Sonar (for SOund NAvigation and Ranging) is a technique that uses sound propagation (usually underwater) to navigate, communicate or to detect other vessels. Photographs Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies Navigation is the process of reading and controlling the movement of a craft or vehicle from one place to another There are two kinds of sonar—active and passive. Sonar may be used as a means of acoustic location. Acoustic location is the art and science of using Sound to determine the distance and direction of something Acoustic location in air was used before the introduction of radar. Acoustic location is the art and science of using Sound to determine the distance and direction of something Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships Sonar may also be used in air for robot navigation while SODAR (an upward looking in-air sonar) is used for atmospheric investigations. SODAR (SOnic Detection And Ranging or sodar is a meteorological instrument also known as a Wind profiler which measures the scattering of sound 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. Infrasound is Sound with a Frequency too low to be heard by the human Ear. The study of underwater sound is known as underwater acoustics or sometimes hydroacoustics. Underwater acoustics is the study of the propagation of Sound in Water and the interaction of the mechanical waves that constitute sound with the water and its boundaries Hydroacoustics is a general term for the study and application of sound in water

History

Contents 1 History 2 Performance factors 2.1 Sound propagation 2.2 Reverberation 2.3 Target characteristics 2.4 Countermeasures 3 Active sonar 3.1 Transponder 3.2 Performance prediction 3.3 Marine animals 3.3.1 Adverse effects 3.3.2 Mitigation methods 3.4 Hand-held sonar for use by a diver 4 Passive sonar 4.1 Identifying sound sources 4.2 Noise limitations 4.3 Performance prediction 5 Warfare 5.1 Anti-submarine warfare 5.2 Torpedoes 5.3 Mines 5.4 Mine countermeasures 5.5 Submarines 5.6 Aircraft 5.7 Underwater communications 5.8 Ocean surveillance 5.9 Underwater security 5.10 Hand-held sonar 5.11 Intercept sonar 6 Civil applications 6.1 Fisheries 6.2 Echo sounding 6.3 Net location 6.4 Ship velocity measurement 6.5 ROV and UUV 6.6 Vehicle location 7 Scientific applications 7.1 Biomass estimation 7.2 Wave measurement 7.3 Water velocity measurement 7.4 Bottom type assessment 7.5 Bottom topography measurement 7.6 Sub-bottom profiling 7.7 Synthetic aperture sonar 7.8 Parametric sonar 8 See also 9 Notes 10 References 11 Bibliography 12 External links

Although some animals have probably used sound for communication and object detection for millions of years, use by humans in the water is initially recorded by Leonardo Da Vinci in 1490: a tube inserted into the water was said to be used to detect vessels by placing an ear to the tube. Leonardo di ser Piero da Vinci ( April 15 1452 – May 2 1519 was an Italian Polymath, having been a scientist Mathematician, Engineer

In the 19th century an underwater bell was used as an ancillary to lighthouses to provide warning of hazards. A lighthouse is a Tower, building or framework designed to emit light from a system of lamps and lenses or in older times from a fire and used as an

The use of sound to 'echo locate' underwater in the same way as bats use sound for aerial navigation seems to have been prompted by the Titanic disaster of 1912. Construction The Titanic was a White Star Line ocean liner built at the Harland and Wolff shipyard in Belfast, Ireland The world's first patent for an underwater echo ranging device was filed at the British Patent Office by English meteorologist Lewis Richardson, one month after the sinking of the Titanic, and a German physicist Alexander Behm obtained a patent for an echo sounder in 1913. A patent is a set of Exclusive rights granted by a State to an inventor or his assignee for a fixed period of time in exchange for a disclosure of an The UK Intellectual Property Office, or UK-IPO is the operating name of what was until April 2 2007 called The Patent Office. Alexander Behm ( 11 November 1880 in Sternberg ( Mecklenburg) – 22 January 1952 in Tarp ( Schleswig-Flensburg Canadian Reginald Fessenden, while working for the Submarine Signal Company in Boston, built an experimental system beginning in 1912, a system later tested in Boston Harbor, and finally in 1914 from the U. Reginald Aubrey Fessenden ( October 6, 1866 &ndash July 22, 1932) born in East Bolton, Quebec, Canada, was S. Revenue (now Coast Guard) Cutter Miami on the Grand Banks off Newfoundland Canada. Newfoundland — ˈn(jufənˌlænd (Terre-Neuve Talamh an Éisc — is a large island 15 km off the east coast of In that test, Fessenden demonstrated depth sounding, underwater communications (Morse Code) and echo ranging (detected an iceberg at two miles (3 km) range). The so-called Fessenden oscillator, at ca. Oscillation is the repetitive variation typically in Time, of some measure about a central value (often a point of Equilibrium) or between two or more different states 500 Hz frequency, was unable to determine the bearing of the berg due to the 3 meter wavelength and the small dimension of the transducer's radiating face (less than 1 meter in diameter). The ten Montreal-built British H class submarines launched in 1915 were equipped with a Fessenden oscillator. Montreal, or Montréal in French ( pronounced in French, in English) is the largest city in the Canadian province of Quebec Boats Group 1 Group 1 was built in Canada at the Canadian Vickers Yards in Montreal before being transported across the Atlantic and deployed ^[1]

During World War I the need to detect submarines prompted more research into the use of sound. World War I (abbreviated WWI; also known as the First World War, the Great War, and the War to End All A submarine is a Watercraft that can operate independently below water as distinct from a Submersible that has only limited underwater capability The British made early use of underwater hydrophones, while the French physicist Paul Langevin, working with a Russian émigré electrical engineer, Constantin Chilowski, worked on the development of active sound devices for detecting submarines in 1915 using quartz. Paul Langevin (23 January 1872 &ndash 19 December 1946 was a prominent French Physicist who developed Langevin dynamics and the Year 1915 ( MCMXV) was a Common year starting on Friday (link will display the full calendar of the Gregorian calendar (or a Common year Although piezoelectric and magnetostrictive transducers later superseded the electrostatic transducers they used, this work influenced future designs. Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to Electrostatics is the branch of Science that deals with the Phenomena arising from what seems to be stationary Electric charges Since Classical Lightweight sound sensitive plastic film and fibre optics have been used for hydrophones (acousto-electric transducers for in-water use), while Terfenol-D and PMN (lead magnesium niobate) have been developed for projectors. A hydrophone (Greek "hydro" = "water" and "phone" = "sound" is a Microphone designed to be used underwater for recording or listening Terfenol-D is an Alloy of the formula Tb (03 Dy (07 Fe (19 developed in the 1950's at the Naval Ordnance Laboratory in America Piezoelectric composite materials are available from several manufacturers including Morgan Electro Ceramics. Morgan Electro Ceramics (MEC is a subsidiary of The Morgan Crucible Company plc, MEC forms part of the Morgan Technical Ceramics (MTC group

