Encyclopedia
SONAR — or
sonar — is a technique that uses
sound propagation under water to
navigate or to detect other vessels. There are two kinds of sonar — active and passive. Sonar is a subcategory of
acoustic location.
History
In 1906,
Lewis Nixon invented the first passive sonar-type listening device, as a way of detecting
icebergs . During
World War I, with the need to detect submarines, interest in sonar increased. The French physicist
Paul Langevin, working with a Russian emigré electrical engineer, Constantin Chilowski, invented the first active sonar-type device for detecting submarines in 1915. Although
piezoelectric transducers later superseded the electrostatic transducers they used, their work influenced the future of sonar designs. In 1916, under the British Board of Inventions and Research, Canadian physicist Robert Boyle took on the project, which subsequently passed to the
Anti- Submarine Detection Investigation Committee, producing a prototype for testing in mid-1917, hence the British acronym
ASDIC.
By 1918, both the
U.S. and Britain had built active systems. The UK tested what they still called ASDIC on
HMS Antrim in 1920, and started production of units 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.
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. The greatest advantage came when it was linked to the
Squid anti-submarine weapon.
Active sonar
Active sonar creates a
pulse of sound, often called a "ping", and then listens for reflections of the pulse. To measure the distance to an object, one measures the time from emission of a pulse to reception. To measure the bearing, one uses several hydrophones, and measures the relative arrival time to each in a process called beamforming.
The pulse may be at constant
frequency or a
chirp of changing frequency. For a chirp, the receiver
correlates the frequency of the reflections to the known chirp. The resultant processing gain allows the receiver to derive the same information as if a much shorter pulse of the same total energy were emitted. In practice, the chirp signal is sent over a longer time interval; therefore the instantaneous emitted power will be reduced, which simplifies the design of the transmitter. In general, long-distance active sonars use lower frequencies. The lowest have a bass "BAH-WONG" sound.
The most 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. 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 to the bottom, it is known as
echo sounding.
Active sonar is also used to measure distance through water between two sonar
transponders. A transponder is a device that can transmit and receive signals , but when it receives a specific interrogation signal it responds by transmitting a specific reply signal. To measure distance, one transponder transmits an interrogation signal and measures the time between this transmission and the receipt of the other transponder's reply. The time difference, scaled by the speed of sound through water and divided by two, is the distance between the two transponders. This technique, when used with multiple transponders, can calculate the relative positions of static and moving objects in water.
Sonar and marine animals - adverse effects
Some marine animals, such as
whales and
dolphins, use
echolocation systems similar to active sonar to locate predators and prey. It is feared that sonar transmitters could confuse these animals and cause them to lose their way, perhaps preventing them from feeding and mating. A recent article on the BBC Web site reports findings published in the journal
Nature to the effect that military sonar may be inducing some whales to experience
decompression sickness .
High-powered sonar transmitters may indirectly harm marine animals, although scientific evidence suggests that a confluence of factors must first be present. In the
Bahamas in 2000, a trial by the
United States Navy of a 230 decibel transmitter in the frequency range 3 – 7 kHz resulted in the beaching of sixteen whales, seven of which were found dead. The Navy accepted blame in a report published in the
Boston Globe on 1 January, 2002.
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. International press coverage of these events can be found at this Web site. A lawsuit was filed in
Santa Monica, California on 19 October, 2005 contending that the U.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. Full text of the lawsuit can be found at this Web site.
An imitation of the
humpback whale's sonar was conducted in the redirection of
Humphrey the whale, who deviated from his normal migration path to enter
San Francisco Bay.
Passive sonar
Passive sonar listens without transmitting. It is 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.
Speed of sound
Sonar operation is affected by sound speed. Sound speed is slower in
fresh water than in
sea water. In all water sound velocity is affected by density . Density is affected by temperature, dissolved molecules , and pressure. The speed of sound is approximately equal to 4388 + + . This is an empirically derived approximation equation that is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. 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. This can frustrate sonar, for a sound originating on one side of the thermocline tends to be bent, or
refracted, off the thermocline. 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. Increased pressure increases the density of the water and raises the sound velocity. Increases in sound velocity cause the sound waves to refract away from the area of higher velocity. The mathematical model of refraction is called
Snell's law.
Sound waves that are radiated down into the ocean bend back up to the surface in great arcs due to the effect of pressure on sound. The ocean must be at least 6000 feet deep, or the sound waves will echo off the bottom instead of refracting back upwards. Under the right conditions these waves will then be focused near the surface and refracted back down and repeat another arc. Each arc is called a convergence zone. Where an arc intersects the surface a CZ is formed. The diameter 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 , depending on the season, forming a pattern of concentric circles around the sound source. Sounds that can be detected for only a few miles in a direct line can therefore also be detected hundreds of miles away. Typically 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.
