It was developed during World War I to counter the growing threat of submarine warfare, with an operational passive sonar system in use by 1918. The first recorded use of the technique was by Leonardo da Vinci in 1490 who used a tube inserted into the water to detect vessels by ear. The study of underwater sound is known as underwater acoustics or hydroacoustics. The acoustic frequencies used in sonar systems vary from very low ( infrasonic) to extremely high ( ultrasonic). The term sonar is also used for the equipment used to generate and receive the sound. Sonar may also be used for robot navigation, and SODAR (an upward-looking in-air sonar) is used for atmospheric investigations. Acoustic location in air was used before the introduction of radar. Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of "targets" in the water. Two types of technology share the name "sonar": passive sonar is essentially listening for the sound made by vessels active sonar is emitting pulses of sounds and listening for echoes. Sonar (sound navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.
Sonar image of the Soviet Navy minesweeper T-297, formerly the Latvian Virsaitis, which was shipwrecked on 3 December 1941 in the Gulf of Finland
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