Sonar is a "method for locating objects in space and under water by means of emitted sound pulses". The word is an English acronym of so and n navigation a nd r anging , which can be translated as sound navigation and distance determination .
Sonar measurement techniques exploit the fact that sound propagates much less lossy under water than in air, especially at high frequencies. For historical reasons, a distinction is made between sonar devices (referred to as “sonars” for short), which are predominantly horizontal, and echo sounders, which are predominantly vertical.
Sound signals can be used for echolocation ( active sonar , including the echo sounder ) or to localize objects that emit sound themselves.
Active sonars use the echo principle like radar systems, i.e. they emit a signal themselves, the echo of which they receive and from which they determine the distance over the transit time of the echo. Echosounders are of this type.
In the literature, the term passive sonar is often used , whereby this information can refer to the passive operating mode of a positioning system that is also capable of active transmission. Only the signals or noises generated independently by objects are received, which protects the camouflaging of the tracking system. The direction of the incident sound can be determined in both operating modes, but precise and reliable distance measurement is often only possible in the active operating mode.
The distinction between "sonar systems", which can have a passive operating mode in addition to the active one, and "listening systems", which only allow passive sound localization, is often not clear, especially in the English-language literature.
The passive tracking ability by water sound was from the universal genius Leonardo da Vinci was first mentioned in 1490: "However, if you dive a tube into the water and keep the other end to your ear, you can ships at great distances to hear." Whether he this has successfully applied it is questionable, since ships without a motor generate comparatively little noise. But perhaps it was possible to hear the drum beat of the clock for the rowers, the rhythmic dipping of the oars, or the creaking of the wooden hull in that way.
Three men are named who at about the same time, completely independently of one another, patented an active system for locating icebergs , which was unsuccessful against icebergs, but was suitable for depth locating as an echo sounder . All three were apparently under the impression of the sinking of the Titanic : these are Alexander Behm (see echo sounder), the Canadian inventor Reginald Fessenden and finally Lewis Fry Richardson .
During the First World War , interest in further developing active sonar to locate submarines increased. In 1915, the French physicist Paul Langevin , together with the electrical engineer Constantin Chilowski, who emigrated from Russia, developed the first active sonar system that was ultimately suitable for locating submarines at a distance of around 1500 m. On the British side, intensive work was carried out on an active sonar under the name ASDIC . There was no operational use of active sonar in the First World War. On the other hand, different versions of passive systems have already been used.
Although the development between the world wars proceeded relatively slowly, sufficiently developed passive and active systems were available during the Second World War . Nevertheless, submarines were initially located mainly when they were surfaced, because at that time they only appeared temporarily in the event of danger or to attack (“submersibles”). The importance of the active sonar was to maintain an even gained contact with a submerged submarine, regardless of its own noises and before the depth charge used its distance, course and speed to determine.
The diving depth of the submarine, on the other hand, could only be estimated because of the refraction of sound in the water and initially the lack of pivoting ability of the sonar systems in the vertical. Sonar contact was mostly lost immediately before depth charges because the attacking ship had to approach at high speed from a distance in order not to be damaged by the depth charges that exploded with a time delay.
Because of the ship's own noise, it was not possible to locate the sonar at medium speed or in the aft area. Even depth charges that have already exploded will disturb them for some time. The range of the sonar systems depends heavily on the water and weather conditions, and it is often difficult to differentiate between real and apparent location targets. This gave the submarines, which had once reached their greatest possible depth, good opportunities to evade depth charges with their own maneuvers and also to escape active sonar location.
The typical active sonar was the search beam sonar ("Search Light Sonar") in the frequency range between 15 kHz and 40 kHz. Short sound impulses, so-called pings, were sent out.
The coverage of a sector with active sonar was very time-consuming because of the signal propagation time (for each angle step). Area surveillance was therefore not very successful. In contrast to this, despite a similar functional principle, area monitoring is possible with radar because electromagnetic waves can be used, which have a propagation speed several orders of magnitude higher in air than sound waves in water.
Once contact has been made with an object, search beam sonar can hold the target well. The "split-beam" technique offers help, with which a two-part receiver can be used to determine whether the target is moving out of the beam axis.
