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Australian Maritime Issues 2007 - Royal Australian Navy

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76 AUSTRALIAN MARITIME ISSUES <strong>2007</strong>: SPC-A ANNUAL<br />

The Sound Surveillance System, or SOSUS, consists<br />

of bottom mounted hydrophone arrays connected by<br />

undersea communication cables to facilities on shore.<br />

The hydrophone arrays are located within the SOFAR,<br />

or deep sound channel, along the continental slope and<br />

seamounts, and therefore this system can detect even<br />

relatively quiet sounds over hundreds of kilometres. 1<br />

This is a typical Lofargram, in which frequency is<br />

portrayed along the horizontal axis and time along the<br />

vertical axis. The presence of spectral energy is indicated<br />

by a darkening of the paper. In this representation, a long,<br />

narrow, vertical line indicates a persistent narrowband<br />

component centered on a single frequency – perhaps a<br />

‘tonal’ caused by rotating machinery – along the bearing<br />

line of interest. Wider gray areas result from broadband<br />

noise sources or the ambient background. 2

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