SONAR

SONAR (underwater sound navigation ranging) can be either of the passive or active type. Passive sonars were developed first and rely upon listening for noise generated by the target vessel, usually submarines (however, submarines also use sonar to detect other ships). The most difficult aspect of passive sonar use is distinguishing target noise from that of the surrounding sea (referred to as ambient noise) and particularly that of the searching platform. Active sonars are popularly characterized by the famous ping known to anybody who has ever watched a Hollywood submarine movie. The ping is a sound wave generated by the searcher that is bounced back off the objects, thus giving the sonar operator a picture of the object in the path of the sound wave.

U.S. sonar development began before World War I when the Submarine Signal Company, formed in 1901, developed steam‐operated underwater warning bells that could be heard for up to 10 nautical miles. By 1912, warning bells were used to supplement the work of lighthouses in marking hazards to navigation off the coasts of North America, South America, Europe, and Asia.

In February 1917, the U.S. Navy Consulting Board created a Subcommittee on Submarine Detection. Two passive sonar detectors developed by a staff member of the Submarine Signal Company, Professor R. A. Fessenden, were installed on navy destroyers, but their performance proved disappointing.

World War II saw active sonar systems predominate in U.S. ships and submarines, in contrast to the Germans, who concentrated on large fixed passive array systems. The American approach helped mitigate the effect of ocean noise that proved such a problem with passive sonars. Navy General Board guidelines of 1938 called for two sonars per destroyer and one unit for lesser craft. However, wartime demands for escort vessels and the low rate of sonar production prevented these guidelines from being followed. Instead, the scarce equipment was put out among destroyer escorts. U.S. submarines typically carried a passive device along with a combined ranging and sounding set.

During the Cold War, passive developments included large arrays of hydrophones mounted conformally along submarine hulls to achieve very well defined and very long range receiving beams; systems for passive range finding; PUFFS (Passive Underwater Fire Control Feasibility Study, a short range triangulation device using three passive sonars mounted along the length of a submarine); and submarine‐towed arrays. The towed array came into use to mitigate the effect of a vessel's own noise upon passive sonar systems; it consists of a string of passive hydrophones towed at some distance behind the ship. A further advantage of the towed array is that it can be made as long as necessary to detect sounds with long (very low frequency) wavelengths.

Today's most advanced U.S. submarines, the SSN‐688I and the SSN‐21, use the AN/BSY‐1 integrated sonar and fire control system that includes both active and passive sonar types. In addition to MAD (magnetic anomaly detector) sensors (a means of locating submarines by detecting changes in the earth's magnetic fields caused by large metal objects), aircraft use small sonobuoys as a means of detecting submarines. Helicopters hover above the ocean surface and dip scanning sonars that emit a ping in all directions at once.
[See also Antisubmarine Warfare Systems; Destroyers and Destroyer Escorts.]

Bibliography

Norman Friedman , U.S. Naval Weapons Systems, 1982; repr. 1985, 1988.

David E. Michlovitz