Passive sonar uses hydrophones to “listen” for underwater sounds. The Surveillance Towed Array Sensor System (SURTASS) is a low-frequency array of hydrophones that is deployed from surface ships and receives acoustic data. SURTASS enables the Navy to make passive acoustic detections of quiet, nuclear- and diesel-powered submarines and to report real-time surveillance information to Navy commanders.

SURTASS consists of a long Y-shaped acoustic array that is towed horizontally behind a surface surveillance ship. The SURTASS array includes environmental sensors, electronic components, and hydrophones that collect underwater sound signals and information about the ocean environment.


Active sonar detects objects underwater by sending out a sound pulse or “ping” that bounces off an object and returns as an echo that can be detected. Some marine mammals use active acoustics to echolocate, which allows them to navigate and find food underwater.

The Navy’s Low-Frequency Active (LFA) sonar system is used when a target is too quiet to be detected by the passive (SURTASS) system alone. LFA sonar is a system of up to 18 acoustic transmitters or projectors suspended vertically by cable beneath a Navy surveillance ship. These “projectors” produce an underwater sonar sound pulse or “ping.” These sonar pings bounce off underwater objects, with the bounced ping being returned to the SURTASS array where it can be analyzed.

LFA sonar includes two types of systems: original LFA and compact LFA (CLFA). CLFA was developed when it became necessary for the Navy to operate in shallower, more coastal waters of the ocean. CLFA consists of smaller, lighter-weight, sonar projectors than the original LFA system and is compact enough to be installed on the VICTORIOUS class surveillance ships. The operational characteristics of CLFA are comparable to the original LFA system. The original LFA sonar system is installed on one Navy ship (USNS IMPECCABLE) while CLFA sonar systems are installed on three Navy ships (USNS VICTORIOUS, USNS ABLE, and USNS EFFECTIVE).

The operational characteristics of both the original and compact LFA sonar systems are:

  • Operational frequency within the 100 to 500 hertz (Hz) range
  • Source level of an individual LFA sonar projector is approximately 215 dB re 1 µPa at 1 m (rms) or less
  • LFA sonar signals or pings are not constant tones but consist of various waveforms that vary in frequency and duration. A complete sequence of LFA sonar transmissions (or waveforms) is referred to as a wavetrain, each of which lasts between 6 and 100 seconds, with an average length of 60 seconds
  • The time between wavetrain transmissions is typically from 6 to 15 minutes
  • Maximum duty cycle of 20% (duty cycle is ratio of sound “on” time to total time).

SURTASS LFA Sonar handling system deployment