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Sonar
A remote sensing technique or device that uses sound waves to detect, locate, and sometimes identify objects in water. The term is an acronym for sound navigation and ranging. There are many applications, using a wide variety of equipment. Naval uses include detection of submarines, sea mines, torpedoes, and swimmers; torpedo guidance; acoustic mines; and navigation. Civilian uses include determining water depth; finding fish; mapping the ocean floor; locating various objects in the ocean, such as pipelines, wellheads, wrecks, and obstacles to navigation; measuring water current profiles; and determining characteristics of ocean bottom sediments. Sound waves rather than electromagnetic waves (for example, radar and light) are used in these applications because theirattenuation inseawater is much less. Some marine mammals use sound waves to find food and to navigate. See alsoAcoustic mine;Acoustic torpedo;Antisubmarine warfare;Echolocation;Marine geology;Marine navigation;Underwater navigation;Underwater sound.
There are two generic types of sonar: active (echolocation) and passive. An active sonar projects a signal (typically a short pulse of sound) into the water in a narrow beam, which propagates at a speed of about 1500 m/s (5000 ft/s). If there is an object (target) in the beam, it reflects a fraction of the sound energy to the sonar, which detects the echo. By measuring the elapsed time between projection and reception, the range to the target can be computed (range = sound speed × travel time ÷ 2).
Direction to the target is determined from the orientation of the sound beam at the time of reception.Passive sonar does not radiate sound but depends on detecting sounds radiated by targets such as submarines and ships. Passive sonar determines direction to a target in the same manner asactive sonar, but range determination is more difficult.
In an elementary active pulse sonar, a pulse signal of certain frequency and duration is generated, amplified, and sent to an electroacoustictransducer, which converts the electrical signal into a sound signal, which then radiates into the water. If the transducer isreciprocal in character (typically the case), it also can be used to sense (detect) the returning echoes. The receiver amplifies the weak echoes and measures the range to each target, as well as the orientation of the receiving beam at the time of reception. This information is displayed in some form of range-direction plot.
Most active sonar transducers are mounted on the hulls of submarines or near the keels of surface ships. Sometimes, transducers are towed at a water depth that provides better operation. There are three basic transducer orientations. In the conventionaldepth sounder, the sound beam is directed downward. Echoes are reflected from the ocean bottom (and from fish that may be in the beam), and the depth of the ocean beneath the sonar can be determined. In the side-scan sonar configuration, the beam is oriented to the side of the ship (normal to the direction of travel) and (usually) slightly downward. As the ship moves forward, a volume of water to the side of the ship is searched. Generally, two sonars are used, one searching to the right and one to the left. Side-scan sonars are well suited to search at a constant speed and along straight lines, such as in mapping the ocean bottom and in general searches of an area. The third, and most popular, sonar configuration involves rotating the sound beam about the vertical axis to search (scan) a sector of the water centered on the sonar platform. See alsoEcho sounder.
The range at which a target can be detected depends on the strength of the projected signal (source level), propagation losses to the target and return, reflection characteristics of the target (target strength), and sensitivity of the receiver. Also, the target echo must be stronger than various masking signals (noise andreverberation), which also are received by the sonar. An important sonar performance characteristic is its ability to locate a target accurately and determine whether an echo is from a single target or from several targets close together. The uncertainty in the direction to the target is approximately the width of the receiving beam.
Passive sonars are used primarily to detect submarines and, to a lesser extent, surface ships. Because passive sonar does not radiate any sound that would reveal its location, it is the primary sensor used by submarines. The major weakness is that it cannot directly measure range to a target. To determine target location, the sonar must take bearings on a target from different locations. Passive sonars depend on detecting noise radiated by targets, a mixture of sounds generated by propellers and hull vibrations (caused by motors, engines, pumps, and hydrodynamic forces). The noise has a continuous spectrum and discrete tones related to rotational speeds of propellers, engines, and so forth. By analysis of the received signals the sonar often can identify the type of target. Most of the radiated energy is in theaudible frequency band and decreases in intensity with increasing frequency.
Most passive sonars use large receiving transducer arrays in order to achieve high sensitivity, discriminate against ambient noise, and determine precisely the direction to a target. In submarines, these arrays may be recessed into the structure or mounted on the hull. Submarines also may tow long slender line arrays. A number of very large fixed receiving arrays, placed on the ocean floor with cables running to shore stations, constantly observe strategically important ocean areas. Detection ranges for large passive sonars vary from hundreds ofkilometers againstnoisy targets under good conditions to a very few kilometers against quiet targets.
The sonobuoy consists of a small surface buoy with ahydrophone array suspended beneath the water. Sounds received by the array are telemetered by radio link to an aircraft overhead. (There are also active sonobuoys.) Sonobuoys have relatively short detection ranges and are used primarily in tactical situations. See alsoSonobuoy;Ultrasonics.
US Military History Companion

