Towed Array Sonar
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A towed array sonar is a system of
hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...
s towed behind a
submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
or a surface ship on a cable. Trailing the hydrophones behind the vessel, on a cable that can be kilometers long, keeps the array's sensors away from the ship's own noise sources, greatly improving its
signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
, and hence the effectiveness of detecting and tracking faint contacts, such as quiet, low noise-emitting submarine threats, or seismic signals. A towed array offers superior resolution and range compared with hull mounted sonar. It also covers the 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.


History

During World War I, a towed sonar array known as the "Electric Eel" was developed by Harvey Hayes, a U.S. Navy physicist. This system is believed to be the first towed sonar array design. It employed two cables, each with a dozen hydrophones attached. The project was discontinued after the war. The U.S. Navy resumed development of towed array technology during the 1960s in response to the development of nuclear-powered submarines by the Soviet Union.


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 submarines typically store towed arrays inside an outboard tube, mounted along the vessel's hull, with an opening on the starboard tail. There is also equipment located in a ballast tank (free flood area) while the cabinet used to operate the system is inside the submarine. 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 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 meters from the ship's
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
there are usually no hydrophones, because their effectiveness would be reduced by noise (
cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...
and hull flow noises), vibration and turbulence generated by the propulsion—which would repeat the same problems of ship-mounted arrays.
Surveillance Towed Array Sensor System The AN/UQQ-2 Surveillance Towed Array Sensor System (SURTASS), colloquially referred to as the ship's "Tail", is a towed array sonar system of the United States Navy. SURTASS Twin-Line consists of either the long passive SURTASS array or the Twi ...
s used by surface ships have a sonar array mounted on a cable, which pulls a depth-adjustable
remote operated vehicle A remotely operated underwater vehicle (technically ROUV or just ROV) is a tethered underwater mobile device, commonly called ''underwater robot''. Definition This meaning is different from remote control vehicles operating on land or in the a ...
(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 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 conduct 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. The array's hydrophones can be used to detect sound sources, but the real value of the array is that the
signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing ''signals'', such as audio signal processing, sound, image processing, images, and scientific measurements. Signal processing techniq ...
technique of
beamforming Beamforming or spatial filtering is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in an antenna array in such a way that signals at particular angles e ...
and
Fourier analysis In mathematics, Fourier analysis () is the study of the way general functions may be represented or approximated by sums of simpler trigonometric functions. Fourier analysis grew from the study of Fourier series, and is named after Josep ...
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 A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports ...
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 Thales Underwater Systems or TUS (formerly Thomson Sintra ASM, Thomson CSF DASM and then Thomson Marconi Sonar) is a subsidiary of the French defense electronics specialist Thales Group. It was created in 2001 and belongs to its naval division. It ...
' CAPTAS-2 (passive and active sonar) claims a detection range up to 60 km and weighs 16t. The heavier CAPTAS-4 weighs 20-34t and claims a detection range up to 150 km.


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. 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 on the order of two meters, and each streamer may be up to 10 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 complicates 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 beamforming math processing. A ship must also limit its overall top speed while a towed array is deployed.
Hydrodynamic In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) and ...
drag increases as a
square function In mathematics, a square is the result of multiplying a number by itself. The verb "to square" is used to denote this operation. Squaring is the same as raising to the power  2, and is denoted by a superscript 2; for instance, the square ...
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 Astern propulsion (as applied to a ship) is a maneuver in which a ship's propelling mechanism is used to develop thrust in a retrograde direction. Astern propulsion does not necessarily imply the ship is moving astern (''in reverse''); astern ...
, or can even be damaged if it bends too tightly.


See also

*
Aperture synthesis Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation an ...
*
Phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
*
FFT A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in the ...
*
Spectrum analyzer A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...
* Synthetic aperture sonar *
Towed pinger locator A towed pinger locator is a water-borne device used to locate the sonar "ping" from the underwater locator beacon which is fitted to the Cockpit Voice Recorders and Flight Data Recorders installed in commercial airliners. They can locate pingers ...


References


External links


Towed Hydrophone Arrays (OSC)
{{hydroacoustics Surveillance Sonar Submarines Multidimensional signal processing