The panorama sonar (English PPI - Plane Position Indication ) only appeared after the Second World War. This means that all-round or sector-wise transmission is carried out, and when receiving, a sector or all-round image of a sonar ping is formed simultaneously (or electronically swiveled very quickly) with an electronic directional formation of the beam through corresponding phase or transit time changes. This was the common technique for the first 20 years after World War II.
A major innovation in the 1970s was the introduction of towed array sonar (TAS). In this passive location system, a chain of hydrophones in an oil-filled hose is dragged along a long cable behind the ship. Due to the oil, the hose floats under water without being buoyant. Mechanical dampers for acoustic decoupling are installed between the hydrophone hose and the towing cable so that vibrations from the ship have as little disruptive influence as possible. With this technology, very low frequencies down to below 100 Hz can be made usable. They are not attenuated much in deep water, but cannot be decoupled with an antenna on the ship and required very large dimensions due to the large wavelength.
The latest development from around 1985 is the active low-frequency towing sonar or LFAS (Low Frequency Active Sonar), which works at frequencies below 2 kHz, sometimes at a few hundred Hz.
During the Cold War , considerable effort was made in developing sonar technology, parallel to the great advances in submarine technology. Even after the end of the Cold War is the sonar technology is of great importance because of the undersea terrorist threat and the development of unmanned underwater vehicles (UUV, Unmanned Underwater Vehicles ), especially the autonomous underwater vehicles (AUV Autonomous Underwater Vehicles ), which increasingly determine the Seekriegssituation.
Known sonar research institutes in the US, the United States Naval Research Laboratory , in NATO , the NATO Undersea Research Center in La Spezia , Italy, and Germany, the Bundeswehr Institute for Underwater Sound and Geophysical .
There are low-frequency (50 Hz to 3 kHz), medium-frequency (3 kHz to 15 kHz) and high-frequency active sonars.
Low-frequency sonars are used as long-range sonars with transmission distances of over 20 km or surveillance systems for submarine hunting , i.e. H. used to locate submarines. This results from the low attenuation of low frequencies. It is usually designed as a towing system. Medium-frequency systems are the traditional underground hunting systems. The torpedo sonar in the head of a torpedo uses 20 kHz to 60 kHz. Mine hunting and mine avoidance sonars use frequencies above 100 kHz because of the high resolution required and the short distance required. Side view sonars are imaging sonar systems that are used in research and mine hunting.
In the civil sector, for example, schools of fish are located with a fish magnifier . The demarcation to the echo sounder is fluid there. In addition to locating the schools of fish, pelagic trawl fishing uses net probes based on the echo sounder principle , which enable both the depth control of the fishing gear and the monitoring of the distance between the shear boards or the net geometry.
The advantage of the active sonar compared to the passive sonar is that it can easily determine the direction as well as the distance to the target and that it is also suitable for targets that do not emit any noise. Therefore, mine hunting and mine avoidance sonars are always active sonars. Their disadvantage is that they reveal the presence of an actively transmitting sonar carrier well beyond the detection range of the system and that they can be a burden for the environment, especially for marine mammals. Submarines use active sonar extremely sparingly, if at all.
So-called search light - ("searchlight" -) sonars are now out of date and are hardly used any more. With this type of system, the transmitter / receiver is rotated mechanically. This means that you can only detect in one direction. Most of today's anti-submarine sonars (except tow sonars) use a circle or part circle system. There the transmitters / receivers are arranged in a circle and several on top of each other. In this way you can monitor all directions at the same time and still detect them in a targeted manner. You can then electronically control individual groups or all of them and send an all-round ping or ping one after the other.
The passive sonar consists of at least one hydrophone (underwater microphone ) and is used to detect underwater noises and other acoustic signals. The passive sonar itself does not emit any sound waves and therefore, unlike the active sonar, cannot be located.
Comprehensive hydrophone arrangements are normally used to determine the direction and to separate the noise that is being sought from other noise sources. This makes it possible to determine the direction of the target, but not its distance. The distance is attempted through complex strategies with algorithms that evaluate one's own movement and the presumed distance of the target with the help of changing the directional bearing (Target Motion Analysis - TMA). Another, more recent approach tries to determine the distance (and depth) of the target from the vertical distribution of the sound field by inverse modeling.
