Towed array sonar: Difference between revisions

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{{Short description|System of hydrophones}}

{{refimprovemore citations needed|date=May 2014}}

[[File:Motte-Picquet-tugged-sonar.jpg|thumb|left|The DUBV 43C towed array sonar of the [[Lamotte-PicquetFrench (Dfrigate 645)La Motte-Picquet|''LamotteLa Motte-Picquet'' (D 645)]].]]

A '''towed array sonar''' is a system of hydrophones[[hydrophone]]s towed behind a [[submarine]] or a surface ship on a cable.<ref>{{cite news|url=http://www.eie.polyu.edu.hk/~enktwong/ktw/TichavskySPT0104.pdf|title=Quasi-Fluid-Mechanics-Based Quasi-Bayesian Cramér–Rao Bounds for Deformed Towed-Array Direction Finding|author=Petr Tichavský and Kainam Thomas Wong|work=IEEE TRANSACTIONS ON SIGNAL PROCESSING|volume=52|edition=1|date=January 2004|page=36|access-date=2010-10-08|archive-date=2016-03-03|archive-url=https://web.archive.org/web/20160303201518/http://www.eie.polyu.edu.hk/~enktwong/ktw/TichavskySPT0104.pdf|url-status=dead}}</ref> Trailing the hydrophones behind the vessel, on a cable that can be kilometers long, keeps the array's sensors away from own-the ship's- own noise sources, greatly improving its [[signal-to-noise ratio]], and hence the effectiveness of detecting and tracking faint contacts, such as a quiet, low noise-emitting submarine threats, or seismic signals.<ref name="Defence Today">{{cite news|url=http://www.ausairpower.net/SP/DT-TAS-Dec-2009.pdf|title=Identification underwaterwith towed array sonar|author=Carlo Kopp|work=Defence Today|date=December 2009|pages=32–33}}</ref>

A towed array offers superior resolution and range compared with hull mounted sonar. It also covers the [[Baffles (submarine)|baffles]], the blind spot of hull mounted sonar. However, effective use of the system limits a vessel's speed and care must be taken to protect the cable from damage.

A towed array offers superior resolution and range compared with hull -mounted sonar. It also covers the [[Baffles (submarine)|baffles]], the blind spot of hull -mounted sonar. However, effective use of the system limits a vessel's speed and care must be taken to protect the cable from damage.

[[File:Motte-Picquet-tugged-sonar.jpg|thumb|left|The DUBV 43C towed array sonar of the [[Lamotte-Picquet (D 645)|''Lamotte-Picquet'' (D 645)]].]]

==History==

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==Current use of towed arrays==

On surface ships, towed array cables are normally stored in drums, then spooled out behind the vessel when in use. U.S. Navy submariessubmarines typically store towed arrays inside an outboard tube, mounted along the vessel's hull, with an opening on the starboard tail.<ref name="Defence Today" /> There is also equipment located in a ballast tank (free flood area) while the cabinet used to operate the system is inside the submarine.<ref>{{Cite web|url=https://www.navy.mil/submit/display.asp?story_id=93664|archive-url=https://web.archive.org/web/20160321203624/http://www.navy.mil/submit/display.asp?story_id=93664|url-status=dead|archive-date=March 21, 2016|title=MARMC Engineers Host NUWC Newport OA-9070E Towed Handling System Training|last=Affairs|first=This story was written by Shelby West, MARMC Public|website=www.navy.mil|language=en|access-date=2020-02-09}}</ref>

Hydrophones in a towed array system are placed at specific distances along the cable, the end elements far enough apart to gain a basic ability to triangulate on a sound source. Similarly, various elements are angled up or down <ref>{{citationCite neededjournal|last=Thomas Folegot, Giovanna Martinelli, Piero Guerrini, J. Mark Stevenson|date=May2008|title=An active acoustic tripwire for simultaneous detection and localization of multiple underwater intruders|journal=The Journal of the Acoustical Society of America|volume=124 |issue=5 |pages=2852–2860 |doi=10.1121/1.2977675 |pmid=19045773 |bibcode=2008ASAJ..124.2852F 2014}},</ref> giving an ability to triangulate an estimated vertical depth of target. Alternatively three or more arrays are used to aid in depth detection.

On the first few hundred metresmeters from the ship's [[propeller]] there are usually no hydrophones, because their effectiveness would be reduced by noise ([[cavitation]] and hull flow noises), vibration and turbulence generated by the propulsion&mdash;which would repeat the same problems of ship-mounted arrays. [[Surveillance Towed Array Sensor System]]s used by surface ships have a sonar array mounted on a cable, which pulls a depth-adjustable [[remote operated vehicle]] (ROV). Another weighted cable may trail from the ROV connector, dropping the towed array to a lower depth. Long seismic streamers have intermediate paravanes along their length which can be used to adjust the depth of the array in real time.