In 1916, under the British Board of Invention and Research, Canadian physicist Robert William Boyle took on the active sound detection project with A B Wood, producing a prototype for testing in mid-1917. Robert William Boyle (born at Carbonear Newfoundland October 2nd 1883 died at London England April 18th 1955 was a Canadian Physicist and Dr Albert Beaumont Wood OBE DSc ( 1890 - 19 July 1964) was a British physicist, known for his pioneering work in the field of This work, for the Anti-Submarine Division, was undertaken in utmost secrecy, and used quartz piezoelectric crystals to produce the world's first practical underwater active sound detection apparatus. To maintain secrecy no mention of sound experimentation or quartz was made - the word used to describe the early work ('supersonics') was changed to 'ASD'ics, and the quartz material 'ASD'ivite. From this came the British acronym ASDIC. In 1939, in response to a question from the Oxford English Dictionary, the Admiralty made up the story that the letters stood for 'Allied Submarine Detection Investigation Committee', and this is still widely believed, though no committee bearing this name has ever been found in the Admiralty archives. The Oxford English Dictionary ( OED) published by the Oxford University Press (OUP is a comprehensive Dictionary of the English ^[2]

By 1918, both the U.S. and Britain had built active systems, though the British were well in advance of the US. Year 1918 ( MCMXVIII) was a Common year starting on Tuesday (link will display the full calendar of the Gregorian calendar (or a Common The United States of America —commonly referred to as the The United Kingdom of Great Britain and Ireland was the formal name of the United Kingdom from 1 January 1801 until 12 April 1927 They tested their ASDIC on HMS Antrim in 1920, and started production in 1922. The 6th Destroyer Flotilla had ASDIC-equipped vessels in 1923. An anti-submarine school, HMS Osprey, and a training flotilla of four vessels were established on Portland in 1924. A flotilla (from Spanish meaning a flota of small ships and this from French flotte) or naval flotilla, is a formation of small The Isle of Portland ( is a limestone tied island long by wide in the English Channel. The U. S. Sonar QB set arrived in 1931.

By the outbreak of World War II, the Royal Navy had five sets for different surface ship classes, and others for submarines, incorporated into a complete anti-submarine attack system. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including The Royal Navy of the United Kingdom is the oldest of the British armed services (and is therefore known as the Senior Service) The effectiveness of early ASDIC was limited by the use of the depth charge as an anti-submarine weapon. Depth Charge is a character in the Beast Wars: Transformers universe This required an attacking vessel to pass over a submerged contact before dropping charges over the stern, resulting in a loss of ASDIC contact in the moments prior to attack. The hunter was effectively firing blind, during which time a submarine commander was able to take evasive action. This situation was remedied by using several ships cooperating and by the adoption of "ahead throwing weapons", such as Hedgehog and later Squid, which projected warheads at a target ahead of the attacker and thus still in ASDIC contact. For the area denial weapon see Czech hedgehog. The Hedgehog was an Anti-submarine weapon developed by the Royal Navy The Squid was a World War II Ship -mounted anti- Submarine Weapon. Developments during the war resulted in British ASDIC sets which used several different shapes of beam, continuously covering blind spots. Later, acoustic torpedoes were used.

At the start of WWII British ASDIC technology was transferred for free to the US. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including Research on ASDIC and underwater sound was expanded in the UK and in the US. Many new types of military sound detection were developed. These included sonobuoys, first developed by the British in 1944, dipping/dunking sonar and mine detection sonar. A sonobuoy (a Portmanteau of Sonar and Buoy) is a relatively small (typically 4⅞ inches or 124 mm in diameter and 36 inches or 910 mm long expendable This work formed the basis for post war developments related to countering the nuclear submarine. A nuclear submarine is a Submarine powered by Atomic energy. Previously conventional submarines used diesel engines that required air for moving on the Work on sonar had also been carried out in the Axis counties, notably in Germany, which included countermeasures. At the end of WWII this German work was assimilated by Britain and the US. Sonars have continued to be developed by many countries, including Russia, for both military and civil uses. In recent years the major military development has been the increasing interest in low frequency active systems.

In World War II, the Americans used the term SONAR for their systems, coined as the equivalent of RADAR. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships In 1948, with the formation of NATO, standardisation of signals led to the dropping of ASDIC in favor of SONAR for all NATO countries. The North Atlantic Treaty

Performance factors

The detection, classification and localisation performance of a sonar depends on the environment and the receiving equipment, as well as the transmitting equipment in an active sonar or the target radiated noise in a passive sonar.

Sound propagation

Sonar operation is affected by variations in sound speed, particularly in the vertical plane. Sound is a vibration that travels through an elastic medium as a Wave. Sound travels more slowly in fresh water than in sea water, though the difference in speeds between fresh and salt water is small. Freshwater is a word that refers to bodies of water such as Ponds lakes rivers and streams containing low concentrations of dissolved Salts and other Total dissolved Seawater is Water from a Sea or Ocean. On average seawater in the world's oceans has a Salinity of about 3 In all water sound speed (sometimes called velocity though this is incorrect) is determined by its bulk modulus and mass density. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different The bulk modulus is affected by temperature, dissolved impurities (usually salinity), and pressure. Salinity is the Saltiness or dissolved salt content of a body of Water. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface The density effect is small. The speed of sound (in feet per second) is approximately equal to:

4388 + (11. Sound is a vibration that travels through an elastic medium as a Wave. 25 × temperature (in °F)) + (0. 0182 × depth (in feet)) + salinity (in parts-per-thousand ).

This is an empirically derived approximation equation that is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. A central concept in Science and the Scientific method is that all Evidence must be empirical, or empirically based that is dependent on evidence Ocean temperature varies with depth, but at between 30 and 100 meters there is often a marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. The thermocline (sometimes metalimnion) is a thin but distinct layer in a large body of water such as an ocean or lake in which temperature changes more rapidly with depth This can frustrate sonar, for a sound originating on one side of the thermocline tends to be bent, or refracted, through the thermocline. Refraction is the change in direction of a Wave due to a change in its Speed. The thermocline may be present in shallower coastal waters. However, wave action will often mix the water column and eliminate the thermocline. Water pressure also affects sound propagation. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Increased pressure increases the sound speed, which causes the sound waves to refract away from the area of higher sound speed. The mathematical model of refraction is called Snell's law. In Optics and Physics, Snell's law (also known as Descartes' law or the law of refraction) is a formula used to describe the relationship

Sound waves that are radiated down into the deep ocean bend back up to the surface in great arcs due to the increasing pressure (and hence sound speed) with depth. The ocean must be at least 6000 feet (1850 meters) deep, or the sound waves will echo off the bottom instead of refracting back upwards, and the reflection loss at the bottom reduces performance. Under the right conditions these sound waves will then be focused near the surface and refracted back down and repeat another arc. Each focus at the surface is called a convergence zone (CZ). This CZ forms an annulus about the sonar. The distance and width of the CZ depends on the temperature and salinity of the water. In the North Atlantic, for example, CZs are found approximately every 33 nautical miles (61 km), depending on the season. A nautical mile or sea mile is a unit of Length. It corresponds approximately to one minute of Latitude along any meridian. Sounds that can be heard from only a few miles in a direct line can therefore also be detected hundreds of miles away. With powerful sonars the first, second and third CZ are fairly useful; further out than that the signal is too weak, and thermal conditions are too unstable, reducing the reliability of the signals. The signal is naturally attenuated by distance, but modern sonar systems are very sensitive, i. e. can detect despite low signal-to-noise ratios.

