PASSIVE SONAR: SURTASS
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 surveillance ships to receive acoustic data. Using SURTASS enables the Navy to detect quiet, nuclear- and diesel-powered submarines and to report real-time surveillance information to Navy commanders.
SURTASS is 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: LFA
Active sonar detects underwater objects by sending out a sound pulse or “ping” that bounces or reflects off an object and returns as an echo that is received by hydrophones or other sensors. Some marine mammals use active acoustics to echolocate, which allows them to navigate, sense their environment, and locate prey (food) underwater.
The Navy uses the LFA sonar system when a target is too quiet to be detected by the passive (SURTASS) system alone. The LFA sonar system is a vertical line array (VLA) consisting of as many as 18 acoustic transmitters or projectors. The LFA sonar VLA is suspended vertically by cable beneath a Navy surveillance ship. The sonar “projectors” produce underwater sonar sound pulses or “pings.” These sonar pings reflect off underwater objects, returning as an echo that is received by the SURTASS array, after which the return echos or sounds can analyzed.
The term LFA sonar is actually inclusive of two types of LFA sonar systems: the original LFA sonar system and the compact LFA (CLFA) sonar system. CLFA sonar was developed when the Navy needed to operate in shallower, more coastal waters of the ocean. The operational characteristics of CLFA sonar are comparable to the original LFA sonar system, but the CLFA sonar system is composed of smaller, lighter-weight sonar projectors than the original LFA sonar system. CLFA sonar systems are compact or small enough to be installed on the VICTORIOUS class of surveillance ships. The original LFA sonar system is currently installed on one Navy surveillance ship (USNS IMPECCABLE), while CLFA sonar systems are installed on three Navy VICTORIOUS-class ships (USNS VICTORIOUS, USNS ABLE, and USNS EFFECTIVE).
The operational characteristics of both the original and compact LFA sonar systems are:
- Frequency range from 100 to 500 hertz (Hz)
- Source level of an individual LFA sonar projector that is approximately 215 dB re 1 µPa at 1 m (rms) or less (the LFA sonar array’s measured sound field would never be higher than the source level of an individual source projector)
- Typical LFA sonar signals are not constant tones but consist of different waveforms that vary in frequency and duration. A complete sequence of LFA sonar transmissions (or waveforms) is referred to as a wavetrain (also called a ping), each of which lasts between 6 and 100 seconds but averaging 60 seconds in length
- 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).
Deployment of the SURTASS LFA sonar handling system