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.
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:. The figure of merit of a passive sonar is:. Modern naval warfare makes extensive use of both passive and active sonar from water-borne vessels, aircraft and fixed installations. Although active sonar was used by surface craft in World War II , submarines avoided the use of active sonar due to the potential for revealing their presence and position to enemy forces.
However, the advent of modern signal-processing enabled the use of passive sonar as a primary means for search and detection operations. In a division of Japanese company Toshiba reportedly sold machinery to the Soviet Union that allowed their submarine propeller blades to be milled so that they became radically quieter, making the newer generation of submarines more difficult to detect.
The use of active sonar by a submarine to determine bearing is extremely rare and will not necessarily give high quality bearing or range information to the submarines fire control team;however, use of active sonar on surface ships is very common.
Active sonar is used by submarines when if the tactical situation dictates it is more important to determine the position of a hostile submarine than conceal their own position. With surface ships it might be assumed that the threat is already tracking the ship with satellite data.
Any vessel around the emitting sonar will detect the emission. Having heard the signal, it is easy to identify the sonar equipment used usually with its frequency and its position with the sound wave's energy. Active sonar is similar to radar in that, while it allows detection of targets at a certain range, it also enables the emitter to be detected at a far greater range, which is undesirable.
Since active sonar reveals the presence and position of the operator, and does not allow exact classification of targets, it is used by fast 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 minimize the risk of detection. Consequently 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 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 the target to be identified. Since any motorized object makes some noise, it may 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" around the ship using it. 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.
This is generally done using a Fourier transform to show the different frequencies making up the sound. Since every engine makes a specific sound, it is straightforward to identify the object. Another use of passive sonar is to determine the target's trajectory. 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 electronic components and 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.
If a submarine's commander 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. 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 pressurized rubber. Such sonars are primarily active in operation.
An example of a conventional hull mounted sonar is the SQS 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. VDS sets are primarily active in operation while towed arrays are passive. 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. Torpedo countermeasures can be towed or free. An early example was the German Sieglinde device while the Bold was a chemical device. Mines may be fitted with a sonar to detect, localize and recognize the required target.
See also Minesweeper ship. 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. 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 AQSA. Dedicated sonars can be fitted to ships and submarines for underwater communication. See also the section on the underwater acoustics page. 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. 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. Sonar can be used to detect frogmen and other scuba divers.
This can be applicable around ships or at entrances to ports. One such device is the Cerberus system. This is a sonar designed to detect and locate the transmissions from hostile active sonars. An example of this is the Type fitted on the British Vanguard class submarines.
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. For example, net sensors take various underwater measurements and transmit the information back to a receiver on board 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.
Echo sounding is a process used to determine the depth of water beneath ships and boats. A type of active sonar, echo sounding is the transmission of an acoustic pulse directly downwards to the seabed, measuring the time between transmission and echo return, after having hit the bottom and bouncing back to its ship of origin. The acoustic pulse is emitted by a transducer which receives the return echo as well. The depth measurement is calculated by multiplying the speed of sound in water averaging 1, meters per second by the time between emission and echo return.
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.
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.
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. These sonars are used for looking ahead of the vehicle. Sonars which act as beacons are fitted to aircraft to allow their location in the event of a crash in the sea.
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.
In , NRDC spearheaded a successful lawsuit against the Navy to restrict the use of low-frequency sonar off the coast of California. In filing their brief, the groups cited Navy documents which estimated that such testing would kill some , marine mammals and cause permanent injury to more than whales, not to mention temporary deafness for at least 8, others.
Environmental groups are still fighting the battle against the sonar, lobbying the government to curtail testing, at least during peacetime, or to at least ramp up testing gradually to give marine wildlife a better chance to flee affected areas. Box , Westport, CT ; earthtalk emagazine. Read past columns at: www. EarthTalk is now a book! Details and order information at: www.
Already a subscriber? Sign in. Petty Officer 2nd Class Cody Greer. Corporal Brooke Dieters. Sergeant Brian Threat. Service Branches Work Environment Radar and sonar operators in the Military primarily work indoors in security-controlled areas. Overview Radar and sonar operators set up, operate, and perform preventive maintenance on sophisticated radar and sonar equipment.
Military Training All enlisted service members complete basic military training, which includes time spent in a classroom and in the field, and covers tactical and survival skills, physical training, military life and customs, and weapons training. Helpful Attributes Ability to concentrate for long periods Ability to work under stress Interest in advanced communications and electronic equipment Detail-oriented.
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