Passive sonar is mainly used by submarines because it does not reveal the location of this submarine through sound emissions. Since around 1975, acoustic tow antennas of up to several kilometers in length have been used to locate submarines from surface ships with the help of the low-frequency radiation, which is difficult to reduce. However, successes in reducing the noise of submarines are now limiting their usability. That is why these towed antennas are supplemented by acoustic transmitters (Active Adjunct), so one goes back to active sonar (when locating using surface ships, not submarines).
In general, surface ships can be located primarily by the cavitation noise caused by the collapse of bubbles caused by the water “tearing” in the negative pressure area of the propeller. However, it (the noise of the drive diesel engine, pump noise, gear noise, all the possible clicks and rattles transients ) and other sounds are located, which is also the classification can provide the type of noise source.
Bi- and multistatic sonar
In order to find the optimal compromise between the properties of the active and passive sonar, one has recently turned to bi- or multistatic sonar. A bistatic sonar is basically an active sonar, but the transmitter is on a different platform than the receiver. Both can be very far apart. The advantage is that the active signal does not reveal the recipient. As a result, the enemy cannot tactically adjust to the situation so easily with military sonar.
The disadvantage is that the transmitter and receiver have to be coordinated in some way in order to take advantage of the distance determination and thus the fast target positioning. It is also much more difficult to gauge the performance of the system and build a reasonable display .
In the case of multistatic sonar, several receivers (each on a separate platform, e.g. ship or submarine) are used for one transmitter, possibly also several transmitters, all of which have to be coordinated with one another. This ultimately leads to distributed systems.
Individual sonar types
In addition to the basic sonar versions mentioned, there are a number of sonars that differ in structure and use:
HMS stands for English Hull Mounted Sonar (hull-mounted sonar). The (submarine) sonar is attached directly to the ship, most often in a special bulge on the bow (bow sonar). This bead has a different shape and a different purpose than the bulbous bow that is widespread today to reduce the flow resistance . The more lenticular sonar bulge is at the front and deeper than the deepest part of the hull in order to achieve a good view to the front and rear.
TAS ( English Towed Array Sonar ), trailing antenna sonar or towed sonar, refers to a passive low frequency sonar for the U-hunting. It is towed behind the ship as a long line antenna, a hose with hydrophones, following a cable. This allows the antenna to be operated at the most favorable depth and is removed from the noise of its own platform.
FAS ( English Flankarray Sonar ), page antenna sonar, refers to a passive sonar at both sides of the fuselage at submarines.
LFA ( English Low Frequency Active Sonar ), low frequency active sonar, called active sonars with low frequencies depending on the type of between about 100 Hz and 3 kHz.
VDS ( English Variable Depth Sonar ): Sonar for variable depths, refers to a towed array sonar, which no long line antenna but a compact device that is towed on a cable behind the ship used in contrast to the recent CAS or LFA.
Mine hunting sonar
Mine hunting sonars (e.g. mine hunting sonar DSQS11M) are high-frequency sonars for the detection and classification of sea mines (ground mines | anchor rope mines), which are then identified optically by mine divers or drones.
Mine avoidance sonar
Mine avoidance sonars are high-frequency active sonars for warning about mines.
A diving sonar English dipping sonar is a sonar that is disconnected from the helicopter. In the past, simple hydrophones were also used, therefore similar to a sonar buoy; today, active sonars similar to a VDS are preferably used.
Sonoboje (sonar buoy)
Sono buoys (sonar buoys) are dropped from aircraft or helicopters for submarine hunting. They hang a hydrophone at a predetermined depth and send received signals back to the aircraft via UHF radio frequencies. There are also more complicated sonoboys with multiple hydrophones for direction formation and active sonoboys.
Passive sonoboys can also be used unnoticed to record acoustic "fingerprints" from surface and underwater vehicles. For this they are also released from ships / boats.
Side viewing sonar
The side scan sonar ( English side-scan sonar ) is an imaging sonar for research and for mine hunting.
Harmful effects on marine mammals
As can be seen from autopsies of stranded marine mammals , a number of whale strandings have been linked to military sonar experiments since 1985 .
