Page 12 - Spring2019

Brian G. Ferguson
Address:
Defence Science and Technology (DST) Group – Sydney Department of Defence Locked Bag 7005 Liverpool, New South Wales 1871 Australia
Email:
Brian.Ferguson@defence.gov.au
Defense Applications of Acoustic Signal Processing
Acoustic signal processing for enhanced situational awareness during military operations on land and under the sea.
Introduction and Context
Warfighters use a variety of sensing technologies for reconnaissance, intelligence, and surveillance of the battle space. The sensor outputs are processed to extract tactical information on sources of military interest. The processing reveals the presence of sources (detection process) in the area of operations, their identities (classification or recognition), locations (localization), and their movement histo- ries through the battle space (tracking). This information is used to compile the common operating picture for input to the intelligence and command decision processes. Survival during conflict favors the side with the knowledge edge and superior technological capability. This article reflects on some contributions to the research and development of acoustic signal-processing methods that benefit warf- ighters of the submarine force, the land force, and the sea mine countermeasures force. Examples are provided of the application of the principles and practice of acoustic system science and engineering to provide the warfighter with enhanced situational awareness.
Acoustic systems are either passive, in that they exploit the acoustic noise radiated by a source (its so-called sound signature), or active, where they insonify the target and process the echo information.
Submarine Sonar
Optimal Beamforming
The digitization (i.e., creating digital versions of the analog outputs of sensors so that they can be used by a digital computing system) of Australia’s submarines occurred 35 years ago with the Royal Australian Navy Research Laboratory un- dertaking the research, development, and at-sea demonstration of advanced next- generation passive sonar signal-processing methods and systems to improve the reach of the sensors and to enhance the situational awareness of a submarine.
A passive sonar on a submarine consists of an array of hydrophones (either hull mounted or towed) that samples the underwater acoustic pressure field in both space and time. The outputs of the spatially distributed sensors are combined by a beamformer, so that signals from a chosen direction are coherently added while the effects of noise and interference from other directions are reduced by destruc- tive interference. The beamformer appropriately weights the sensor outputs before summation so as to enhance the detection and estimation performance of the pas- sive sonar system by improving the output signal-to-noise ratio. This improvement in the signal-to-noise ratio relative to that of a single sensor is referred to as the array gain (see Wage, 2018; Zurk, 2018).
After transformation from the time domain to the frequency domain, the hy- drophone outputs are beamformed in the spatial frequency domain to produce a frequency-wave number power spectrum. (The wave number is the number of
10 | Acoustics Today | Spring 2019
https://doi.org/10.1121/AT.2019.15.1.12