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Examining The Sound Navigation Technique Of Sonar Engineering Essay

Sonar means sound navigation and ranging is a method that uses sound propagation (usually underwater, just as Submarine navigation) to navigate, talk to or identify other vessels. Two types of technology share the name "sonar": passive sonar is essentially listening for the sound made by vessels; active sonar is emitting pulses of sounds and listening for echoes. Sonar can be utilized as a way of acoustic location and of measurement of the echo characteristics of "targets" in the. Acoustic location in air was used prior to the introduction of radar. Sonar could also be used in air for robot navigation, and SODAR upward looking in-air sonar) can be used for atmospheric investigations. The word sonar is also used for the gear used to create and have the sound. The acoustic frequencies found in sonar systems vary from suprisingly low (infrasonic) to extremely high (ultrasonic). The analysis of underwater sound is known as underwater acoustics or hydro acoustics

History

Although some animals (dolphins and bats) have used sound for communication and object detection for an incredible number of years, use by humans in this particular is in the beginning recorded by Leonardo Da Vinci in 1490: a tube inserted in to the water was reported to be used to discover vessels by putting an ear to the tube. [citation needed]

In the 19th century an underwater bell was used as an ancillary to lighthouses to provide warning of hazards.

The use of sound to 'echo locate' underwater in the same way as bats use sound for aerial navigation appears to have been prompted by the Titanic disaster of 1912. The world's first patent for an underwater echo ranging device was filed at the British Patent Office by English meteorologist Lewis Richardson per month after the sinking of the Titanic, and a German physicist Alexander Behm obtained a patent for an echo sounder in 1913. 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. S. Revenue (now Coast Guard) Cutter Miami on the Grand Banks off Newfoundland Canada. For the reason that test, Fessenden demonstrated depth sounding, underwater communications (Morse Code) and echo ranging (detecting an iceberg at two miles (3 km) range). The so-called Fessenden oscillator, at ca. 500 Hz frequency, was unable to determine the bearing of the berg due to the 3 meter wavelength and the tiny 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.

During World War I the need to discover submarines prompted more research in to the use of sound. The British made early use of underwater hydrophones, as the French physicist Paul Langevin, dealing with a Russian immigrant electrical engineer, Constantin Chilowski, worked on the introduction of active sound devices for detecting submarines in 1915 using quartz. Although piezoelectric and magnetostrictive transducers later superseded the electrostatic transducers they used, this work influenced future designs.

Performance factors

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

Sound propagation

Sonar procedure is affected by variations in sound speed, particularly in the vertical plane. Sound travels more slowly in fresh water than in sea water, though the difference is small. The speed is determined by the water's bulk modulus and mass density. The bulk modulus is afflicted by temperature, dissolved impurities (usually salinity), and pressure. The density effect is small. The speed of sound (in feet per second) is approximately:

4388 + (11. 25 - temperature (in F)) + (0. 0182 - depth (in feet)) + salinity (in parts-per-thousand ).

This empirically derived approximation equation is fairly accurate for normal temperatures, concentrations of salinity and the range of all ocean depths. Ocean temperature varies with depth, but at between 30 and 100 meters there is usually a marked change, called the thermo cline, dividing the warmer surface water from the cold, still waters that define all of those other ocean. This may frustrate sonar, just because a sound originating using one side of the thermo cline is commonly bent, or refracted, through the thermo cline. The thermo cline may be present in shallower coastal waters. However, wave action will often mix this particular column and eliminate the thermo cline. Water pressure also influences sound propagation: higher pressure increases the sound speed, which in turn causes the sound waves to refract from the region of higher sound speed. The mathematical style of refraction is named Snell's law.

If the sound source is deep and the conditions are right, propagation might 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 without losses at the boundaries. Similar propagation can occur in the 'surface duct' under suitable conditions. Yet, in this case there are reflection losses at the surface.

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

Sound propagation is afflicted by absorption in this itself as well as at the top and bottom. This absorption depends upon frequency, with a number of different mechanisms in sea water. Long-range sonar uses low frequencies to minimize absorption effects.

The sea contains many sources of noise that interfere with the required target echo or signature. The main noise sources are waves and shipping. The motion of the receiver through this can also cause speed-dependent low frequency noise.

Scattering

When active sonar can be used, scattering occurs from small objects in the sea as well as from underneath and surface. This is often a major way to obtain interference. This acoustic scattering is analogous to the scattering of the light from a car's headlights in fog: a high-intensity pencil beam will penetrate the fog somewhat, but broader-beam headlights emit much light in unwanted directions, a lot of which is scattered back to the observer, overwhelming that reflected from the prospective ("white-out"). For analogous reasons active sonar needs to transmit in a narrow beam to minimize scattering.