Changing an ROV's depth allows a towed array to be deployed in different [[Thermocline|thermal layers]], giving a surface anti-submarine warfare (ASW) vessel a view above and below the layer. This compensates for density and temperature differences, which ductconduct sound above or below a thermal layer by reflection. By dropping the array's 'tail' below the layer, a surface ASW platform can better detect a quiet, submerged contact hiding in cold water below a warm upper layer. A submarine can likewise monitor surface combatants by floating the tail of its array above a thermal layer while lurking below.

[[Image:Akula class submarine.JPG|right|thumb|When not deployed, an [[Akula -class submarine|''Akula's']]'s towed array is stored in a teardrop shaped container mounted on top of the vertical fin]]

The array's hydrophones can be used to detect sound sources, but the real value of the array is that the [[signal processing]] technique of [[beamforming#Sonar beamforming|beamforming]] and [[Fourier analysis]] can be used not only to calculate the distance and the direction of a sound source, but also to identify the type of ship by the distinctive, acoustic signature of its machinery noises. For this, the relative positions of the hydrophones need to be known, usually possible only when the cable is in a straight line (stable), or when a self-sensing system (see [[strain gauge]]s) or GPS or other methods embedded in the cable, and reporting relative position of hydrophone elements, is used to monitor the shape of the array and correct for curvature.

As an example, [[Thales Underwater Systems]]' CAPTAS-2 (passive and active sonar) claims a detection range up to 60 km and weighs 16t.<ref>{{cite web|url=https://www.thalesgroup.com/sites/default/files/database/document/2020-10/UWS_BL_Datasheet_CAPTAS-2_2020.pdf|title=CAPTAS-2}}</ref> The heavier CAPTAS-4 weighs 20-34t and claims a detection range up to 150 km.<ref>{{cite web|author=Luca Peruzzi|url=https://www.thalesgroup.com/sites/default/files/database/document/2020-10/UWS_BL_Datasheet_CAPTAS-4-2020.pdf|title=CAPTAS-4}}</ref><ref>{{cite web|title=Towed Active Sonar Systems – a Growing Industry|url=https://euro-sd.com/2022/05/articles/exclusive/26297/towed-active-sonar-systems-a-growing-industry/|author=Luca Peruzzi|date=25 May 2022 }}</ref>

===Use in geophysics===

Towed array systems are also used by the oil and gas industry for seismic exploration of geological formations under the sea bed.<ref>{{cite book|url=httphttps://books.google.com/books?id=WZuBmNOLGo8C&pg=PA524&lpg=PA524&dqq=towed+array+sonar+history&sourcepg=bl&ots=xIUJwq17Wx&sig=0Qx6w5AwvRskfsLR8S7vJkMHzi8&hl=en&sa=X&ei=lf9wU-aCJobisATh44DYAQ&ved=0CGAQ6AEwDg#v=onepage&q=towed%20array%20sonar%20history&f=falsePA524|title= Advanced Topics in Science and Technology in China: Digital Sonar Design in Underwater Acoustics: Principles and Applications|author=Qihu Li|date=2012|publisher=Zhejiang University Press|page=524|ISBNisbn=978-7-308-07988-4}}</ref> The systems used are similar in concept to the naval ones, but are typically longer and with more streamers in a given array (6 or more in some cases). Typical hydrophone spacing along each streamer is ofon the order of two meters, and each streamer may be up to 10&nbsp;km long. Sometimes streamers are flown at different heights, to give a so-called 3D array.

===Limitations===

Effective use of the towed array system requires a vessel to maintain a straight, level course over a data sampling interval. Maneuvering, or changing course, disturbs the array and interruptscomplicates analysis of the sampled data stream. These periods of instability are closely tested during sea trials and known by the crew's officers and enlisted sonar experts. Modern systems compensate by constantly self-measuring the relative positions of the array, element to element, reporting back data that can be automatically corrected for curvatures by computers as part of the beam formingbeamforming math processing.

A ship must also limit its overall top speed when while a towed array is deployed. [[hydrodynamics|Hydrodynamic]] [[Drag (physics)|drag]] increases as a [[square function]] of velocity, and could tear the cable or damage its mooring hardware. Furthermore, a minimum speed may have to be established depending on the buoyancy of the towed array (military arrays are ballasted to sink, geophysics arrays are supposed to be neutrally buoyant at about 10m). The array could also be damaged by contact with the seafloor or if the vessel operates [[astern propulsion]], or can even be damaged if it bends too tightly.

== See also ==

*[[Aperture synthesis]]

*[[Phased array]]

*[[Fast Fourier transform|FFT]]

*[[Phased array]]

*[[Spectrum analyzer]]

*[[Sonar]]

*[[Synthetic aperture sonar]]

*[[Towed pinger locator]]

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[[Category:Surveillance]]

[[Category:Sonar]]

[[Category:SubmarinesSubmarine components]]

[[Category:Multidimensional signal processing]]