Editing Sonar (section)

== Military applications ==
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 1987 a division of Japanese company [[Toshiba]] reportedly<ref>{{Cite web|url=https://foxtrotalpha.jalopnik.com/how-the-soviet-akula-changed-submarine-warfare-1819380681|title=How The Soviet Akula Changed Submarine Warfare|website=Foxtrot Alpha|date=13 October 2017 |language=en-us|access-date=2020-01-15}}</ref> 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 and is used by submarines when the tactical situation dictates that 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 as 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 [[sonobuoy]]s 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 that 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. On a submarine, 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 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. Databases of unique engine sounds are part of what is known as ''acoustic intelligence'' or ACINT.

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.

=== Anti-submarine warfare ===
[[File:Variabel dybde sonar (VDS) på et af søværnets skibe af Thetis-klassen.jpg|thumb|upright|Variable depth sonar and its winch]]
Until recently, ship sonars were usually made 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-56.

Because of the problems of ship noise, towed sonars are also used.  These have the advantage of being able to be placed deeper in the water, but have limitations on their use in shallow water.  These are called towed arrays (linear) or variable depth sonars (VDS) with 2/3D arrays.  A problem is that the winches required to deploy/recover them are large and expensive.  VDS sets are primarily active in operation, while towed arrays are passive.

An example of a modern active-passive ship towed sonar is [[Sonar 2087]] made by [[Thales Underwater Systems]].

=== Torpedoes ===
Modern torpedoes are generally fitted with an active/passive sonar.  This may be used to home directly on the target, but [[wake homing]] 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 (decoy)|''Bold'']] was a chemical device.  A widely used US device was the towed [[AN/SLQ-25 Nixie]] while the [[mobile submarine simulator]] (MOSS) was a free device. A modern alternative to the Nixie system is the [[SSTD|UK Royal Navy S2170 Surface Ship Torpedo Defence]] system.

=== Mines ===
Mines may be fitted with a sonar to detect, localize and recognize the required target.  An example is the [[CAPTOR mine]].

=== Mine countermeasures ===
Mine countermeasure (MCM) sonar, sometimes called "mine and obstacle avoidance sonar (MOAS)", is a specialized type of sonar used for detecting small objects.  Most MCM sonars are hull mounted but a few types are VDS design.  An example of a hull mounted MCM sonar is the Type 2193 while the SQQ-32 mine-hunting sonar and Type 2093 systems are VDS designs.

=== Submarine navigation ===
{{Main|Submarine navigation}}
Submarines rely on sonar to a greater extent than surface ships as they cannot use radar in water.  The sonar arrays may be hull mounted or towed.  Information fitted on typical fits is given in {{sclass|Oyashio|submarine|1}} and {{sclass|Swiftsure|submarine|1}}.

=== Aircraft ===
[[File:SH-3H HS-15 lowers AQS-13 sonar 1979.JPEG|thumb|[[AN/AQS-13]] dipping sonar deployed from an [[H-3 Sea King]]]]

Helicopters can be used for antisubmarine warfare by deploying fields of active-passive sonobuoys or can operate dipping sonar, such as the [[AQS-13]].  Fixed wing aircraft can also deploy sonobuoys and have greater endurance and capacity to deploy them.  Processing from the sonobuoys or [[Geophysical MASINT#Airborne dipping sonar|dipping sonar]] can be on the aircraft or on ship.  Dipping sonar has the advantage of being deployable to depths appropriate to daily conditions. Helicopters have also been used for mine countermeasure missions using towed sonars such as the [[AQS-20A]].

===Underwater communications===
Dedicated sonars can be fitted to ships and submarines for underwater communication.