Target characteristics

The sound reflection characteristics of the target of an active sonar, like a submarine, are known as its target strength. A complication is that echoes are also extracted from other objects in the ocean such as whales, wakes, schools of fish and rocks.

Passive sonar detects the target's radiated noise characteristics. The radiated spectrum comprises a continuing spectral range of noise with peaks at certain frequencies which may be used for classification.

Countermeasures

1) Active countermeasures may be launched by the submarine under attack to improve the noise level, provide a large false target, and obscure the signature of the submarine itself.

2) Passive countermeasures include:

There is a mounting noise-generating device on isolating devices.

We use a sound-absorbent coating on the hulls of submarines, for example anechoic tiles.

Active sonar 

Active sonar uses a sound transmitter and a receiver. When the two are in the same place it is monostatic operation. Once the transmitter and receiver are separated it is bistatic operation. When more transmitters (or more receivers) are utilized, again spatially separated, it is multistate operation. Most sonar's are used monostatically with the same array often being used for transmission and reception. Active son buoy 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. This pulse of sound is normally created electronically utilizing a sonar Projector consisting of a signal generator, power amplifier and electro-acoustic transducer/array. A beam former is usually employed to concentrate the acoustic power into a beam, which may be swept to hide the mandatory search angles. Generally, the electro-acoustic transducers are of the Tonpilz type and their design may be optimized to attain maximum efficiency above the widest bandwidth, to be able to optimize performance of the overall system. Occasionally, the acoustic pulse may be created by other means, e. g.

(1) Chemically using explosives

(2) Air guns

(3) Plasma sound sources.

To gauge the distance for an object, enough time from transmission of your pulse to reception is measured and converted into a range by knowing the speed of sound. To gauge the bearing, several hydrophones are employed, and the set measures the relative arrival time to each, or with a range of hydrophones, by measuring the relative amplitude in beams formed through an activity called beam forming. Usage of a wide range reduces the spatial response so that to provide wide cover multibeam systems are being used.

The targets signal (if present) as well as noise is then passed through various forms of signal processing, which for simple sonar's may be just energy measurement. It is then presented for 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 sonar this function may be carried out by software. Further processes may be completed to classify the target and localize it, as well as measuring its velocity.

The pulse may be at continuous frequency or a chirp of changing frequency (to permit pulse compression on reception). Simple sonar's generally use the former with a filter wide enough to hide possible Doppler changes due to focus on movement, while more technical ones generally are the latter technique. Since digital processing became available pulse compression has usually been implemented using digital correlation techniques. Military sonar's often have multiple beams to provide all-round cover while simple ones only cover a narrow arc, however the beam may be rotated, relatively slowly, by mechanical scanning.

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

One of the useful small sonar is comparable in appearance to a waterproof flashlight. The top is pointed into the water, a button is pressed, and the device displays the length to the target. Another variant is a "fish finder" that presents a small display with shoals of fish. Some civilian sonars approach active military sonar's in capability, with quite exotic three-dimensional displays of the region close to the boat.

When active sonar can be used to gauge the distance from the transducer to the bottom, it is known as echo sounding. 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 any hydrophone (underwater acoustic microphone) and projector (underwater acoustic speaker). A transducer is a tool that can transmit and receive acoustic signals ("pings"). Whenever a hydrophone/transducer receives a specific interrogation signal it responds by transmitting a particular reply signal. To measure distance, one transducer/projector transmits an interrogation signal and measures enough time between this transmission and the receipt of the other transducer/hydrophone reply. Enough time difference, scaled by the speed of sound through water and divided by two, is the distance between your two platforms. This system, when used with multiple transducers/hydrophones/projectors, can calculate the relative positions of static and moving objects in water.

In combat situations, an active pulse can be detected by an opponent and will reveal a submarine's position.

A very directional, but low-efficiency, type of sonar makes use of a complex nonlinear feature of water known as non-linear sonar, the virtual transducer being known as a parametric array.

Project ARTEMIS

Project ARTEMIS was one-of-a-kind low-frequency sonar for surveillance that was deployed off Bermuda for several years in the first 1960s. The active portion was deployed from a global War II tanker, and the obtaining array was a built into a set position with an offshore bank.

Transponder

This can be an active sonar device that receives a stimulus and immediately retransmits the received signal or a predetermined one.