If the sound source is deep and the conditions are right, propagation may occur in the 'deep sound channel'. This provides extremely low propagation loss to a receiver in the channel. This is because of sound trapping in the channel with no losses at the boundaries. Similar propagation can occur in the 'surface duct' under suitable conditions. However in this case there are reflection losses at the surface.

In shallow water propagation is generally by repeated reflection at the surface and bottom, where considerable losses can occur.

Sound propagation is also affected by absorption in the water itself as well as at the surface and bottom. Absorption refers to the absorption of sound waves by a material This absorption is frequency dependent, with several different mechanisms in sea water. Thus sonars required to operate over long ranges tend to utilise low frequencies to minimise absorption effects.

The sea contains many sources of noise that interfere with the desired target echo or signature. The main noise sources are due to waves and shipping. A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. Shipping is physical process of Transporting goods and Cargo. The motion of the receiver through the water can also cause low frequency noise, which is speed dependent.

Reverberation

When active sonar is used, scattering occurs from small objects in the sea as well as from the bottom and surface. This can be a major source of interference but does not occur with passive sonar. This scattering effect is different from that in room reverberation which is a reflection phenomenon. An analogy for reverberation is the scattering of a car's headlights in fog or mist. A high-intensity pencil beam will penetrate the fog; main headlights are less directional and result in "white-out" where the returned reverberation dominates. Similarly, to overcome reverberation, an active sonar needs to transmit in a narrow beam.

Target characteristics

The target of a sonar, such as a submarine, has two main characteristics that influence the performance of the sonar. For active sonar it is its sound reflection characteristics, known as its target strength. For passive sonar the target's radiated noise characteristics are critical. The radiated spectrum in general will consist of an unresolved continuum of noise with spectral lines in it, the lines being used for classification.

Echoes are also obtained from other objects in the sea such as whales, wakes, schools of fish and rocks.

Countermeasures

Active (powered) countermeasures may be launched by a submarine under attack to raise the noise level and/or provide a large false target. Passive (ie non-powered) countermeasures include mounting noise generating devices on isolating devices and coating the hull of submarines.

Active sonar

Active sonar uses a sound transmitter and a receiver. When the two are in the same place it is monostatic operation. When the transmitter and receiver are separated it is bistatic operation. When more transmitters (or more receivers) are used, again spatially separated, it is multistatic operation. Most sonars are used monostatically with the same array often being used for transmission and reception, though when the platform is moving it may be necessary to consider a single transmitter/receiver as being operated bistatically. Active sonobuoy fields may be operated multistatically.

Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections (echo) of the pulse. In Medicine, a person's pulse is the throbbing of their arteries. Reflection is the change in direction of a Wave front at an interface between two different media so that the wave front returns into the medium from which In Audio signal processing and Acoustics, an echo (plural echoes) is a reflection of sound arriving at the listener some time after the direct This pulse of sound is generally created electronically using a Sonar Projector consisting of a signal generator, power amplifier and electro-acoustic transducer/array, possibly with a beamformer. Sonar (which started as an Acronym for sound navigation and ranging) is a technique that uses Sound propagation (usually underwater to navigate However, it may be created by other means, e. g. chemically using explosives or by using heat sources in thermoacoustics. Thermoacoustic hot air engines ( Sonic heat pump and refrigeration or thermoacoustic heat pump and refrigeration) of which nearly all are thermoacoustic stirling

To measure the distance to an object, the time from transmission of a pulse to reception is measured and converted into a range by knowing the speed of sound. To measure the bearing, several hydrophones are used, and the set measures the relative arrival time to each, or with an array of hydrophones, by measuring the relative amplitude in beams formed through a process called beamforming. In Navigation, a bearing is the direction one object is from another object A hydrophone (Greek "hydro" = "water" and "phone" = "sound" is a Microphone designed to be used underwater for recording or listening Beamforming is a Signal processing technique used in sensor arrays for directional signal transmission or reception Use of an array reduces the spatial response so that to provide wide cover multibeam systems are used. Multibeam Echosounders also known as Swathe or Swath echosounders originated in the late 1950s The target signal (if present) together with noise is then passed through various forms of signal processing, which for simple sonars may be just energy measurement. Signal processing is the analysis interpretation and manipulation of signals Signals of interest include sound, images, biological signals such as It is then presented to some form of decision device that calls the output either the required signal or noise. This decision device may be an operator with headphones or a display, or in more sophisticated sonars this function may be carried out by software. Further processes may be carried out to classify the target and localise it, as well as measuring its velocity.

The pulse may be at constant frequency or a chirp of changing frequency (to allow pulse compression on reception). Frequency is a measure of the number of occurrences of a repeating event per unit Time. A chirp is a signal in which the Frequency increases ('up-chirp' or decreases ('down-chirp' with time Pulse compression is a Signal processing technique mainly used in Radar, Sonar and Echography to augment the range resolution as Simple sonars generally use the former with a filter wide enough to cover possible Doppler changes due to target movement, while more complex ones generally include the latter technique. Today, pulse compression is usually achieved using digital correlation techniques. Military sonars often have multiple beams to provide all-round cover while simple ones only cover a narrow arc. Originally the latter was often scanned around mechanically but this was a slow process.

Particularly when single frequency transmissions are used, the Doppler effect may be used to measure the radial speed of a target. The difference in frequency between the transmitted and received signal is measured and converted into a velocity. Since Doppler shifts can be introduced by either receiver or target motion, allowance has to be made for the radial speed of the searching platform.

One useful small sonar looks roughly like a waterproof flashlight. One points the head into the water, presses a button, and reads a distance. Another variant is a "fishfinder" that shows a small display with shoals of fish. A fishfinder is a type of fathometer, both being specialized types of Echo sounding systems a type of Active SONAR. A shoal or sandbar (also called sandbank) is a somewhat Linear Landform within or extending into a body of Water, Some civilian sonars approach active military sonars in capability, with quite exotic three-dimensional displays of the area near the boat. However, these sonars are not designed for stealth.

When active sonar is used to measure the distance from the transducer to the bottom, it is known as echo sounding. Echo sounding is the technique of using Sound pulses directed from the surface or from a submarine vertically down to measure the distance to the bottom by means of sound waves Similar methods may be used looking upward for wave measurement.

Active sonar is also used to measure distance through water between two sonar transducers or a combination of a hydrophone (underwater acoustic microphone) and projector (underwater acoustic speaker). A transducer is a device that can transmit and receive acoustic signals ("pings"). When a hydrophone/transducer receives a specific interrogation signal it responds by transmitting a specific reply signal. To measure distance, one transducer/projector transmits an interrogation signal and measures the time between this transmission and the receipt of the other transducer/hydrophone reply. The time difference, scaled by the speed of sound through water and divided by two, is the distance between the two platforms. This technique, when used with multiple transducers/hydrophones/projectors, can calculate the relative positions of static and moving objects in water.

