Surveillance Towed Array Sensor System (SURTASS) Low Frequency Active (LFA) Sonar
Overview | Related Publications
The SURTASS LFA sonar system is a long-range, low frequency (between 100 and 500 Hertz) sonar that has both active and passive components. The sonar's detection capability does not rely on noise generated by the target, but rather on the use of active sounds or pulses originating from the system. The active component of the system is a set of low frequency (LF) acoustic transmitting source elements (called projectors) suspended from a cable from underneath a ship. The projectors are devices that produce the active sound or pulse. The purpose of SURTASS LFA sonar is to provide the Navy with a reliable and dependable system for long-range detection of quieter, harder-to-find submarines. LF sound travels in seawater more effectively and for greater distances than higher frequency sound used by most other active sonars. The SURTASS LFA sonar system would meet the Navy's need for improved detection and tracking of new generation submarines at a longer range. This would maximize the opportunity for U.S. armed forces to safely react to, and defend against, potential submarine threats while remaining a safe distance beyond a submarine's effective weapons range.
The typical SURTASS LFA sonar signal is not a constant tone, but rather a transmission of various waveforms that vary in frequency and duration. A complete sequence of sound transmissions is referred to as a "ping" and can last for as short as 6 seconds (sec) to as long as 100 sec. The time between pings is typically from 6 to 15 minutes. Average duty cycle (ratio of sound "on" time to total time) can be controlled but is less than 20 percent; typical duty cycle is between 10 and 20 percent. The passive or listening component of the system is SURTASS, which detects returning echoes from submerged objects, such as submarines, through the use of hydrophones. The hydrophones are mounted on a horizontal array that is towed behind the ship.
The MMPA allows for the incidental harassment, called a take, of marine mammals under section 101(a)(5) of the MMPA. The U.S. Navy requested an incidental take authorization for operating SURTASS LFA sonar for a five-year period from NOAA Fisheries because this sonar system has the potential to harass marine mammals incidental to its operation. As part of its review under the MMPA, NOAA Fisheries held two public comment periods, including public hearings, and conducted a detailed review of the 280 issues identified by the public regarding the SURTASS LFA sonar's impact on marine mammals and other marine life. As a result, NOAA Fisheries granted the U.S. Navy an authorization to harass small numbers of marine mammals over the five-year period incidental to operating a maximum of two SURTASS LFA sonar systems in the Atlantic and Pacific Oceans. Implementation of a monitoring and mitigation program that has been determined to be more than 95 percent effective at preventing marine mammal injury ensures that the takings will have no more than a negligible impact on affected marine mammal species and stocks.
NMFS publications regarding the Navy's request for a small take authorization:
- Advanced Notice of Proposed Rulemaking [pdf] [25 KB] (October 22, 1999)
- Proposed Rule [pdf] [684 KB] (March 19, 2001)
- 2003 LFA Letter of Authorization Chouest [pdf] [46 KB]
- 2003 LFA Letter of Authorization Impeccable [pdf] [23 KB]
- LFA Letter of Authorization [pdf] [45 KB]
- LFA Press Release [pdf] [24 KB]
- LFA Executive Summary [pdf] [33 KB]
- LFA History [pdf] [9 KB]
- Questions and Answers About Surtass LFA Sonar [pdf] [16 KB]
- Questions and Answers About Ocean Noise [pdf] [12 KB]
- SURTASS LFA: Technical Background and Marine Mammal Scientific Research Presentation [pdf] [1.8 MB]
- MMPA Small Take Authorization Presentation [pdf] [653KB]
- LFA Final Rule [pdf] [2.4 MB]
- Frequently asked Questions [pdf] [13 KB]
