Explain and introduce
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Sonar is an electronic device that uses underwater sound waves to detect, locate and communicate underwater targets. It is the most widely used and important device in hydroacoustics. It is the translation of the word SONAR, which is the abbreviation of sound navigation and ranging.
Sonar technology has a history of 100 years. It was invented by Lewis Nixon of the British Navy in 1906. The first sonar instrument he invented was a listening device, which was mainly used to detect icebergs. This technology was applied to the battlefield in World War I to detect submarines hidden underwater.
At present, sonar is the main technology used by navies of various countries for underwater surveillance, which is used to detect, classify, locate and track underwater targets. Conduct underwater communication and navigation, and ensure the tactical maneuvering of warships, anti-submarine aircraft and anti-submarine helicopters and the use of underwater weapons. In addition, sonar technology is also widely used in torpedo guidance, mine fuze, fish exploration, offshore oil exploration, ship navigation, underwater operation, hydrological survey and submarine geological and geomorphological survey.
Like the development of many technologies, the demand of society and the progress of technology promote the development of sonar technology.
principle of operation
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Sound wave is an important means of observation and measurement. Interestingly, the word "sound" in English means "sound" as a noun and "detection" as a verb, which shows the close relationship between sound and detection.
Only sound waves have unique conditions for observation and measurement in water. This is because the action distance of other detection means is very short, and the penetration ability of light in water is very limited. Even in the clearest seawater, people can only see objects in the range of ten meters to dozens of meters. Electromagnetic waves decay too fast in water, and the shorter the wavelength, the greater the loss. Even if high-power low-frequency electromagnetic waves are used, they can only propagate for tens of meters. But the attenuation of sound wave propagation in water is much smaller. A few kilograms of bombs exploded in the deep-sea passage, and signals could be received 20 thousand kilometers away. Low-frequency sound waves can also penetrate the strata thousands of meters under the sea and obtain information in the strata. So far, there is no more effective means to measure and observe in water than sound waves.
Structure and classification
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Sonar device generally consists of array, electronic cabinet and auxiliary equipment. The array is made up of underwater acoustic transducers arranged according to a certain geometric pattern, and its shape is usually spherical, cylindrical, flat or linear, and it can be divided into receiving array, transmitting array or receiving array. Electronic cabinets generally have subsystems of transmitting, receiving, displaying and controlling. Auxiliary equipment includes power supply equipment, connecting cables, underwater junction boxes and repeaters, lifting, slewing, pitching, retracting, dragging, suspending and releasing devices matched with sonar array transmission control, sonar dome, etc.
Transducer is an important device in sonar, which converts acoustic energy into mechanical energy, electrical energy, magnetic energy and other forms of energy. It has two purposes: one is to emit sound waves underwater, which is called "transmitting transducer", which is equivalent to a loudspeaker in the air; The second is to receive sound waves underwater, which is called "receiving transducer", which is equivalent to a microphone in the air (commonly known as "microphone" or "microphone"). Transducers are often used to transmit and receive sound waves at the same time in practical use, and the transducers specially used for receiving are also called "hydrophones". The working principle of transducer is that some materials expand and contract under the action of electric field or magnetic field, resulting in piezoelectric effect or magnetostrictive effect.
The classification of sonar can be divided into different kinds of sonar according to its working mode, equipment object, tactical use, array carrying mode and technical characteristics. For example, it can be divided into active sonar and passive sonar according to the working mode; According to the equipment object, it can be divided into surface ship sonar, submarine sonar, aerial sonar, portable sonar and coastal sonar.
Active sonar: Active sonar technology means that sonar actively emits sound waves to "illuminate" the target, and then receives the echo reflected by the underwater target to determine the parameters of the target. Most of them use pulse system, and some use continuous wave system. It evolved from a simple echo detection instrument. It actively emits ultrasonic waves, and then collects and calculates the echoes. It is suitable for detecting icebergs, reefs, sunken ships, seawater depth, fish schools, mines and hidden submarines with engines turned off.
Passive sonar: Passive sonar technology refers to sonar passively receiving radiated noise generated by underwater targets such as ships and signals sent by underwater acoustic equipment to determine the direction of the targets. It evolved from a simple hydrophone. It monitors the noise emitted by the target and judges the position and some characteristics of the target. It is especially suitable for submarines that can't expose themselves through sound, but want to detect the activities of enemy ships.
