Proposed Action

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 non-Arctic western and central North Pacific and eastern Indian Oceans. The U.S. Navy currently has four surveillance ships that utilize SURTASS LFA sonar systems. 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. 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.

Description of the SURTASS LFA Sonar System

SURTASS LFA sonar is a long-range system that transmits in the low-frequency band (between 100 and 500 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 (Figure 1-2).

Sonar is an acronym for sound navigation and ranging, and its definition includes any system that uses underwater sound, or acoustics, for observations and communications. Sonar systems are used for many purposes, ranging from commercial “fish finders” to military anti-submarine warfare systems used for detection and classification of submarines. The two basic types of sonar used in the SURTASS LFA sonar system are passive and active sonar. Passive sonar detects sound created by a source. This is a one-way transmission of sound waves through water from the source to the receiver. Passive sonar is similar to people hearing sounds that are transmitted through the air to the human ear. Very simply, passive sonar “listens” without transmitting any sound signals. Active sonar detects objects by creating a sound pulse or “ping” that is transmitted from the sonar through the water, reflects off a target object, and returns in the form of an echo to be detected by a receiver. Active sonar is a two-way transmission of sound waves through water (sound source to reflector to receiver).

Figure 1-2. Schematic of a SURTASS LFA Sonar System Deployed from a T-AGOS Vessel Including the Passive SURTASS Horizontal Line Array (Receive Array) of Hydrophones and the Active Vertical Line Array of LF Sonar Projectors (Source Array).