The Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) sonar system is an antisubmarine warfare device designed by the US Navy. The system has both passive and active components. The passive part of the system is called SURTASS. It consists of a long array of hydrophones to listen for underwater sounds, particularly submarines. When the submarine is too quiet to be heard by SURTASS, LFA sonar is employed. LFA is a set of acoustic transmitters suspended on a cable beneath a ship. These transmitters produce sound between 100 and 500 Hz with a source level of approximately 215 underwater dB at 1 meter. The combined system has an effective source level from 230 to 240 underwater dB at 1 meter. These sound pulses reflect off submerged objects and are heard on SURTASS. Using sounds with this intensity and frequency allows for a long detection range. The objective of this system is to locate submarines far enough away so that they are not within firing range of the ship or any other ships it may be protecting.
There has been much concern about the potential risks of SURTASS LFA to marine animals and divers. LFA has 18 transmitters, each with a source level of approximately 215 underwater dB. Sounds received from these transmitters are potentially threatening to animals that rely on sound for survival. The Navy has spent over $16 million dollars conducting scientific research on the potential effects of LFA on marine mammals and developing an Environmental Impact Statement (EIS), including a marine mammal mitigation system. The Navy has put restrictions on the use of SURTASS LFA so that received levels would be below 180 underwater dB within 22 km (12 nm) of all coastlines and in offshore biologically important areas. To see further information regarding the marine mammal research of SURTASS LFA and the operation systems of SURTASS LFA go to Why the United States Needs SURTASS LFA.