Acronym for "sound navigation and ranging". A system using reflected sound waves to determine the position of some target.

Passive Sonar
The noise produced by the target is observed using a reciever or number of receivers from which the location can be calculated by triangulation.

Active Sonar
A pulse is emitted at the transmitter that travels to the target. A fraction of the signal is reflected by the target and this propagates back to the receiver. The time taken for the round trip is used to calculate the distance to the target. The received level is used to indicate the type of target (eg size or density).

See also: Sound.

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