In wartime, emission of an active pulse is so compromising for a submarine's stealth that it is considered a very severe breach of tactics.

A very directional, yet low-efficiency type of sonar (used by fisheries, military, and for port security) makes use of a complex nonlinear feature of the water known as non-linear sonar, the virtual transducer being known as a parametric array. The parametric array is a nonlinear transduction mechanism that generates narrow nearly sidelobe free beams of low frequency sound through the mixing and interaction of high

Transponder

This is an active sonar device that receives a stimulus and immediately (or with a delay) retransmits the received signal or a predetermined one.

Performance prediction

A sonar target is small relative to the sphere, centred around the emitter, on which it is located. "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe Therefore, the power of the reflected signal is very low, several orders of magnitude less than the original signal. An order of magnitude is the class of scale or magnitude of any amount where each class contains values of a fixed ratio to the class preceding it Even if the reflected signal was of the same power, the following example (using hypothetical values) shows the problem: Suppose a sonar system is capable of emitting a 10000 W/m² signal at 1m, and detecting a 0. 001 W/m² signal. At 100 m the signal will be 1 W/m² (due to the inverse-square law). In Physics, an inverse-square law is any Physical law stating that some physical Quantity or strength is inversely proportional If the entire signal is reflected from a 10 sq m target, it will be at 0. 001 W/m² when it reaches the emitter, ie just detectable. However, the original signal will remain above 0. 001 W/m² until 300 m. Any 10 sq m target between 100 and 300 m using a similar or better system would be able to detect the pulse but would not be detected by the emitter. The detectors must be very sensitive to pick up the echoes. Since the original signal is much more powerful, it can be detected many times further than twice the range of the sonar (as in the example).

In active sonar there are two performance limitations, due to noise and reverberation. In general one or other of these will dominate so that the two effects can be initially considered separately.

In noise limited conditions at initial detection:-

SL - 2TL + TS - (NL -DI) = DT

where SL is the source level, TL is the transmission loss (or propagation loss), TS is the target strength, NL is the noise level, DI is the directivity index of the array (an approximation to the array gain) and DT is the detection threshold. Underwater acoustics is the study of the propagation of Sound in Water and the interaction of the mechanical waves that constitute sound with the water and its boundaries Underwater acoustics is the study of the propagation of Sound in Water and the interaction of the mechanical waves that constitute sound with the water and its boundaries Underwater acoustics is the study of the propagation of Sound in Water and the interaction of the mechanical waves that constitute sound with the water and its boundaries In Telecommunication, noise level is the Noise power, usually relative to a reference In MIMO Communication systems array gain means a power gain of transmitted signals that is achieved by using multiple-antennas at transmitter and/or receiver Sensory threshold is a theoretical concept used in Psychophysics.

In reverberation limited conditions at initial detection (neglecting array gain):-

SL - 2TL + TS = RL + DT

where RL is the reverberation level and the other factors are as before. Reverberation is the persistence of Sound in a particular space after the original sound is removed

Marine animals

Adverse effects

High-powered sonar transmitters may harm marine animals, although the precise mechanisms for this are not well understood. Some marine animals, such as whales and dolphins, use echolocation systems similar to active sonar to locate predators and prey. Whales are marine mammals which are neither Dolphins (ie members of the families Delphinidae or Platanistoidae) nor Porpoises Orcas Dolphins are Marine mammals that are closely related to Whales and Porpoises There are almost forty species of dolphin in seventeen genera. Echolocation, also called biosonar, is the biological Sonar used by several Animals such as Dolphins Shrews most Bats It is conjectured that sonar transmitters could confuse these animals and cause them to lose their way, perhaps preventing them from feeding and mating.

In 1996 twelve Cuvier's beaked whales beached themselves alive along the coast of Greece while NATO was testing active low and mid-range frequency sonar, according to a paper published in the journal Nature in 1998. The author established for the first time the link between atypical mass strandings of whales and the use of military sonar by concluding there was better than a 99. 9% likelihood that sonar testing caused that stranding. ^[3] A NATO panel investigated the above stranding and concluded the whales were exposed to 150-160 dB re 1 μPa of low and mid-range frequency sonar. This level is about 66 dB less (more than a million times lower intensity) than the threshold for hearing damage specified by a panel of marine mammal experts. ^[4]

It has been suggested that military sonar may induce whales to panic and surface too rapidly leading to a form of decompression sickness. Decompression sickness (DCS, the diver’s disease, the bends, caisson disease is the name given to a variety of symptoms suffered by a person This was first raised by a paper published in the journal Nature in 2003. It reported acute gas-bubble lesions (indicative of decompression sickness) in whales that beached shortly after the start of a military exercise off the Canary Islands in September 2002. The Canary Islands ( English pronunciation kəˈnæriː ˈaɪləndz Spanish: Islas Canarias, ˈizlas kaˈnarjas are a Spanish ^[5]

In the Bahamas in 2000, a sonar trial by the United States Navy of transmitters in the frequency range 3–8 kHz at a source level of 223–235 decibels re 1 μPa (scaled to a distance of 1 m) resulted in the beaching of seventeen whales, seven of which were found dead. The Bahamas, officially the Commonwealth of The Bahamas, is an independent sovereign English -speaking country consisting of two thousand Cays and The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. The decibel ( dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity relative to The Navy accepted blame in a report, which found the dead whales to have experienced acoustically-induced hemorrhages and bleeding around the animals' ears and eyes. Bleeding, technically known as hemorrhaging / haemorrhaging (see American and British spelling differences) is the loss of Blood from ^[6] The resulting disorientation may have led to the stranding. ^[7]

A kind of sonar called mid-frequency sonar has been correlated with mass cetacean strandings throughout the world's oceans, and has therefore been singled out by environmentalists as causing the death of marine mammals. The Order Cetacea (sɪˈteɪʃiə L cetus, whale includes Whales Dolphins and Porpoises Cetus is ^[8] A lawsuit filed by the Natural Resources Defense Council in Santa Monica, California on 20 October 2005 contended that the U. The Natural Resource Defense Council ( NRDC) is a New York City -based Non-profit, Non-partisan international environmental Advocacy Events 1740 - Maria Theresa takes the throne of Austria. France, Prussia, Bavaria and Saxony Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. S. Navy has conducted sonar exercises in violation of several environmental laws, including the National Environmental Policy Act, the Marine Mammal Protection Act, and the Endangered Species Act. The National Environmental Policy Act ( NEPA) is a United States environmental law that was signed into law on January 1, 1970 by U The Marine Mammal Protection Act of 1972 prohibits with certain exceptions the taking of Marine mammals in United States waters and by U The Endangered Species Act of 1973 ( et seq or ESA is the most wide-ranging of the dozens of United States environmental laws passed in the 1970s ^[9]