===Ocean surveillance===
The United States began a system of passive, fixed ocean surveillance systems in 1950 with the classified name [[SOSUS|Sound Surveillance System (SOSUS)]] with [[American Telephone and Telegraph Company]] (AT&T), with its [[Bell Laboratories]] research and [[Western Electric]] manufacturing entities being contracted for development and installation. The systems exploited the [[SOFAR channel]], also known as the deep sound channel, where a sound speed minimum creates a [[waveguide]] in which low frequency sound travels thousands of miles. Analysis was based on an AT&T sound spectrograph, which converted sound into a visual [[spectrogram]] representing a [[time–frequency analysis]] of sound that was developed for speech analysis and modified to analyze low-frequency underwater sounds. That process was [[Low Frequency Analyzer and Recorder (LOFAR)|Low Frequency Analysis and Recording]] and the equipment was termed the Low Frequency Analyzer and Recorder, both with the acronym LOFAR. LOFAR research was termed ''Jezebel'' and led to usage in air and surface systems, particularly sonobuoys using the process and sometimes using "Jezebel" in their name.<ref name=Whitman>{{cite magazine |last=Whitman |first=Edward C. |date=Winter 2005 |title=SOSUS The "Secret Weapon" of Undersea Surveillance |magazine=Undersea Warfare |volume=7 |issue=2 |url=https://www.public.navy.mil/subfor/underseawarfaremagazine/Issues/Archives/issue_25/sosus.htm |access-date=5 January 2020 |archive-date=24 March 2020 |archive-url=https://web.archive.org/web/20200324114806/https://www.public.navy.mil/subfor/underseawarfaremagazine/Issues/Archives/issue_25/sosus.htm |url-status=dead }}</ref><ref name=ICAA>{{cite web |title=Integrated Undersea Surveillance System (IUSS) History 1950 - 2010 |publisher=IUSS/CAESAR Alumni Association |url=http://www.iusscaa.org/history.htm |access-date=22 May 2020}}</ref><ref>{{cite journal |last1=Munk |first1=Walter H. |last2=Spindel |first2=Robert C. |last3=Baggeroer |first3=Arthur |last4=Birdsall |first4=Theodore G. |date=20 May 1994  |journal=Journal of the Acoustical Society of America |title=The Heard Island Feasibility Test |volume=96  |issue=4 |pages=2330–2342 |publisher=Acoustical Society of America |doi=10.1121/1.410105 |bibcode=1994ASAJ...96.2330M |url=https://docs.google.com/viewer?url=http%3A%2F%2Fbrigus.physics.mun.ca%2F~zedel%2FP6317%2Fpapers%2Fheard_island.pdf |access-date=26 September 2020}}</ref><ref>{{cite book |last1=Lieberman |first1=Philip |last2=Blumstein |first2=Sheila E.  |date= 4 February 1988|title=Speech Physiology, Speech Perception, and Acoustic Phonetics |location=Cambridge, Cambridgeshire, UK/New York |publisher=Cambridge University Press  |isbn=0521308666 |lccn=87013187  |pages=51–52 |url=https://books.google.com/books?id=c07XORQ3NdkC&pg=PA51 |access-date=22 May 2020}}</ref> The proposed system offered such promise of long-range submarine detection that the Navy ordered immediate moves for implementation.<ref name=ICAA/><ref name=CUS>{{cite web |title=Origins of SOSUS |publisher=Commander, Undersea Surveillance |url=https://www.public.navy.mil/subfor/cus/Pages/sosus_origins.aspx |access-date=22 May 2020 |archive-date=7 August 2020 |archive-url=https://web.archive.org/web/20200807123302/https://www.public.navy.mil/subfor/cus/Pages/sosus_origins.aspx |url-status=dead }}</ref>

[[File:Watch floor.gif|thumb|Lofargram writers, one for each array beam, on a NAVFAC watch floor]]
Between installation of a test array followed by a full scale, forty element, prototype operational array in 1951 and 1958 systems were installed in the Atlantic and then the Pacific under the unclassified name ''Project Caesar''. The original systems were terminated at classified shore stations designated Naval Facility (NAVFAC) explained as engaging in "ocean research" to cover their classified mission. The system was upgraded multiple times with more advanced cable allowing the arrays to be installed in ocean basins and upgraded processing. The shore stations were eliminated in a process of consolidation and rerouting the arrays to central processing centers into the 1990s. In 1985, with new mobile arrays and other systems becoming operational the collective system name was changed to Integrated Undersea Surveillance System (IUSS). In 1991 the mission of the system was declassified. The year before IUSS insignia were authorized for wear. Access was granted to some systems for scientific research.<ref name=Whitman/><ref name=ICAA/>

A similar system is believed to have been operated by the Soviet Union.

===Underwater security===
{{main|Anti-frogman techniques#Detection}}
Sonar can be used to detect [[frogmen]] and other [[scuba diver]]s.  This can be applicable around ships or at entrances to ports.  Active sonar can also be used as a deterrent and/or disablement mechanism.  One such device is the [[Anti-frogman techniques#Cerberus|Cerberus]] system.

[[File:US Navy 030223-N-7479T-011 An AN-PQS-2A, a handheld sonar device used to aid in the search of the Toledo Bend Reservoir for debris from the Space Shuttle Columbia.jpg|thumb|upright|AN/PQS-2A handheld sonar, shown with detachable flotation collar and magnetic compass]]

=== Hand-held sonar ===
Limpet mine imaging sonar (LIMIS) is a hand-held or [[Remotely operated underwater vehicle|ROV]]-mounted imaging sonar designed for patrol divers (combat [[frogmen]] or [[clearance diver]]s) to look for [[limpet mine]]s in low [[visibility]] water.

The LUIS is another imaging sonar for use by a diver.

Integrated navigation sonar system (INSS) is a small flashlight-shaped handheld sonar for divers that displays range.<ref>{{cite journal |url=http://archive.rubicon-foundation.org/7497 |title=Very High Resolution Imaging Diver Held Sonar |journal=Report to the Office of Naval Research |year=2002 |author=Lent, K |access-date=2008-08-11 |url-status=usurped |archive-url=https://web.archive.org/web/20081008092104/http://archive.rubicon-foundation.org/7497 |archive-date=2008-10-08 }}</ref><ref>{{cite web|url=http://archive.rubicon-foundation.org/7498|title=Diver Charting and Graphical Display|publisher=Texas Univ at Austin Applied Research Labs|author=Krueger, Kenneth L.|date=2003-05-05|access-date=2009-01-21|url-status=usurped|archive-url=https://web.archive.org/web/20090813191011/http://archive.rubicon-foundation.org/7498|archive-date=2009-08-13}}</ref>

=== Intercept sonar ===
This is a sonar designed to detect and locate the transmissions from hostile active sonars.  An example of this is the Type 2082 fitted on the British {{sclass|Vanguard|submarine|1}}s.