Passive sonar

Passive sonar listens without transmitting. It is often used in military settings, though it is also found in science applications, e. g. , detecting fish for presence/absence studies in various aquatic environments - see also passive acoustics and passive radar. In the very broadest usage, this term can encompass almost any analytical technique involving remotely made sound, though as well as restricted to techniques applied within an aquatic environment.

Identifying sound sources

Passive sonar has a wide variety of techniques for identifying the source of the detected sound. For instance, U. S. vessels usually operate 60 Hz alternating current power systems. 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. This can help to recognize its nationality, as most European submarines have 50 Hz power systems. Intermittent sound sources (like a wrench being dropped) may also be detectable to passive sonar. Until fairly recently, a skilled trained operator discovered signals, but now computers can do this.

Passive sonar systems may have large sonic databases, however the sonar operator usually finally classifies the signals manually. A computer system frequently uses these databases to identify classes of ships, actions (i. 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 united states Office of Naval Intelligence.

Noise limitations

Passive sonar on vehicles is usually severely limited because of noise generated by the vehicle. Because of this, 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 this, and these cavitations have a definite sound.

The sonar hydrophones may be towed behind the ship or submarine in order to reduce the effect of noise produced by the watercraft itself. Towed units also combat the thermo cline, as the unit may be towed above or below the thermo cline.

The display of most passive sonar's 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. Newer displays are generated by the computers, and mimic radar-type plan position indicator displays.

Performance prediction

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

SL  ' TL = NL  ' DI + DT

where SL is the foundation 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 passive sonar is:

FOM = SL + DI  ' (NL + DT).

Warfare

Modern naval warfare makes considerable use of both passive and active sonar from water-borne vessels, aircraft and fixed installations. The relative usefulness of active versus passive sonar depends on the radiated noise characteristics of the prospective, generally a submarine. Although in WW II active sonar was employed by surface craft-submarines avoided emitting pings which revealed their occurrence and position-with the advent of modern signal-processing passive sonar became preferred for initial detection. Submarines were then suitable for quieter operation, and active sonar is currently more used. In 1987 a division of Japanese company Toshiba reportedly sold machinery to the Soviet Union that allowed it to mill submarine propeller blades in order that they became radically quieter, creating a huge security issue with their newer generation of submarines.

Active sonar gives the exact bearing to a target, and sometimes the range. Active sonar works the same manner as radar: a sign 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 a great many other directions. Some of the energy will travel back to the emitting source. The echo will allow the sonar system or technician to calculate, with many factors such as the frequency, the vitality of the received signal, the depth, this inflatable water temperature, the positioning of the reflecting object, etc. Active sonar is utilized when the platform commander determines that it is more important to determine the position of a possible threat submarine than it is to conceal his own position. With surface ships it could be assumed that the threat has already been tracking the ship with satellite data. Any vessel around the emitting sonar will find the emission. Having heard the signal, it is easy to recognize the sonar equipment used and its own position. Active sonar is similar to radar in that, although it allows detection of targets at a certain range, it also permits the emitter to be detected at a lot better range, which is undesirable.

Since active sonar reveals the presence and position of the operator, and will not allow exact classification of targets, it is used by fast (planes, helicopters) and by noisy platforms but rarely by submarines. When active sonar can be used by surface ships or submarines, it is typically activated very briefly at intermittent periods to reduce the risk of detection. Consequently active sonar is generally considered a backup to passive sonar. In aircraft, active sonar can be used in the form of disposable son buoys that are dropped in the aircraft's patrol area or near possible enemy sonar contacts.

Passive sonar has several advantages. Most of all, it is silent. If the prospective radiated noise level is high enough, it can have a larger range than active sonar, and allows the mark to be identified. Since any motorized object makes some noise, it could in principle be detected, depending on the level of noise emitted and the ambient noise level in the area, as well as the technology used. To simplify, passive sonar "sees" about the ship using it. Over a submarine, nose-mounted passive sonar detects in directions around 270, devoted to the ship's alignment, the hull-mounted array of about 160 on each side, and the towed selection of a full 360. The invisible 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). That is generally done using a Fourier transform to show the different frequencies creating the sound. Since every engine makes a particular sound, it is straightforward to identify the thing. Databases of unique engine sounds are part of what's known as acoustic intelligence or ACINT.