On November 13, 2007, a United States appeals court restored a ban on the U. Events 1002 - English king Ethelred orders the killing of all Danes in England, known today as the St Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. S. Navy's use of submarine-hunting sonar in training missions off Southern California until it adopted better safeguards for whales, dolphins and other marine mammals. On 16 January, 2008, President George W. Events 27 BC - The title Augustus is bestowed upon Gaius Julius Caesar Octavian by the Roman Senate. Bush exempted the US Navy from the law and argued that naval exercises are crucial to national security. Bush said that "the use of mid-frequency active sonar. . . [is] in the paramount interest of the United States. " California Coastal Commissioner Sara Wan commented that "Both the court and the Coastal Commission have said that the Navy can carry out its mission as well as protect the whales. This is a slap in the face to Californians who care about the oceans. "^[10] On 4 February, 2008, a Federal judge ruled that despite President Bush's decision to exempt it, the Navy must follow environmental laws placing strict limits on mid-frequency sonar. Events 211 - Roman Emperor Septimius Severus dies leaving the Roman Empire in the hands of his two quarrelsome sons In a 36-page decision, U. S. District Judge Florence-Marie Cooper wrote that the Navy is not "exempted from compliance with the National Environmental Policy Act" and the court injunction creating a 12 nautical-mile no-sonar zone off Southern California. ^[11][12] On 29 February, 2008, a three-judge federal appeals court panel upheld the lower court order requiring the Navy to take precautions during sonar training to minimize harm to marine life. ^[6]

Mitigation methods

Environmental impacts of the operation of active sonar are required to be carried out by US law. Procedures for minimising the impact of sonar are developed in each case where there is significant impact.

Hearing damage can be reduced by limiting the sound exposure received by an animal. The maximum sound exposure level recommended by Southall et al ^[13] for cetaceans is 183 dB re 1 μPa² s for behavioural effects and 198 dB re 1 μPa² s for hearing damage. The Order Cetacea (sɪˈteɪʃiə L cetus, whale includes Whales Dolphins and Porpoises Cetus is

Hand-held sonar for use by a diver

• The LIMIS (= Limpet Mine Imaging Sonar) is a hand-held or ROV-mounted imaging sonar for use by a diver. Remotely operated underwater vehicles (ROVs is the common accepted name for tethered underwater Robots in the offshore industry Its name is because it was designed for patrol divers (combat frogmen or Clearance Divers) to look for limpet mines in low visibility water. A frogman is someone who is trained to dive or swim in a military capacity often in combat A Clearance Diver was originally a specialist naval diver who used Explosives Underwater to remove obstructions to make harbours and shipping channels safe A naval mine is a self-contained Explosive device placed in water to destroy Ships or Submarines Unlike Depth charges mines are deposited In Meteorology, visibility is a measure of the Distance at which an object or light can be clearly discerned Links:
  • [1] Abstract of article by the International Society for Optical Engineering ^[14]
  • [2] Used to find debris from the Space Shuttle Columbia crash
  • [3] Used in fish passage research at hydropower facilities
• The LUIS (= Lensing Underwater Imaging System) is another imaging sonar for use by a diver. SPIE is a Not-for-profit international society for the exchange collection and dissemination of knowledge in Optics, Photonics, and Imaging engineering Space Shuttle Columbia ( NASA Orbiter Vehicle Designation: OV-102) was the first spaceworthy Space shuttle in NASA 's Fishways, most commonly called fish ladders but also known as fish passes and in Australia also referred to as fish steps, are structures on or around artificial Hydropower, hydraulic power or water power is power that is derived from the Force or Energy of moving water which may Links:
  • [4] Use counting salmon in a river
• There is or was a small flashlight-shaped handheld sonar for divers, that merely displays range.

• For the INSS = Integrated Navigation Sonar System see:-
  • an image. Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set.
  • short description
  • description

Passive sonar

Passive sonar listens without transmitting. It is often employed in military settings, although it is also used in science applications, e. g. detecting fish for presence/absence studies in various aquatic environments - see also passive acoustics and passive radar. Passive acoustics is the action of listening for sounds often at specific frequencies or for purposes of specific analyses Passive radar systems (also referred to as passive coherent location and passive covert radar) encompass a class of Radar systems that detect and track In the very broadest usage, this term can encompass virtually any analytical technique involving remotely generated sound, though it is usually restricted to techniques applied in an aquatic environment.

Identifying sound sources

Passive sonar has a wide variety of techniques for identifying the source of a detected sound. For example, U. S. vessels usually operate 60 Hz alternating current power systems. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant If transformers or generators are mounted without proper vibration insulation from the hull or become flooded, the 60 Hz sound from the windings can be emitted from the submarine or ship. A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors In Electricity generation, an electrical generator is a device that converts Mechanical energy to Electrical energy, generally using Electromagnetic Vibration refers to mechanical Oscillations about an equilibrium point. A hull is the body of a Ship or Boat. It is a central concept in floating vessels as it provides the Buoyancy that keeps the vessel from sinking A submarine is a Watercraft that can operate independently below water as distinct from a Submersible that has only limited underwater capability This can help to identify its nationality, as most European submarines have 50 Hz power systems. Intermittent sound sources (such as a wrench being dropped) may also be detectable to passive sonar. A wrench or spanner is a Tool used to provide a Mechanical advantage in applying Torque to turn bolts, nuts or Until fairly recently, the identification of a signal was carried out by an operator using experience and training. Now computers may be utilised in this process.

Passive sonar systems may have large sonic databases, however final classification is generally performed manually by the sonar operator. A Computer Database is a structured collection of records or data that is stored in a computer system A computer system frequently uses these databases to identify classes of ships, actions (i. A computer is a Machine that manipulates data according to a list of instructions. e. , the speed of a ship, or the type of weapon released), and even particular ships. Publications for classification of sounds are provided by and continually updated by the U.S. Office of Naval Intelligence. The Office of Naval Intelligence ( ONI) was established in the United States Navy in 1882

Noise limitations

Passive sonar on vehicles is usually severely limited because of noise generated by the vehicle. For this reason, many submarines operate nuclear reactors that can be cooled without pumps, using silent convection, or fuel cells or batteries, which can also run silently. This article is a subarticle of Nuclear power. A nuclear reactor is a device in which Nuclear chain reactions are initiated controlled Convection in the most general terms refers to the movement of molecules within Fluids (i A fuel cell is an electrochemical conversion device It produces electricity from Fuel (on the Anode side and an oxidant (on the In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy Vehicles' propellers are also designed and precisely machined to emit minimal noise. A propeller is essentially a type of fan which transmits power by converting Rotational motion into Thrust for propulsion of a vehicle such as an High-speed propellers often create tiny bubbles in the water, and this cavitation has a distinct sound. Cavitation is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure

The sonar hydrophones may be towed behind the ship or submarine in order to reduce the effect of noise generated by the watercraft itself. A hydrophone (Greek "hydro" = "water" and "phone" = "sound" is a Microphone designed to be used underwater for recording or listening Towed units also combat the thermocline, as the unit may be towed above or below the thermocline. The thermocline (sometimes metalimnion) is a thin but distinct layer in a large body of water such as an ocean or lake in which temperature changes more rapidly with depth The thermocline (sometimes metalimnion) is a thin but distinct layer in a large body of water such as an ocean or lake in which temperature changes more rapidly with depth

The display of most passive sonars used to be a two-dimensional waterfall display. The spectrogram is the result of calculating the Frequency spectrum of Windowed frames of a compound signal. The horizontal direction of the display is bearing. The vertical is frequency, or sometimes time. Another display technique is to color-code frequency-time information for bearing. More recent displays are generated by the computers, and mimic radar-type plan position indicator displays. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships The plan position indicator (PPI, is the most common type of Radar display.