In December 2001, the US Navy admitted complicity in the stranding and death of several marine mammals in March 2000. The interim report, which she co-authored, concluded that the animals were killed or injured by the active sonar of some Navy ships.
The active low-frequency sonar systems (Low Frequency Active Sonar, LFAS) used in the military sector can, with their sound pressure of up to 240 decibels, frighten and stun marine mammals such as whales and dolphins and, presumably , kill them through the subsequent rapid changes in depth ( decompression sickness ). Sperm whales can generate similarly high sound pressures. Location signals of up to 180 decibels were measured behind her head, the sound pressure level in front of her head is probably up to 40 decibels higher. More than 180 decibels are given for blue whales.
With this information it must be taken into account that a different reference value (1 µPa ) is used for the dB unit for pressure with water- borne noise than for air-borne noise (20 µPa). For identical absolute pressures, the specified water-borne sound pressure level is exactly 26 dB higher, a sound pressure level of 26 dB specified for water corresponds to a sound pressure level of 0 dB for air (roughly the human hearing threshold). (The reference values for intensity levels differ even more, the reference value for air is 10 −12 watt / m² and for water 6.7 · 10 −19 watt / m², so that with the same absolute value there is a numerical difference of 61.7 dB .)
Nevertheless, the animals examined show severe physiological damage, including cerebral haemorrhage, vascular injuries, vesicle formation in the blood and cardiovascular collapse. A high number of unreported cases must also be assumed, since animals that die in the open sea sink to the sea floor and remain undetected.
- Radar grammetry
- Sound surveillance system
- Human echolocation
- Lateral line organ - fish absorb pressure fluctuations triggered by movements
- Robert J. Urick: Principles of Underwater Sound. 2nd edition. McGraw-Hill Book Company, New York NY 1975, ISBN 0-07-066086-7 .
- Heinz G. Urban: Handbook of water-borne sound technology. STN Atlas Electronics, Bremen 2000.
- Gerhard Aretz: Sonar in theory and practice for underwater applications. Monsenstein and Vannerdat, Münster 2006, ISBN 3-86582-393-9 .
- Philippe Blondel, Bramley J. Murton: Handbook of seafloor sonar imagery. Wiley et al. a., Chichester et al. a. 1997, ISBN 0-471-96217-1 ( Wiley Praxis Series in Remote Sensing ).
- Harrison T. Loeser (Ed.): Sonar engineering Handbook. Peninsula Publishing, Los Altos CA 1992, ISBN 0-932146-02-3 .
- Are the sonars leaving the ship? In: Hansa - International Maritime Journal . January 2003, p. 38-42 .
- whalesong.info sound sample of a sonar ( MP3 )
- Underwater noise: whales in constant stress . Greenpeace, November 22, 2007
- Why whales strand - the deadly hunt for submarines . Sounds Of The Seas, 2011
- ↑ Sonar . duden.de; Retrieved August 13, 2011.
- ↑ Scanmar | Trålsonde. Retrieved October 5, 2018 (American English).
- ↑ Thünen Institute: network probe systems. Retrieved October 5, 2018 .
- ↑ U-boat tracking device kills whales . telepolis , January 14, 2002; Retrieved September 2, 2013.
- ^ Joint Interim Report, Bahamas Marine Mammal Stranding, Event of 15-16 March 2000 , ( Memento of October 4, 2013 in the Internet Archive ) (PDF; 1.6 MB) bahamaswhales.org, December 2001; Retrieved September 2, 2013.
- ↑ Ulf Marquardt: Hell Noise in the Realm of Silence (PDF). WDR, October 9, 2007, accessed May 9, 2017 .
- ↑ With the microphone into the deep sea . NZZ.ch, November 20, 2002; Retrieved September 28, 2011.
- ↑ Our animal friends - real super ears . Planet school; Retrieved September 28, 2011.
- ↑ a b Reinhard Lerch, Gerhard Martin Sessler, Dietrich Wolf: Technical acoustics . Basics and Applications. Springer, Berlin / Heidelberg 2009, ISBN 978-3-540-23430-2 , pp. 539 (Chapter 17.1 Sound Propagation in Water).