Another use of passive sonar is to determine the target's trajectory. This technique 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 differing times, and comparing the motion your 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 needs high-tech electronic components and it is costly. It is generally deployed on expensive ships in the form of arrays to enhance 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. When a submarine's commander believes he is alone, he might bring his boat nearer to the surface and be better to detect, or go deeper and faster, and therefore make sounder.

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

Anti-submarine warfare

Variable Depth Sonar and its own winch until recently, ship sonar's were usually with hull mounted arrays, either amidships or at the bow. It had been soon found after their initial use a method of reducing flow noise was required. The first were made of canvas over a framework, and then steel ones were used. Now domes are usually manufactured from reinforced plastic or pressurized rubber. Such sonar's are mostly active functioning. An example of conventional hull mounted sonar is the SQS-56.

Because of the issues of ship noise, towed sonar's are also used. These also have the advantage of having the ability to be located deeper in this inflatable water. However, there are limitations on their use in shallow water. They are called towed arrays (linear) or variable depth sonar's (VDS) with 2/3D arrays. Issues would be that the winches required to deploy/recover they are large and expensive. VDS sets are generally active in operation while towed arrays are passive. A good example of a modern active/passive ship towed sonar is Sonar 2087 created by Thales Underwater Systems.

Torpedoes

Modern torpedoes are generally fitted with active/passive sonar. This may be used to home on the mark, but wake following torpedoes are also used. An early exemplory case of an acoustic homer was the Mark 37 torpedo.

Torpedo countermeasures can be towed or free. An early on example was the German Sieglinde device as the Pillenwerfer was a chemical device. A trusted US device was the towed Nixie while MOSS submarine simulator was a free of charge device. Today's option to the Nixie system is the united kingdom Royal Navy S2170 Surface Ship Torpedo Defense system.

Mines

Mines may be fitted with a sonar to detect, localize and recognize the mandatory target. More info is given in acoustic mine and an example is the CAPTOR mine.

Mine countermeasures

Mine Countermeasure (MCM) Sonar, sometimes called "Mine and Obstacle Avoidance Sonar (MOAS)", is a specialized kind of sonar used for detecting small objects. Most MCM sonar's are hull mounted but a few types are VDS design. A good example of a hull mounted MCM sonar is the Type 2193 as the SQQ-32 Mine-hunting sonar and Type 2093 systems are VDS designs. See also Minesweeper (ship)

Submarine navigation

Submarines rely on sonar to a larger extent than surface ships as they can not use radar at depth. The sonar arrays may be hull mounted or towed. Information fitted on typical fits is given in Yoshiro class submarine and Swift sure class submarine.

Aircraft

Helicopters can be used for antisubmarine warfare by deploying fields of active/passive son buoys or can operate dipping sonar, including the AQS-13. Fixed wing aircraft can also deploy son buoys and have greater endurance and capacity to deploy them. Processing from the son buoys or dipping sonar can be on the aircraft or on ship. Helicopters are also used for mine countermeasure missions using towed sonar's including the AQS-20A

Ocean surveillance

For a long time, america operated a huge 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). A similar system is believed to have been operated by the Soviet Union. As permanently mounted arrays in the deep ocean were utilized, these were in very quiet conditions such a long time ranges could be performed. Signal processing was completed using powerful computers ashore. Along with the ending of the Cold War a SOSUS array has been turned over to scientific use.

Underwater security

Sonar can be used to identify frogmen and other scuba divers. This can be applicable around ships or at entrances to ports. Active sonar can be used as a deterrent and/or disablement mechanism. One such device is the Cerberus system.

Hand-held sonar

Limpet Mine Imaging Sonar (LIMIS) is hand-held or ROV-mounted imaging sonar designed for patrol divers (combat frogmen or clearance divers) to consider limpet mines in low visibility water. The LUIS is imaging sonar for use by the diver. Integrated Navigation Sonar System (INSS) is small flashlight-shaped handheld sonar for divers that display range.

Intercept sonar

This is sonar made to detect and locate the transmissions from hostile active sonar's. A good example of this is actually the Type 2082 fitted on the British Vanguard class submarines.

Uses in daily life

Fisheries

Fishing can be an important industry that is seeing growing demand, but world catch tonnage is falling because of this of serious resource problems. The industry faces another of continuing worldwide consolidation until a spot of sustainability can be reached. However, the consolidation of the fishing fleets are driving increased demands for advanced fish finding electronics such as sensors, sounders and sonar's. Historically, fishermen have used a number of techniques to find and harvest fish. However, acoustic technology has been one of the main driving forces behind the development of the present day commercial fisheries.

