Welcome to the U.S. Navy’s website for Surveillance Towed Array Sensor System (SURTASS) Low Frequency Active (LFA) Sonar.

This website is a resource for information concerning the potential environmental effects of training and testing, and more specifically military readiness activities associated with the SURTASS LFA sonar system.

In 2019, the Department of the Navy, with the National Marine Fisheries Service (NMFS) as a cooperating agency, prepared a Supplemental Environmental Impact Statement/Supplemental Overseas Environmental Impact Statement (SEIS/SOEIS) for SURTASS LFA sonar in accordance with the National Environmental Policy Act and Executive Order 12114 The 2019 SEIS/SOEIS supported authorization by NMFS,  under the Marine Mammal Protection Act (MMPA) and its implementing regulations, to allow the incidental takes of threatened and endangered marine species under the Endangered Species Act. It is anticipated that a supplemental National Environmental Policy Act (NEPA) analysis will begin in summer 2024 to support renewal of current federal regulatory permits and authorizations that expire in August 2026.

Study Area

The Pacific (PAC) SURTASS LFA Study Area (see Figure 1) consists of a large portion of the western North Pacific Ocean and eastern Indian Ocean. The Study Area is the same as that defined in the 2019 SEIS/SOEIS. 

Figure 1. PAC SURTASS LFA Study Area

Proposed Action

The purpose of the Navy’s Proposed Action is to perform training and testing activities that ensure the Navy remains proficient in the use of SURTASS LFA sonar. Additionally, the Proposed Action includes the at-sea sonar testing of up to one new T-AGOS vessel per year. T-AGOS ships are operated by Military Sealift Command (MSC) to support the anti-submarine warfare mission of the commanders of the Atlantic and Pacific Fleets. The need for the Proposed Action is to maintain a system and crews capable of detecting at long ranges the increasingly technologically advanced foreign submarine presence that threatens our national security.

The U.S. Navy proposes to continue utilizing SURTASS LFA sonar systems onboard U.S. Navy surveillance ships for training and testing conducted under the authority of the Secretary of the Navy in the Pacific and Indian oceans. The U.S. Navy currently has four surveillance ships that utilize SURTASS LFA sonar systems: U.S. Naval Ship (USNS) VICTORIOUS (T-AGOS 19); USNS ABLE (T-AGOS 20); USNS EFFECTIVE (T-AGOS 21); and USNS IMPECCABLE (T-AGOS 23). The Navy intends to develop and field additional SURTASS LFA sonar equipped vessels, either to replace or complement the Navy current SURTASS LFA sonar equipped fleet.

SURTASS LFA sonar is a long-range system that transmits in the low-frequency band (below 1,000 Hertz [Hz]) that is composed of both active and passive components. The active component is the LFA sonar source array while the passive component is the SURTASS receive array.

The active component of the SURTASS LFA sonar system, LFA, is employed when active sound signals are needed to detect and track underwater targets of interest. LFA sonar complements SURTASS passive activities by actively acquiring and tracking submarines when they are in quiet operating modes, measuring accurate target range, and re-acquiring lost contacts.

LFA sonar consists of a vertical source array of sound-producing elements that are suspended by cable under one of the T-AGOS vessels (Figure 2-1). These elements, called projectors, are devices that produce the active sonar sound pulses or pings. To produce a ping, the projectors transform electrical energy to mechanical energy (i.e., vibrations), which travel as pressure disturbances in water. The LFA sonar source is a vertical line array (VLA) consisting of up to 18 source projectors. Each LFA source projector transmits sonar beams that are omnidirectional (360 degrees) in the horizontal, with a narrow vertical beamwidth that can be steered above or below the horizontal. The source frequency ranges between 100 and 500 Hz.

Project Schedule and Public Involvement

Public involvement is a fundamental aspect of the NEPA process, and there are opportunities for the public to participate in the development of the SEIS/OEIS. The Navy welcomes and appreciates the public’s participation. The planned project schedule is outlined below. Please note that this schedule may change as the project progresses; in that case this page will be updated accordingly.