Home >Library >Military >US Military History CompanionSONAR
(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, usuallysubmarines (however, submarines also use sonar to detect other ships). The most difficult aspect ofpassive 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 aHollywood 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, andAsia.
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 sawactive 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 theCold 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 rangetriangulation 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 (magneticanomaly 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 alsoAntisubmarine Warfare Systems;Destroyers and Destroyer Escorts.]
Bibliography
Norman Friedman, U.S. Naval Weapons Systems, 1982; repr. 1985, 1988
US Military Dictionary

Home >Library >Military >US Military Dictionarysonar
[?sō?när]
?sō?närn. 1. a system for the detection of objects under water and for measuring the water’s depth by emitting sound pulses and detecting or measuring their return after being reflected.
2. an apparatus used in this system.
See the Introduction, Abbreviations and Pronunciation for further details.
Britannica Concise Encyclopedia

Home >Library >Reference >Britannica Concise Encyclopediasonar
Technique for detecting and determining the distance and direction of underwater objects by tracking acoustic echoes. The name derives from sound navigation ranging. Sound waves emitted by or reflected from an object are detected by sonar apparatus and analyzed for information. In active sonar a sound wave is generated that spreads outward and is reflected back by a target object. Passive systems consist simply of receiving sensors that pick up the noise produced by the target (such as a submarine or torpedo). A third kind of sonar, used in communication systems, requires a projector and receiver at both ends. Sonar was first used to detect submarines in 1916. Modern nonmilitary uses include fish finding, depth sounding, mapping of the ocean floor, Doppler navigation (seeDoppler effect), and searching for wrecks or other objects in the oceans.
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Columbia Encyclopedia

Home >Library >Reference >Columbia Encyclopediasonar (sō‘när) , device used underwater for locating submerged objects and for submarine communication by means of sound waves. The term sonar is an acronym for sound navigation ranging. The main component of sonar equipment is anelectroacoustic transducer that is in direct contact with the water. It is suspended from the hull of a ship or on a cable from a low-flying helicopter. The transducer converts electric energy into acoustic energy (thus acting as a projector), much as does a loudspeaker, and converts acoustic energy into electric energy (serving as ahydrophone), as does a microphone. A pulse of electric energy vibrates the diaphragm of the projector, sending sound waves through the water. These waves are concentrated into a sound beam, which scans the water when the projector is rotated. After the sound wave is emitted, the projector is converted into a hydrophone and listens for an echo. The cycle is repeated periodically. A returning echo is converted into an electric current by the transducer and may be interpreted (for range, bearing, and the nature of the target) aurally or by a cathode-ray tube, as is done with radar signals. The various types of sonar in use can be put into three classes: direct listening, communications, and echo ranging. In direct listening, the object under observation generates the sounds that are received. In communications and echo ranging the sonar must generate its own signals. Sonar operates in the 10- to 50-kilocycle acoustical frequency range. It is used for communication between submerged submarines or between a submarine and a %2