Mid-frequency Active Sonar

Mid-frequency active (MFA) sonars are used by navies around the world primarily to detect submarines. The U.S. Navy classifies MFA sonars as those that emit frequencies between 1 kHz and 10 kHz while some other countries classify MFA with different frequency ranges.

Image of the USS Cowpens CG-63 in dry dock. The hull mounted mid-frequency sonar is in the bulbous projection at the bow. Photo by the U.S. Navy, public domain.

One modern U.S. hull-mounted MFA sonar, the SQS-53C, is used on many U.S. Navy destroyers and frigates. The active sonar projector array is arranged in a curved geometry permitting a variety of transmit beams (for more about projector arrays see Technology > Projector Array). The SQS-53C sonar operates at 2.6 kHz and/or 3.3 kHz and often at a nominal source level of 235 dB (1µPa at 1m).

A sonar waveform with three sonar pulses. One pulse is a frequency modulated pulse (FM) and two are continuous wave pulses (CW), a 1 kHz bandwidth from 3 to 4 kHz and duration 2.5 sec. Image from Naval Information Warfare Center Pacific, Technical Report 6843, Figure 1. Note that the data in this figure is for illustrative purposes only and does not represent any real-world sonar signal.

The sonar waveform from the SQS-53C system can last several seconds (waveform duration) with a waveform interval period of between twenty and thirty seconds. MFA sonars can produce a variety of signals, for example, a single frequency tone (continuous wave), multiple frequency tones, or a frequency modulated pulse (upsweep or downsweep). Each sonar pulse type is designed for specific situations such as detecting movement or detecting an object in a reverberant environment.

Sonar Technicians monitor contacts on an AN/SQQ-89V15 Surface Anti Submarine Combat System, which includes MFA sonar, aboard the guided missile destroyer USS Momsen (DDG 92). U.S. Navy photo by Mass Communication Specialist 2nd Class James R. Evans, public domain.

Modern MFA sonars are integrated aboard U.S. Navy ships as part of an antisubmarine system used for search, detection, localization, and tracking as well as providing information for weapons control and guidance. As seen in the figure above, the system displays images and audio. A group of U.S. Navy ships will often operate a variety of sonar systems simultaneously, even coordinating the duty cycle so no two systems emit at the same time.

Additional Links on DOSITS

Additional Resources