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Warfare

Modern naval warfare makes extensive use of sonar. 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 modern signal processing passive sonar was preferred for initial detection. As the submarines have become quieter, active operation is now more used. In 1987 a division of Toshiba sold the machinery to Russia that allowed them to mill the submarine propeller blades so that they became radically quieter, creating a huge security issue with their newer generation of submarines.

Active sonar is extremely useful, since it gives the exact bearing to a target (and sometimes the range). 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. Active sonar is used when the platform commander determines that it is more important to determine the position of a possible threat submarine than it is to reveal his own position. With surface ships it might be assumed that the threat is already tracking the ship with satellite date. 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 exact classification and is very noisy, this type of detection is used by fast platforms (planes, helicopters) and by noisy platforms (most surface ships) but very rarely by submarines. Ballistic missile submarines do not even have active sonar, since they never want to risk detection. 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 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. Since every engine makes a specific noise, it is straightforward to identify the object. The classified databases of unique noises are part of what is known as acoustic intelligence or ACINT.

Another use of the 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 components (band-pass filters, receivers) 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.

özet: bu denizaltılar ful pasif sonar kullanıyor sanırsam.

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