Sonar (for SOun NAvigation and Ranging) is a technique that uses sound propagation (usually underwater) to navigate, communicate or to detect other vessels. There are two kinds of sonar—active and passive. Sonar may be used as a means of acoustic location. Acoustic location in air was used before the introduction of radar. Sonar may also be used in air for robot navigation while SODAR (an upward looking in-air sonar) is used for atmospheric investigations. The term sonar is also used for the equipment used to generate and receive the sound. The frequencies used in sonar systems vary from infrasonic to ultrasonic. The study of underwater sound is known as underwater acoustics or sometimes hydroacoustics.