Towed sonar arrays (the 80s)
Around 1980, a new principle of submarine detection was started. The submarines were no longer tracked by actively transmitting sonar signals, but by listening with sensitive hydrophones to the noise caused by the submarine. These hydrophones were then towed far behind the ship in a hose (length of 150 metres) attached to a long cable (1200 metres). This allows one to listen to submarines at very large distances and keeps the array’s hydrophones away from the ship’s noise sources. However, this method has the disadvantage that if one detects a submarine, one does not know whether it is on the right or the left side. To solve this problem, not one hose was dragged but two hoses next to each other. The sound strikes one hose earlier than the other. In this way, one can determine whether the sound source is at the left or the right. Towed array sonar, however, only works well with advanced signal processing capabilities.
In one study, 32 acoustic sensors were installed at equal distances in the hose supplemented by twelve non-acoustic sensors. Each sensor is connected to the deck unit with a wire connection of more than 1200 metres. Because this large number of connections made the system vulnerable, a solution was required: the transfibre system. At the end of the hose, a multiplexer unit (“pod”) was installed which supplies the sensor signals with a high sampling frequency in time-division multiplex to a single optical glass fibre that transfers the signal to the ship via the 1200 metre-long towing cable, where a deck unit converts the received signal back into a set of individual sensor signals.