Page 12 - Spring 2019
P. 12

Defens Applications of
Acoustic Signal Processing
Brian G. Ferguson Acoustic signal processing for enhanced situational awareness during
Address: military operations on land and under the sea.
Defence Science and Technology (DST) ln and context
Group A S‘/finer . . . . . .
Department “Defmm Warfighters use a variety of sensing technologies for reconnaissance, intelligence,
Locked Bag 7005 and surveillance of the battle space. The sensor outputs are processed to extract
Liverpmll New scum wales 1871 tactical information on sources of  interest. The processing‘re_vea1s-the
Ausmlia presence of sources (detection procus) in the area of operations, thetr identities
(classification or recognition), locations (localization), and their movement histo-
E""'"-‘ ries through the battle space (tracking). "fliis information is used to compile the
B"i3“~F5l'E“5”“@d5f¢“C5»E”"-3“ 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 thatbeneiit 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.
Stlan-Iarina Sonar
Optimal Eeamfarming
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 beamforrner, 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 bearnformed in the spatial frequency domain to produce
a frequency-wave number power spectrum. (The wave number is the number of
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