Performance prediction

Unlike active sonar, only one way propagation is involved. Because of the different signal processing used, the minimum detectable signal to noise ratio will be different. The equation for determining the performance of a passive sonar is:

SL - TL = NL - DI + DT

where SL is the source level, TL is the transmission loss, NL is the noise level, DI is the directivity index of the array (an approximation to the array gain) and DT is the detection threshold. The figure of merit of a passive sonar is:

FOM = SL + DI - (NL + DT). A Figure of merit is a quantity used to characterize the performance of a device relative to other devices of the same type

Warfare

Modern naval warfare makes extensive use of sonar. Naval warfare is Combat in and on Seas Oceans or any other major bodies of water such as large Lakes and wide Rivers History The two types described before are both used, from various platforms, i. e. water-borne vessels, aircraft and fixed installations. The usefulness of active versus passive sonar depends on the radiated noise characteristics of the target, generally a submarine. Although in WWII active sonar was mainly used, except by submarines, with the advent of noisy nuclear submarines passive sonar was preferred for initial detection. As the submarines have become quieter, active operation is now more used.

Active sonar is extremely useful, since it gives the exact position of an object. Active sonar works the same way as radar: a signal is emitted. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships The sound wave then travels in many directions from the emitting object. When it hits an object, the sound wave is then reflected in many other directions. Some of the energy will travel back to the emitting source. The echo will enable the sonar system or technician to calculate, with many factors such as the frequency, the energy of the received signal, the depth, the water temperature, etc. , the position of the reflecting object. Using active sonar is somewhat hazardous however, since it does not allow the sonar to identify the target, and any vessel around the emitting sonar will detect the emission. Having heard the signal, it is easy to identify the type of sonar (usually with its frequency) and its position (with the sound wave's energy). Moreover, active sonar, similar to radar, allows the user to detect objects at a certain range but also enables other platforms to detect the active sonar at a far greater range.

Since active sonar does not allow an exact identification and is very noisy, this type of detection is used by fast platforms (planes, helicopters) and by noisy platforms (most surface ships) but rarely by submarines. When active sonar is used by surface ships or submarines, it is typically activated very briefly at intermittent periods, to reduce the risk of detection by an enemy's passive sonar. As such, active sonar is normally considered a backup to passive sonar. In aircraft, active sonar is used in the form of disposable sonobuoys that are dropped in the aircraft's patrol area or in the vicinity of possible enemy sonar contacts. A sonobuoy (a Portmanteau of Sonar and Buoy) is a relatively small (typically 4⅞ inches or 124 mm in diameter and 36 inches or 910 mm long expendable

Passive sonar has several advantages. Most importantly, it is silent. If the target radiated noise level is high enough, it can have a greater range than active sonar, and allows an identification of the target. Since any motorized object makes some noise, it may be detected eventually. It simply depends on the amount of noise emitted and the amount of noise in the area, as well as the technology used. To simplify, passive sonar "sees" around the ship using it. On a submarine, the nose mounted passive sonar detects in directions of about 270°, centered on the ship's alignment, the hull-mounted array of about 160° on each side, and the towed array of a full 360°. The no-see areas are due to the ship's own interference. Once a signal is detected in a certain direction (which means that something makes sound in that direction, this is called broadband detection) it is possible to zoom in and analyze the signal received (narrowband analysis). This is generally done using a Fourier transform to show the different frequencies making up the sound. This article specifically discusses Fourier transformation of functions on the Real line; for other kinds of Fourier transformation see Fourier analysis and Since every engine makes a specific noise, it is straightforward to identify the object.

Another use of the passive sonar is to determine the target's trajectory. Trajectory is the path a moving object follows through space The object might be a Projectile or a Satellite, for example This process is called Target Motion Analysis (TMA), and the resultant "solution" is the target's range, course, and speed. TMA is done by marking from which direction the sound comes at different times, and comparing the motion with that of the operator's own ship. Changes in relative motion are analyzed using standard geometrical techniques along with some assumptions about limiting cases.

Passive sonar is stealthy and very useful. However, it requires high-tech components (band-pass filters, receivers) and is costly. A band-pass filter is a device that passes frequencies within a certain range and rejects ( Attenuates frequencies outside that range It is generally deployed on expensive ships in the form of arrays to enhance the detection. Surface ships use it to good effect; it is even better used by submarines, and it is also used by airplanes and helicopters, mostly to a "surprise effect", since submarines can hide under thermal layers. A submarine is a Watercraft that can operate independently below water as distinct from a Submersible that has only limited underwater capability If a submarine captain believes he is alone, he may bring his boat closer to the surface and be easier to detect, or go deeper and faster, and thus make more sound.

Examples of sonar applications in military use are given below. Many of the civil uses given in the following section may also be applicable to naval use.

Anti-submarine warfare

Until recently, ship sonars were usually with hull mounted arrays, either amidships or at the bow. It was soon found after their initial use that a means of reducing flow noise was required. The first were made of canvas on a framework, then steel ones were used. Now domes are usually made of reinforced plastic or pressurised rubber. Such sonars are primarily active in operation. An example of a conventional hull mounted sonar is the SQS-56.

Because of the problems of ship noise, towed sonars are also used. These also have the advantage of being able to be placed deeper in the water. However, there are limitations on their use in shallow water. These are called towed arrays (linear) or variable depth sonars (VDS) with 2/3D arrays. A problem is that the winches required to deploy/recover these are large and expensive. VDS sets are primarily active in operation while towed arrays are passive.

An example of a modern active/passive ship towed sonar is Sonar 2087 made by Thales Underwater Systems. Sonar 2087 is a Towed array sonar system for Royal Navy Type 23 frigates manufactured by Thales Underwater Systems. Thales Underwater Systems (TUS, formerly known as Thomson Marconi Sonar, is an international defence manufacturer specialising in Sonar systems for Submarines

Torpedoes

Modern torpedoes are generally fitted with an active/passive sonar. This may be used to home directly on the target, but wake following torpedoes are also used. An early example of an acoustic homer was the Mark 37 torpedo. The Mark 37 torpedo is a Torpedo with electrical propulsion developed for the US Navy after World War II.

