The U.S. Navy's SURTASS LFA is an active sonar system under development since the late 1980's, for employment from a small number of auxiliary ships assigned to conduct undersea surveillance to detect, classify and track potential threat submarines. SURTASS LFA system development is important because traditional passive systems are facing increasing challenges in their detection capabilities due to improved submarine quieting technologies. SURTASS LFA hardware consists of an array of acoustic transmitting components suspended an average of 100 meters beneath the host ship, which travels at a maximum of 3-4 knots This vertical array of transducers transmits waveforms (100-500 Hz) which are much longer than other active sonar systems. Echoes reflected off objects such as submarines are then detected on passive towed horizontal line arrays (HLA), and processed and evaluated to identify and classify them. The U.S. Navy will soon be at the stage of transitioning the system from a test and evaluation status to the fleet for worldwide employment to enhance undersea warfare capabilities. At present, the U.S. Navy has only one system. If proposals are carried through to completion, a total of four systems will be procured, two each for the Atlantic and Pacific fleets. High sound levels can potentially be harmful, either by affecting hearing or by causing other physiological effects; however, the combination of high transmission losses in the marine environment and the implementation of proven mitigation measures (visual and acoustic monitoring, source ramp- up, sound pressure level monitoring, shut-down criteria), reduce the potential for harmful sound levels from the SIJRTASS LFA system reaching marine animals and divers to a negligible level. During the development of the SURTASS LFA system, the U.S. Navy prepares various environmental analyses prior to sea tests, which are coordinated with the appropriate agencies responsible for wildlife protection. In July, 1996, the U.S. Navy announced a proposal for operational employment of the system and the initiation of a worldwide environmental impact statement (EIS) to evaluate the potential environmental effects of such deployment. The EIS is one element of a synergistic plan composed of three primary thrusts; 1) a comprehensive scientific research program (SRP) under the aegis of some of the world's most prominent marine bioacousticians, using the SURTASS LFA system as a scientific tool to collect much-needed data on the potential effects of low frequency sound on marine mammals; 2) an intensive diver risk assessment involving in-water low frequency sound measurements with human subjects under the direction of the U.S. Navy Submarine Medical Research Laboratory; and 3) the EIS, which will be available to the public in draft form in 1998.
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