Sonar - SOund, NAvigation and Ranging
The word Sonar is an American term first used in World War II , it is an acronym for SOund, NAvigation and Ranging. The British also call Sonar, ASDICS, which stands for Anti-Submarine Detection Investigation Committee. Later developments of Sonar included the echo sounder, or depth detector, rapid-scanning Sonar, side-scan Sonar, and WPESS (within-pulseectronic-sector-scanning) Sonar.
There are two major kinds of sonar, active and passive.
Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections of the pulse. The pulse may be at constant frequency or a chirp of changing frequency. If a chirp, the receiver correlates the frequency of the reflections to the known chirp. The resultant processing gain allows the receiver to derive the same information as if a much shorter pulse of the same total power were emitted. In general, long-distance active sonars use lower frequencies. The lowest have a bass "BAH-WONG" sound. To measure the distance to an object, one measures the time from emission of a pulse to reception. Passive sonars listen without transmitting. They are usually military (although a few are scientific). Passive sonar systems usually have large sonic databases. A computer system frequently uses these databases to identify classes of ships, actions (i.e. the speed of a ship, or the type of weapon released), and even particular ships.