Torpedo countermeasures can be towed or free. An early example was the German Sieglinde device while the Pillenwerfer was a chemical device. Pillenwerfer or BOLD was a German SONAR decoy used by U-boats during the Second World War from 1942 onwards A widely used US device was the towed Nixie while MOSS submarine simulator was a free device. Mosses are small soft Plants that are typically 1–10  cm (0 A modern alternative to the Nixie system is the UK Royal Navy S2170 Surface Ship Torpedo Defence system. Disambiguation Also used for the Space Science and Technology Department of the United Kingdom 's Science and Technology Facilities Council.

Mines

Mines may be fitted with a sonar to detect, localise and recognise the required target. Further information is given in acoustic mine and an example is the CAPTOR mine. An acoustic mine is a type of Naval mine which monitors audio activity in its vicinity The CAPTOR is the United States Navy 's primary anti-submarine Naval mine.

Mine countermeasures

Mine Countermeasure (MCM) Sonar, sometimes called "Mine and Obstacle Avoidance Sonar (MOAS)", is a specialised type of sonar used for detecting small objects. Most MCM sonars are hull mounted but a few types are VDS design. An example of a hull mounted MCM sonar is the Type 2193 while the SQQ-32 Mine-hunting sonar and Type 2093 systems are VDS designs. The AN/SQQ-32 is a mine -hunting Sonar system Developed by Raytheon for the United States Navy, it includes an active sonar for detecting objects

Submarines

Submarines rely on sonar to a greater extent than surface ships as they cannot use radar at depth. The sonar arrays may be hull mounted or towed. Information fitted on typical fits is given in Oyashio class submarine and Swiftsure class submarine. Boats There is a total of 11 boats in the class - the last boat was commissioned in 2008 Incidents On 28 May 2008, HMS Superb of the Swiftsure class collided with a rock while submerged in the Red Sea.

Aircraft

Helicopters can be used for antisubmarine warfare by deploying fields of active/passive sonobuoys or can operate dipping sonar, such as the AQS-13. A sonobuoy (a Portmanteau of Sonar and Buoy) is a relatively small (typically 4⅞ inches or 124 mm in diameter and 36 inches or 910 mm long expendable Fixed wing aircraft can also deploy sonobuoys and have greater endurance and capacity to deploy them. Processing from the sonobuoys or dipping sonar can be on the aircraft or on ship. Helicopters have also been used for mine countermeasure missions using towed sonars such as the AQS-20A

Underwater communications

Dedicated sonars can be fitted to ships and submarines for underwater communication. See also the section on the underwater acoustics page. Underwater acoustics is the study of the propagation of Sound in Water and the interaction of the mechanical waves that constitute sound with the water and its boundaries

Ocean surveillance

For many years, the United States operated a large set of passive sonar arrays at various points in the world's oceans, collectively called Sound Surveillance System (SOSUS) and later Integrated Undersea Surveillance System (IUSS). The United States of America —commonly referred to as the SOSUS, an acronym for SOund SUrveillance System, was a chain of underwater listening posts located across the northern Atlantic Ocean A similar system is believed to have been operated by the Soviet Union. As permanently mounted arrays in the deep ocean were utilised, they were in very quiet conditions so long ranges could be achieved. Signal processing was carried out using powerful computers ashore. With the ending of the Cold War a SOSUS array has been turned over to scientific use.

In the United States Navy, a special badge known as the Integrated Undersea Surveillance System Badge is awarded to those who have been trained and qualified in its operation. The Integrated Undersea Surveillance System insignia is a military badge of the United States Navy which has been in existence

Underwater security

Sonar can be used to detect frogmen and other scuba divers. A frogman is someone who is trained to dive or swim in a military capacity often in combat Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set. This can be applicable around ships or at entrances to ports. Active sonar can also be used as a deterrent and/or disablement mechanism. One such device is the Cerebus system. Anti-frogman techniques are security methods developed to protect Watercraft, Ports and installations and other sensitive resources both in or nearby vulnerable

See Underwater Port Security System and Anti-frogman techniques#Ultrasound weapon. See UIS and IAS for other meanings of these initials For other diver-detector sonars see:CategoryDiver-detector sonars. Anti-frogman techniques are security methods developed to protect Watercraft, Ports and installations and other sensitive resources both in or nearby vulnerable

Hand-held sonar

Limpet Mine Imaging Sonar (LIMIS) is a hand-held or ROV-mounted imaging sonar designed for patrol divers (combat frogmen or clearance divers) to look for limpet mines in low visibility water. Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set. Remotely operated underwater vehicles (ROVs is the common accepted name for tethered underwater Robots in the offshore industry A frogman is someone who is trained to dive or swim in a military capacity often in combat A Clearance Diver was originally a specialist naval diver who used Explosives Underwater to remove obstructions to make harbours and shipping channels safe A naval mine is a self-contained Explosive device placed in water to destroy Ships or Submarines Unlike Depth charges mines are deposited In Meteorology, visibility is a measure of the Distance at which an object or light can be clearly discerned

The LUIS is another imaging sonar for use by a diver.

Integrated Navigation Sonar System (INSS) is a small flashlight-shaped handheld sonar for divers that displays range. ^[15][16]

Intercept sonar

This is a sonar designed to detect and locate the transmissions from hostile active sonars. An example of this is the Type 2082 fitted on the Vanguard class submarines. Design The Vanguard s were designed from the outset as an unlimited-range nuclear powered ballistic missile submarine unlike the previous ''Resolution'' class

Civil applications

Fisheries

Fishing is an important industry that is seeing growing demand, but world catch tonnage is falling as a result of serious resource problems. For the computer security term see Phishing. Fishing is the activity of catching Fish. The industry faces a future of continuing worldwide consolidation until a point of sustainability can be reached. Sustainability, in a general sense is the capacity to maintain a certain process or state indefinitely However, the consolidation of the fishing fleets are driving increased demands for sophisticated fish finding electronics such as sensors, sounders and sonars. Historically, fishermen have used many different techniques to find and harvest fish. However, acoustic technology has been one of the most important driving forces behind the development of the modern commercial fisheries.

Sound waves travel differently through fish than through water because a fish's air-filled swim bladder has a different density than seawater. gas bladder (also fish maw, less accurately swim bladder or air bladder) is an internal organ that contributes to the ability of a Fish This density difference allows the detection of schools of fish by using reflected sound. Acoustic technology is especially well suited for underwater applications since sound travels farther and faster underwater than in air. Today, commercial fishing vessels rely almost completely on acoustic sonar and sounders to detect fish. Fishermen also use active sonar and echo sounder technology to determine water depth, bottom contour, and bottom composition.