Installation and application
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Traditionally, the main position of submarine sonar installation is at the forefront. Because the modern submarine relies heavily on the detection effect of passive sonar, the huge radio device not only increases the diameter of the submarine, but also makes the torpedo tube originally in this position give up and retreat to both sides.
Other sonar types installed on submarines include passive sonar listening devices installed in other parts of the hull. Using the same signal received from different positions, shallow positioning can be carried out quickly after computer processing and calculation, which is beneficial to submarines with large hull because of long measurement baseline and high accuracy.
Another kind of sonar is called "towed sonar", because this sonar device is connected to the submarine by cable, and the sonar body is towed far behind the submarine for detection. The use of towed sonar greatly enhances the submarine's ability to detect in all directions and at different depths, especially at the tail end of the submarine. This is because the tail of the submarine is also part of the power output. Due to the interference of water flow sound, the sonar located in front can't hear the signal in this area, forming a blind spot. After using towed sonar, this blind area can be eliminated and the targets hidden in this area can be found.
Interestingly, sonar is not a patent of human beings. Many animals have their own "sonar". Bats emit 10-20 ultrasonic pulses per second with their throats and receive echoes with their ears. With this "active sonar", they can detect tiny insects and wire obstacles with a thickness of 0. 1 mm ... moths and other insects also have "passive sonar", which can clearly hear the ultrasonic waves of bats 40 meters away, so they often avoid attacks. But some bats can use high-frequency ultrasound or low-frequency ultrasound that insects can't hear, so the hit rate of catching insects is still very high. It seems that animals are also engaged in "sonar warfare" like humans! Marine mammals such as dolphins and whales have "underwater sonar", which can generate very definite signals to explore food and communicate with each other.
Dolphin sonar has a high sensitivity. It can find a metal wire with a diameter of 0.2 mm and a nylon rope with a diameter of 1 mm several meters away, can distinguish two signals with a time difference of 200 burs, can find fish schools hundreds of meters away, and can walk through a pool full of bamboo poles flexibly and quickly blindfolded without touching. Dolphin sonar has a strong "target recognition" ability, which can not only identify different fish and distinguish different materials such as brass, aluminum, bakelite and plastic, but also distinguish the echo of its own voice from the sound waves played back by the person who recorded its voice. The anti-jamming ability of dolphin sonar is also amazing. If there is noise interference, it will increase the call intensity over the noise so that its judgment will not be affected. And dolphin sonar also has the ability to express feelings. It has been proved that dolphins are animals with "language", and their "dialogue" is carried out through their sonar system. In particular, among the four remaining freshwater dolphins in the world, the most precious baiji in the middle and lower reaches of the Yangtze River in China has a clear "division of labor" in its sonar system, which is used for positioning, communication and alarm, and has a special function of phase modulation through frequency modulation.
Many kinds of whales use sound to detect and communicate, the frequency is much lower than that of dolphins, and the range is much farther. Other marine mammals, such as seals and sea lions, also send out sonar signals for detection.
Animals that have lived in the depths of the extremely dark ocean all their lives have to use sonar and other means to search for prey and avoid attacks. The performance of their sonar is far beyond the capabilities of modern human technology. Solving these mysteries of animal sonar has always been an important research topic of modern sonar technology.
influencing factor
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Besides the technical conditions of sonar itself, there are also external conditions that affect the performance of sonar. Direct factors include propagation attenuation, multipath effect, reverberation interference, ocean noise, self-noise, target reflection characteristics or radiation noise intensity. Most of them are related to marine environmental factors. For example, acoustic waves are influenced and restricted by the uneven distribution of seawater medium, sea surface and seabed, which will produce refraction, scattering, reflection and interference, sound ray bending, signal fluctuation and distortion, change the propagation path, and appear acoustic shadow area, which will seriously affect the range and measurement accuracy of sonar. Modern sonar can properly select the working depth and pitch angle of the array according to the propagation conditions formed by the change of sound velocity-depth in the sea area, and use different propagation paths of sound waves (direct sound, seabed reflection, convergence area and deep-sea channel) to overcome the adverse effects of underwater acoustic propagation conditions and improve the sonar detection distance. For another example, the self-noise of the carrier platform is mainly related to the speed. The higher the speed, the greater the self-noise, and the closer and farther the sonar works. The greater the reflection ability of the target, the farther it will be found by the other active sonar; The greater the intensity of the target radiation noise, the farther it will be found by the passive sonar of the other side.