The VDS uses large transducers that are towed from a surface vessel on a cable with an adjustable scope. As the extent of the shadow zones decreases when the operational depth of the sonar increases, a variable depth sonar (VDS) has turned out to be very useful. In deeper waters the negative sound velocity gradient causes downward refraction of the signals and the formation of shadow zones, where submarines may hide. The performance of a hull-mounted sonar on a surface vessel is strongly dependent on the propagation conditions for its acoustic signals near the sea surface. On submarines the active sonar is frequently considered to be the secondary sonar as the use of the active sonar will inform the submarine environment about the presence and position of the submarine. The frequencies used by active sonar range from a few kilohertz and down to and below 1 kHz, and the active sonar can be hull-mounted in a dome below the hull of a surface vessel, be hull-mounted in a bow bulb on a surface vessel or a submarine, or be in a “fish” towed at variable depths behind a surface vessel.
The active sonar of surface vessels is normally used for detection, surveillance, tracking, and identification of submarines. Bjørnø, in Applied Underwater Acoustics, 2017 14.12.2 Active Sonar
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