Towed array sonar
Encyclopedia
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
array that is towed behind a submarine
Submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...
or surface ship
Ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...
. It is basically a long cable, up to 5 km, with hydrophone
Hydrophone
A hydrophone 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 electricity when subjected to a pressure change...
s that is trailed behind the ship when deployed. The hydrophones are placed at specific distances along the cable. On the first few hundred metres near the ship's propeller
Propeller
A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...
there are usually none since their effectiveness would be reduced by noise, vibration and turbulence generated by the propulsion. Surface ships often have the sonar array mounted on a cable which pulls a tow vehicle (an ROV - see Surveillance Towed Array Sensor System
Surveillance Towed Array Sensor System
The AN/UQQ-2 Surveillance Towed Array Sensor System is a towed array sonar system of the United States Navy.SURTASS began as development program in 1973 using the new research vessel Moana Wave. In 1980 SURTASS passed OPEVAL...
) behind the mother ship, or on another cable which trails from the ROV connector. By changing the ROV's depth, the sensor can be deployed in different thermal layers
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
.
Signal processing
Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
technique of beamforming and interferometry
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
can be used to calculate the distance and the direction of a sound source. For this, the relative positions of the hydrophones need to be known, usually only guaranteed when the cable is in a straight line, or else a GPS is used to monitor the shape of the array. Therefore, a vessel using a towed array will need to travel straight and level lest a change of course disturbs the array and reduces its effectiveness. This requirement is reduced in modern systems by sensors which constantly measure the relative positions of the array. Also it has to reduce its speed as the hydrodynamic drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
might tear the cable - this can also happen if the array makes contact with the seafloor or the submarine operates astern propulsion
Astern propulsion
Astern propulsion is a maneuver in which a ship's propelling mechanism is used to develop thrust in a retrograde direction...
.
Despite all those disadvantages, a towed array is useful since it offers better resolution and range compared to a hull mounted sonar and it covers the baffles
Baffles (submarine)
The baffles is the area in the water directly behind a submarine or ship through which a hull-mounted sonar cannot hear. This blind spot is caused by the need to insulate a sonar, commonly mounted near the bow, from the noise of the vessel's machinery....
, the blind spot of hull mounted sonar.
See also
- Aperture synthesisAperture synthesisAperture 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...
- Phased arrayPhased arrayIn wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
- SonarSonarSonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
- Surveillance Towed Array Sensor SystemSurveillance Towed Array Sensor SystemThe AN/UQQ-2 Surveillance Towed Array Sensor System is a towed array sonar system of the United States Navy.SURTASS began as development program in 1973 using the new research vessel Moana Wave. In 1980 SURTASS passed OPEVAL...
(SURTASS) - Synthetic aperture sonarSynthetic aperture sonarSynthetic aperture sonar is a form of sonar in which sophisticated post-processing of sonar data are used in ways closely analogous to synthetic aperture radar. Synthetic aperture sonars combine a number of acoustic pings to form an image with much higher resolution than conventional sonars,...
- Towed Hydrophone Arrays (OSC)
External links
- P. Tichavsky & K. T. Wonghttp://www.eie.polyu.edu.hk/~enktwong/, “Quasi-Fluid-Mechanics-Based Quasi-Bayesian Cramer-Rao Bounds for Deformed Towed-Array Direction-Finding,” IEEE Transactions on Signal Processing, vol. 52, no. 1, pp. 36-47, January 2004. http://www.eie.polyu.edu.hk/~enktwong/ktw/TichavskySPT0104.pdf