Sonar

Sonar (short for so und n avigation a nd r anging) is a device using the special properties of the propagation of sound in the water to detect and locate objects underwater. His invention stems from the work of Paul Langevin and Constantin Chilowski during the First World War. It is used:

by naval forces to detect submarines (and surface vessels by submarines), as well as mines and objects placed on the bottom by the buildings of mine hunting, but also for guiding torpedoes during their underwater path to the target.

for fishing for detecting schools of fish;

for navigation and maritime river to measure depth, even professionals of hydrography use to map the ocean floor and other bodies of water, this is called bathymetric sonar.

The sonar can be active (emission of a sound and listen to the echo) or passive (listening to sounds).

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Echo sonar (info)

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In the field of biology also known as sonar about means of echolocation available to whales and bats, both of which use a particular protein, the prestine.

Active sonar

The first military use of active sonar was the ASDIC (acronym for A llied S ubmarine D detection I nvestigation C ommittee). The first ultrasonic transducers quartz were developed in 1917 by the Canadian Robert William Boyle and the very first ASDIC were mounted on some ships a few months before the end of the conflict.

Active sonar emits a sound pulse and its echo listening on the obstacles she encounters. The transmitting and receiving antennas are called basis signals and are formed from many hydrophone.

The antenna is: is directional, and the operator mechanically oriented in a particular direction it sends and listens; is fixed and omnidirectional, then the transmission is in either omnidirectional beam is electronically formed in a given direction. At the reception, the direction of the echo is determined by an electronic process (measurement of phase shifts on each hydrophone).

The distance is obtained by measuring the time between transmission and reception of the echo (the velocity of propagation of sound in sea water being about 1500 m / s).

The sonar transmitter frequency is selected depending on its use. High frequencies (tens or hundreds of kHz) are rapidly absorbed by seawater (several hundred meters), but instead allow the detection of small objects and can thus achieve real images. Used at a frequency of 14-22 kHz for ASDIC the Second World War, since they are used for hydrographic echo sounders, sonars fishing for finding mines, for the detection of torpedoes. The pulse can have a constant frequency, but also a slightly variable and modulated frequency for a finer detection of the echo (Chirp)

Further down in frequency, the detection distances are great, but you lose finesse and antennas become very large and heavy. In practice, very low-frequency active sonar (ATBF) is down much below 3 kHz. Detection ranges do not exceed a few tens of kilometers (in the areas of convergence).

Because the other difficulty is that sea water is not a homogeneous medium. On the one hand, the propagation is affected by bottom topography, marine animals and plankton. On the other hand, the speed of sound varies depending on the temperature (the bathythermie) and pressure (and marginally salinity). These parameters vary with depth, but between 30 and 100 meters is usually a marked limit, called the thermocline, which divides the hot water from the surface of the cold bottom water. The wave generated by a sonar of a side of the thermocline is reflected by it. This phenomenon does not exist in the shallow coastal waters, but the echoes on the bottom are so very annoying. Beyond the thermocline, the temperature is constant, but the waves are subject to the influence of the pressure that will “recover” to the surface, creating convergence zones (ZC). The sound waves then do not follow straight paths in the vertical plane (and also long-distance on the horizontal plane), creating as sound bathythermal conditions “gray areas”, used by submarines and areas annular detections (ZC) in favor of detection. For this reason, the buildings of anti submarine warfare use both hull-mounted sonar and towed sonars, including the “fish “is immersed within a few hundred meters to” light up “from below the thermocline. These buildings also use passive sonar towed.

Indeed, the implementation of an active sonar makes it detectable and identifiable bearer, since issuing a very distinctive sound. It is for this reason hardly used by submarines, which implement several types of passive sonar.

The helicopter anti-submarine warfare also implement active sonar called “wet” medium frequency, they immerse treuillent and hovering.

Smaller pollsters resemble a sealed flashlight. The user in the water peak, press a button and can then be read remotely. The sonar fishing is a variant to see schools of fish on the screen. Some pollsters or sonars may be similar to those of military sonar capabilities, with three-dimensional representations of the area under the boat.

