LOFAR Sonobuoy
The U.S. Navy first used Low Frequency Analysis and Ranging (LOFAR) sonobuoys in the 1950's. The Navy uses LOFAR buoys to detect and classify submarines. However, researchers in fisheries and marine mammal projects have also used them. The LOFAR buoy is a passive acoustic sonobuoy that can detect acoustic energy from 5 Hz to 40 kHz. This buoy can work at depths of up to 400 feet and for up to eight hours.
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| LOFAR sonobuoy. Photo courtesy of the Federation of American Scientists |
LOFAR buoys use a signal processing technique that detects and identifies a submarine's signature sound in background noise. Each type of submarine produces a unique signature, a specific low frequency sound that is hard to detect in the background noise. The LOFAR system compares the average background noise to the sounds it is receiving, particularly focusing on the lower frequencies where submarine noise differs most from background noise. This allows the LOFAR sonobuoys to detect the submarine's signature more efficiently than if it looked for it over all frequencies, at much greater distances before background noise drowns it out. This is different from a simple listening device in which submarines are detected by listening to the signal-to-noise ratio (the amount of noise from the submarine versus the amount of background noise of the ocean). As the submarine comes closer, the signal-to-noise ratio increases (more submarine noise than background noise) and as it moves further away, the signal-to-noise ratio drops until only background noise is heard.
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