonar simply listens for sounds given off by objects such as a ship’s propellar. It can determine the direction in which an object is located but not its distance. But it has one advantage; it does not give off any sound that another sonar unit might detect. These sonars also help in identifying a target because the noise a ship gives off may reveal what type of ship it is. Submarines generally use passive sonar for locating enemy ships.