Sound waves travel differently through fish than through water just because a fish's air-filled swim bladder has another density than seawater. This density difference allows the detection of schools of fish by using reflected sound. Acoustic technology is particularly perfect 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 ascertain water depth, bottom contour, and bottom composition.

Cabin display of fish finder sonar Companies such as Ray marine UK makes a number of sonar and acoustic instruments for the deep sea commercial fishing industry. For instance, net sensors take various underwater measurements and transmit the info back again 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 also received by the hull mounted hydrophone. The analog signals are decoded and converted by an electronic acoustic receiver into data which is transmitted to a bridge computer for graphical display on a high resolution monitor.

Echo sounding

An echo-sounder sends an acoustic pulse directly downwards to the seabed and records the returned echo. The sound pulse is produced by the transducer that emits an acoustic pulse and then "listens" for the return signal. Enough time for the signal to return is recorded and changed into a depth measurement by calculating the speed of sound in water. As the speed of sound in water is just about 1, 500 meters per second, the time interval, measured in milliseconds, between your 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 resulted in the introduction of other acoustic instruments that operate in a similar fashion to echo-sounders but, because their function is slightly different from the initial style of the echo-sounder, have been given different terms.

Net location

The net sounder can be an echo sounder with a transducer installed on the headline of the net rather than on the bottom of the vessel. Nevertheless, to support the length from the transducer to the display unit, which is a lot greater than in a normal echo-sounder, several refinements have to be made. Two main types are available. The first is the cable enter which the signals are sent along a cable. In this case there must be the provision of an cable drum 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 your net and hull mounted receiver/hydrophone on the vessel. In this case no cable drum is required but complex electronics are needed at the transducer and receiver.

The display on a net sounder shows the distance of the web from underneath (or the surface), as opposed to the depth of water much like the echo-sounder's hull-mounted transducer. Fixed to the headline of the web, the footrope can usually be seen which gives a sign of the net performance. Any fish passing in to the net may also be seen, allowing fine adjustments to be produced to catch the most fish possible. In other fisheries, where in fact the amount of fish in the web is important, catch sensor transducers are mounted at various positions on the cod-end of the web. As the cod-end fills up these catch sensor transducers are triggered one by one which information is transmitted acoustically to show 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 aspect transducers, function similar to sonar than an echo sounder and show slices of the region before the net and not merely the vertical view that the initial net sounders used.

The sonar can be an echo-sounder with a directional capability that can show fish or other objects round the vessel good

Ship velocity measurement

Sonar's have been developed for measuring a ship's velocity either in accordance with the water or to underneath.

Scientific applications 

Biomass estimation

Detection of fish, and other marine and aquatic life, and estimation their individual sizes or total biomass using active sonar techniques. As the sound pulse travels through water it encounters objects that are of different density or acoustic characteristics than the surrounding medium, such as fish, that reflect sound back toward the sound source. These echoes provide information on fish size, location, abundance and behavior. Data is usually processed and analyzed using a variety of software such as Echo view.

Wave measurement

An upward looking echo sounder attached to underneath or on the platform enable you to make measurements of wave height and period. From this statistics of the surface conditions at a spot can be derived.

Water velocity measurement

Special short range sonar's have been developed to permit measurements of water velocity.

Bottom type assessment

Sonar's have been developed that can be used to characterize the ocean bottom into, for example, mud, sand, and gravel. Relatively simple sonar's such as echo sounders can be promoted to seafloor classification systems via add-on modules, converting echo parameters into sediment type. Different algorithms exist, nonetheless they are all based on changes in the energy or form of the reflected sounder pings. Advanced substrate classification analysis can be achieved using calibrated echo sounders and parametric or fuzzy-logic analysis of the acoustic data

Bottom topography measurement

Side-scan sonar's may be used to derive maps of the topography of an area by moving the sonar across it right above the bottom. Low frequency sonar's such as GLORIA have been used for continental shelf wide surveys while high frequency sonar's are being used for more descriptive 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 sonar's have been built-in the laboratory plus some have entered use within mine-hunting and search systems. A conclusion of their procedure is given in synthetic aperture sonar.

Parametric sonar

Parametric sources use the non-linearity of water to create the difference frequency between two high frequencies. A virtual end-fire array is formed. Such a projector has advantages of broad bandwidth, narrow beam width, and when fully developed and carefully measured it has no apparent side lobes: see parametric array. Its major disadvantage is suprisingly low efficiency of only a few percent. P. J. Westerville's seminal 1963 JASA paper summarizes the trends involved.

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