Towed array sonar

A towed array sonar is a sonar array that is towed behind a submarine (STSA; Submarine Towed Sonar Array) or surface ship such as a (SURTASS). It is basically a long cable, up to 6 km, with an array of hydrophones that is trailed behind the ship when deployed&mdash;that gets the sensitive sensors away from own-ships-noise sources greatly improving Signal-to-noise ratio, and hence effectiveness and so detection and tracking performance versus faint contacts, such as a quiet, low noise-emitting submarine threat, or seismic signals.

The hydrophones 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 3 or more arrays are used to aid in depth detection. On the first few hundred metres nearest the ship's propeller there are usually no hydrophones since 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. Surface ships most often have the sonar array mounted on a cable which pulls a depth adjustable tow vehicle (an minimal function type of remote operated vehicle (ROV) - see Surveillance Towed Array Sensor System) behind the mother ship, or on another weighted cable which trails from the ROV connector, dropping the towed array lower. Long seismic streamers have intermediate paravanes along their length which can be used to adjust the depth of the array in real time.

By changing the ROV's depth, the sensor can be deployed in different thermal layers, allowing the surface ASW vessel to enjoy the same benefits of being able to see above and below the layer, which density and temperature differences act to duct sound above or below by reflection. By getting the 'tail' below the layer, an surface ASW platform can better detect a quiet submerged contact hiding deep below the layer. By floating the tail, above the layer, a submarine hiding below it can get a better picture of the position of surface combatants in a fleet above it.



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 and Fourier analysis can be used not only to calculate the distance and the direction of a sound source, but display its acoustic signature for determining the type of ship based on machinery noise. For this, the relative positions of the hydrophones need to be known, usually only guaranteed when the cable is in a straight line (stable), or else a self-sensing system (see strain gauges) embedded in the cable and reporting relative position of hydrophone elements is used to monitor the shape of the array, and correct for curvature.

Therefore, a vessel using a towed array will need to travel straight and level over a data sampling interval, lest a change of course disturbs the array and bollixes up the ability to rely on the sampled data stream&mdash; a ship's turn maneuver consequently reduces its effectiveness as a reliable information source for a time. These periods of instability are closely tested during sea trials and known by the crews officers and enlisted sonar experts. This requirement is reduced in modern systems by sensors which constantly self-measure the relative positions of the array element to element, reporting back data that can be corrected for curvatures automatically by computers as part of the beam forming math processing. A ship using a towed array also has to limit its overall top speed when it is deployed, as the hydrodynamic drag which increase as a square function of velocity, might mount sufficiently to tear the cable or rip or deform its mooring hardware - this can also happen if the array makes contact with the seafloor or if the vessel operates astern propulsion.

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, the blind spot of hull mounted sonar.