Towed array sonar

A towed array sonar is a system of hydrophones towed behind a submarine or a surface ship on a cable. Trailing the hydrophones behind the vessel, on a cable that can be kilometres long, keeps the array's sensors away from 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 quiet, low noise-emitting submarine threats, or seismic signals. A towed array sonar is a system of hydrophones towed behind a submarine or a surface ship on a cable. Trailing the hydrophones behind the vessel, on a cable that can be kilometres long, keeps the array's sensors away from 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 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. 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. 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. 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 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—which would repeat the same problems of ship-mounted arrays. Surveillance Towed Array Sensor Systems 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 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 duct 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 technique of 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 gauges) 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.