Identifying sound sources
Military sonar has a wide variety of techniques for identifying a detected sound. For example, U.S. vessels usually operate 60 Hz
alternating current power systems. If
transformers are mounted without proper vibration insulation from the hull, or flooded, the 60 Hz sound from the windings and generators can be emitted from the
submarine or ship, helping to identify its nationality. In contrast, most European submarines have 50 Hz power systems. Intermittent noises may also be detectable to sonar.
Passive sonar systems may have large sonic databases, however most classification is performed manually by the sonar operator. A computer system frequently uses these databases to identify classes of ships, actions , and even particular ships. Publications for classification of sounds are provided by and continually updated by the U.S. Office of Naval Intelligence.
Noise
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. Vehicles'
propellers are also designed and precisely machined to emit minimal noise. High-speed propellers often create tiny bubbles in the water, and this
cavitation has a distinct sound.
The sonar hydrophones may be towed behind the ship or submarine in order to reduce the effect of noise generated by the watercraft itself. Towed units also combat the thermocline, as the unit may be towed above or below the thermocline.
For many years, the
United States operated a large set of passive sonar arrays at various points in the world's oceans, collectively called
SOSUS. As permanently mounted arrays in the deep ocean, they were very quiet.
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.
The display of most passive sonars used to be a two-dimensional waterfall display. 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.
Sonar in warfare
Modern naval warfare makes extensive use of sonar. The two types described before are both used, but from different platforms, i.e., types of water-borne vessels.
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. 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 and its position . 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.
A sonar target is small relative to the sphere, centered around the emitter, on which it is located. Therefore, the power of the reflected signal is very low, several orders of magnitude less than the original signal. Even if the reflected signal was of the same power, the following example shows the problem: Suppose a sonar system is capable of emitting a 20 W signal and detecting a 5 W signal. Now, suppose that at 500 m the signal is 10 W . If the entire signal is reflected, it will be at 5 W when it reaches the emitter. Any further target will produce an echo that will fall below 5 W before it reaches the emitter. However, the original signal will remain above 5 W until 1000 m. Any target between 500 and 1000 m using a similar or better system would be able to detect the pulse but would not be detected by the emitter. Combined with the very low power of real reflected signals, this effect becomes even more prominent. 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 .
Since active sonar does not allow an exact identification and is very noisy, this type of detection is used by fast platforms and by noisy platforms 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.
Passive sonar has fewer drawbacks. Most importantly, it is silent. Generally, it has a much 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 it is possible to zoom in and analyze the signal received . This is generally done using a Fourier transform to show the different frequencies making up the sound. Since every engine makes a specific noise, it is easy to identify the object.
Another use of the passive sonar is to determine the target's trajectory. This process is called Target Motion Analysis , 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 and is costly. 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. 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.
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 sonar operation and warfare.
Fisheries Acoustics
Fisheries hydroacoustics uses Sonar to detect fish. 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. The basic components of the scientific echosounder hardware function is to transmit the sound, receive, filter and amplify, record, and analyze the echoes. While there are many manufacturers of commercially available “fish-finders”, quantitative hydroacoustic analyses require that measurements are made with scientific-quality echo sounder equipment, having high signal to noise ratios, and ability for easy calibration.
Vertical, or down-looking hydroacoustics has become increasingly important to the assessment of marine fish, anadromous and land-locked salmonids , and lake and reservoir fishes .
Hydroacoustics provides a repeatable, non-invasive method of collecting high-resolution , continuous data along transects in three dimensions . MacLennan and Simmonds as well as Brandt give a thorough introduction in the use of
hydroacoustics for measuring fish abundances and distributions.
Fisheries Applications
Fishing is an important industry that is seeing growing demand, but world catch tonnage is falling as a result of serious resource problems. The industry faces a future of continuing worldwide consolidation until a point of sustainability can be reached. 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. 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 HTI , Wesmar, and Simrad make a variety of sonar and acoustic instruments for the deep sea commercial fishing industry. 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 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/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.
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 , rather than the depth of water as with the echo-sounder's hull-mounted transducer. 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.
Fisheries Acoustics References
- Fisheries Acoustics Research at University of Washington http://www.acoustics.washington.edu/
- NOAA Protocols for Fisheries Acoustics Surveys http://www.st.nmfs.gov/st4/protocol/Acoustic_protocols.pdf
- Fisheries Acoustics http://store.blackwell-professional.com/063205994x.html
- "ACOUSTICS IN FISHERIES AND AQUATIC ECOLOGY" http://www.ifremer.fr/sympafae/
- "Hydroacoustic Protocol - Lakes, Reserviors and Lowland Rivers" http://www.pnamp.org//web/workgroups/FPM/meetings/2005_1205/2005_1202Hydroacoustics-Lakes.doc
Notes
In World War II, the Americans used the term
SONAR for their system. The British still called their system
ASDIC. In 1948, with the formation of
NATO, standardization of signals led to the dropping of ASDIC in favor of sonar.
See also
References
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External links