Sonar and marine animals

Some marine animals, such as whales and toothed dolphins, use of systems echolocation similar to active sonar to locate predators and prey. Currently, there are doubts about the safety of sonar on these animals. They are accused of including them to lose their way, or even prevent them from feeding or breeding. A recent article on the BBC website (see below) reports a study of the journal Nature on the effects on the whales from military sonar, which would cause discomfort relief (followed by grounding).

High power sonar can also kill marine animals. The Bahamas in 2000, an experience of the U.S. Navy with a transmitter at 230 dB in the frequency range of 3000 at 7 000 Hz caused the stranding of whales sixteen, seven of which were found dead. The U.S. Navy recognized its responsibility in this grounding in a report in the Boston Globe on January 1, 2002.

Passive sonar

The principle of passive sonar is listening to sounds without issue. They are generally used in military and scientific applications.

Listening to sounds is the main activity of a submarine on patrol, he listens: sounds emitted by boat propellers and hulls movements or motors of maritime patrol aircraft and helicopters at low altitude; emissions sonar; its own to reduce noise, keep quiet and keep a good listener.

These sounds are detected, analyzed, followed by a series of hydrophones, more or less specialized in terms of their frequency of collection, on the same hull of the submarine or a towed linear antenna (a ” flute “) of very low frequency listening (ETBF) (tens to hundreds of Hz). These devices can detect a presence at distances exceeding one hundred kilometers.

The display of most passive sonars are traditionally in the form of a “waterfall” two-dimensional, with the x-axis direction (the azimuth) of the sound and orderly time (or frequency analysis spectral). Some displays use colors. The most recent use a representation based on that of radar.

Passive sonar measuring a direction, the distance is obtained by calculating the kinematic and on the buzzer and the submarine. In the event of detection of sonar emissions, the distance may also be determined by the time intervals between successive echoes due to reflection on the bottom and the surface.

For listening, the submarine has a great advantage over his opponent surface: it is not disturbed by the sound of waves or (low speed) its own noise, which allows him to exploit specific properties of sound propagation in the sea Also, the oceanography military studied water bodies, their temperatures and salinity, currents and eddies, in order to determine the conditions favorable to the detection diving and prone areas at the discretion of submarines.

Aircraft maritime patrol helicopters and anti-submarine warfare also use passive sonar as dropped sonobuoys and the signals are transmitted by radio waves and received and analyzed by the aircraft or relayed to a center ground.

Identifications noisemakers

Specialized staff is trained to listen (colloquially called “the golden ears”), a large practice and a lot of training enable them to recognize the type of ship, or to identify individually, calculate its speed and detect its route changes. Passive sonar also use very large sound databases and computer analyzes the frequency spectrum of the noise source to identify it. However, the performance of the human ear is still superior in many cases to that of the computer, and both ways are complementary.

Noise

Passive sonars are usually very limited because of the sounds that are emitted into the building itself. For this reason, surface ships can not use that as towed linear antenna (it is thus decoupled from the shell).

By design, the submarines are quiet, and auxiliary engines are connected to the chassis (“suspended”) through pads (generally elastomeric), dissipating vibration (noise) in the form of heat. Submarines are subject to special care to avoid any vibration by choosing inherently very quiet, by hydrodynamic forms neat equipment, avoiding the generation of intrusive disruptions in water and discrete thrusters. The propellers are subject to careful design to reduce noise emissions, including those related to the phenomenon of cavitation (formation of bubbles on the propeller blades during their rapid rotation in low immersion). External acoustic coating is often arranged on the outer shell to absorb sound (called masking material) and / or decrease the echo of a pulse of an opposing sonar (called anechoic coating).

Sonar data exchange developed in a navy

Data generated from active sonar and liabilities are translated at the combat system or mission platforms, by creating objects following tactics:

Underwater tracks

Contacts lost

the Datums

benchmarks ASW

points “reference buoys”

the position of sonar buoys with their course and speed of drift

the round detections

the presence area of submarine AMI , Zone NOTACK.

These data are exchanged within the naval force by tactical data links, the link 11, link 16 and link 22.

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