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These portable hydrophones have a radio link to transmit the ocean sounds to the ship from as far away as 20 km. The buoy and radio antenna stay at the surface while the hydrophone can be set to drop to a preset depth up to 1000 feet. A magnetic compass and a set of acoustic particle motion sensors allow determination of the compass direction to each sound from each sonobuoy to within a few degrees. These are a valuable tool in correlating visual and acoustic behavior of whales. The compass bearing to the sounds are displayed in near real time on a computer display on the ship. The range to which whales can be heard depends on how rough and noisy the sea is near the sonobuoy, but is commonly up to 20 km and in certain environments up to 100 km. |
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Deploying sonobuoys
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These expendable hydrophones have a radio link to transmit ocean sounds to the ship from as far away as 20 km. |
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The spectrogram of a blue whale call (left figure) is analyzed over the portion highlighted in yellow to obtain the bearing plot (right figure) which shows the whale to be at a magnetic bearing of about 95 degrees from the sonobuoy and the ship to be on a bearing of about 315 degrees.
Error in DIFAR bearings can be estimated using blue whale calls from a whale at known locations, the offset shown here being the magnetic declination. The absolute errors depend on knowledge of magnetic declination, which is often corrected using known bearings to the research vessel. The relative error in this case has a standard deviation of just over 2 degrees.
Considering the typical baseline lengths of less than 15 km, DIFAR localization is much more accurate than time of arrival methods for low-frequency whale calls and requires only two sensors for localization. Description of the use of DIFAR sonobuoys in whale research has been described in an article published in the scientific journal, Canadian Acoustics.