A towed array sonar is a sonar array that is towed behind a submarine or surface ship. It is basically a long cable, up to 5 km, with hydrophones 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 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) 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.
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 interferometry 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 might tear the cable - this can also happen if the array makes contact with the seafloor or the submarine 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.