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 their attenuation in seawater is much less. Some marine mammals use sound waves to find food and to navigate. See also Acoustic 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 as active 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 electroacoustic transducer, which converts the electrical signal into a sound signal, which then radiates into the water. If the transducer is reciprocal 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 conventional depth 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 also Echo 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 and reverberation), 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 the audible 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 of kilometers against noisy targets under good conditions to a very few kilometers against quiet targets.
The sonobuoy consists of a small surface buoy with a hydrophone 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 also Sonobuoy; Ultrasonics.