Companies such as Raymarine UK, Marport Canada, Wesmar, Furuno, Krupp, and Simrad make a variety of sonar and acoustic instruments for the deep sea commercial fishing industry. The deep sea, or deep layer, is the lowest layer in the Ocean, existing below the Thermocline, at a depth of 1000 fathoms or more For example, net sensors take various underwater measurements and transmit the information back to a receiver onboard a vessel. Each sensor is equipped with one or more acoustic transducers depending on its specific function. Data is transmitted from the sensors using wireless acoustic telemetry and is received by a hull mounted hydrophone. The analog signals are decoded and converted by a digital acoustic receiver into data which is transmitted to a bridge computer for graphical display on a high resolution monitor. An analog or analogue signal is any continuous signal for which the time varying feature (variable of the signal is a representation of some other Information graphics or infographics are visual representations of Information, Data or Knowledge.

Echo sounding

An echo-sounder sends an acoustic pulse directly downwards to the seabed and records the returned echo. The sound pulse is generated by a transducer that emits an acoustic pulse and then “listens” for the return signal. The time for the signal to return is recorded and converted to a depth measurement by calculating the speed of sound in water. As the speed of sound in water is around 1,500 metres per second, the time interval, measured in milliseconds, between the pulse being transmitted and the echo being received, allows bottom depth and targets to be measured.

The value of underwater acoustics to the fishing industry has led to the development of other acoustic instruments that operate in a similar fashion to echo-sounders but, because their function is slightly different from the initial model of the echo-sounder, have been given different terms.

Net location

The net sounder is an echo sounder with a transducer mounted on the headline of the net rather than on the bottom of the vessel. Nevertheless, to accommodate the distance from the transducer to the display unit, which is much greater than in a normal echo-sounder, several refinements have to be made. Two main types are available. The first is the cable type in which the signals are sent along a cable. In this case there has to be the provision of a cable drum on which to haul, shoot and stow the cable during the different phases of the operation. The second type is the cable less net-sounder – such as Marport’s Trawl Explorer - in which the signals are sent acoustically between the net and hull mounted receiver/hydrophone on the vessel. In this case no cable drum is required but sophisticated electronics are needed at the transducer and receiver.

The display on a net sounder shows the distance of the net from the bottom (or the surface), rather than the depth of water as with the echo-sounder's hull-mounted transducer. A transducer is a device usually electrical, electronic, Electro-mechanical, Electromagnetic, Photonic, or Photovoltaic Fixed to the headline of the net, the footrope can usually be seen which gives an indication of the net performance. Any fish passing into the net can also be seen, allowing fine adjustments to be made to catch the most fish possible. In other fisheries, where the amount of fish in the net is important, catch sensor transducers are mounted at various positions on the cod-end of the net. As the cod-end fills up these catch sensor transducers are triggered one by one and this information is transmitted acoustically to display monitors on the bridge of the vessel. The skipper can then decide when to haul the net.

Modern versions of the net sounder, using multiple element transducers, function more like a sonar than an echo sounder and show slices of the area in front of the net and not merely the vertical view that the initial net sounders used.

The sonar is an echo-sounder with a directional capability that can show fish or other objects around the vessel.

Ship velocity measurement

Sonars have been developed for measuring a ship's velocity either relative to the water or to the bottom.

ROV and UUV

Small sonars have been fitted to Remotely Operated Vehicles (ROV) and Unmanned Underwater Vehicles (UUV) to allow their operation in murky conditions. These sonars are used for looking ahead of the vehicle. The Long-Term Mine Reconnaissance System is an UUV for MCM purposes. The Long-Term Mine Reconnaissance System (LMRS is a torpedo tube-launched and tube-recovered underwater search and survey Unmanned undersea vehicle (UUV capable of performing

Vehicle location

Sonars which act as beacons are fitted to aircraft to allow their location in the event of a crash in the sea. Short and Long Baseline sonars may be used for caring out the location, such as LBL.

Scientific applications

Biomass estimation

Main article: Bioacoustics

Biomass estimation uses sonar to detect fish, etc. Bioacoustics is a cross-disciplinary Science that combines Biology and Acoustics. As the sound pulse travels through water it encounters objects that are of different density than the surrounding medium, such as fish, that reflect sound back toward the sound source. These echoes provide information on fish size, location, and abundance.

Wave measurement

An upward looking echo sounder mounted on the bottom or on a platform may be used to make measurements of wave height and period. From this statistics of the surface conditions at a location can be derived.

Water velocity measurement

Special short range sonars have been developed to allow measurements of water velocity.

Bottom type assessment

Sonars have been developed that can be used to characterise the sea bottom into, for example, mud, sand, and gravel. Relatively simple sonars such as echo sounders can be promoted to seafloor classification systems via add-on modules, converting echo parameters into sediment type. Different algorithms exist, but they are all based on changes in the energy or shape of the reflected sounder pings.

Bottom topography measurement

Side-scan sonars can be used to derive maps of the topography of an area by moving the sonar across it just above the bottom. Side-scan sonar (also sometimes called side scan sonar, sidescan sonar, side looking sonar, side-looking sonar and bottom classification Low frequency sonars such as GLORIA have been used for continental shelf wide surveys while high frequency sonars are used for more detailed surveys of smaller areas.

Sub-bottom profiling

Powerful low frequency echo-sounders have been developed for providing profiles of the upper layers of the ocean bottom.

Synthetic aperture sonar

Various synthetic aperture sonars have been built in the laboratory and some have entered use in mine-hunting and search systems. An explanation of their operation is given in synthetic aperture sonar. Synthetic aperture sonar (SAS is a form of Sonar in which sophisticated post-processing of sonar data are used in ways closely analogous to Synthetic aperture radar

Parametric sonar

Parametric sources use the non-linearity of water to generate the difference frequency between two high frequencies. A virtual end-fire array is formed. Such a projector has advantages of broad bandwidth, narrow beamwidth, and when fully developed and carefully measured it has no obvious sidelobes: see Parametric array. The parametric array is a nonlinear transduction mechanism that generates narrow nearly sidelobe free beams of low frequency sound through the mixing and interaction of high Its major disadvantage is very low efficiency of only a few percent. ^[17]. P. J. Westervelt's seminal (and brief!) 1963 JASA paper summarizes the trends involved. In theory, a parametric receiver is possible but there is no known implementation.

See also

Acoustic Doppler Current Profiler Acoustic Tags Animal echolocation Baffles (submarine) Beached whale Beamforming Fish finder Hydroacoustics Ocean acoustic tomography Radar

Passive Radar Scientific Echosounder Side-scan sonar SOFAR channel Sonar 2087 Sonobuoy Sound Synthetic aperture sonar Towed array sonar Underwater acoustics Upward looking sonar

Notes

References

Bibliography

• Hackmann, Willem D. Seek & Strike: Sonar, Anti-submarine Warfare, and the Royal Navy, 1914-54. (London: HMSO, 1984)
• Hackmann, Willem D. "Sonar Research and Naval Warfare 1914-1954: A Case Study of a Twentieth-Century Science. ” Historical Studies in the Physical and Biological Sciences’’ 16#1 (1986) 83-110

External links

• Sonar Tutorial for Robots
• Profiling Sonar: Inspecting In-Service, Submerged Pipes
• Sonars and the marine environment by Norwegian Defence Research Establishment (FFI)
• Single Beam Sonars
• Sonar "Primer"

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