The unmanned submarine deployed from the Australian Navy Ship Ocean Shield had to return to the surface yesterday due to the huge pressure at which it was operating deep under the southern Indian Ocean.

The Australian-led search effort for the missing Malaysia Airlines Flight 370 reported that after completing around six hours of its mission, Bluefin-21 exceeded its operating depth limit of 4,500 metres and that a built-in safety feature forced it to re-surface. The six hours of data captured by the submarine is however being extracted and analysed.

Searchers are confident they know the approximate position of the aircraft wreckage 1,550 km (963 miles) northwest of Perth, and are continuing on the basis of four acoustic signals detected since April 5.

“Despite the lack of further detections, the four signals previously acquired taken together constitute the most promising lead we have in the search for MH370,” said Air Chief Marshal Angus Houston. The batteries in the black boxes are now two weeks past their 30-day expected life and sonar detection and camera equipment on the Bluefin-21 submarine are now being used in a race to find the aircraft.

While Bluefin-21 is planned to redeploy later today when weather conditions permit, it underlines the challenge of searching for the missing Malaysia Airlines Flight 370.

Steve Winter is an international aviation consultant, technologist and a former naval consultant has worked with the British Royal Navy to develop both new sonar technologies and the modelling of oceanic sound propagation, detection, and tracking.

“I would now compare the search to trying to find a person lost in the Grand Canyon using a moving microphone on a slow-moving helicopter, at night, at a height of 1000 feet or more over the canyon. Very challenging,” says Winter.

He reckons the signals initially detected by Ocean Shield were encouraging and that the search effort could now be quite confident based on the Joint Acoustic Analysis Centre (JACC) analysis that they are from a black-box-type pinger, or pingers.

“The information about the detected 33.331 kHz frequency of the signal is interesting, but incomplete,” points out Winter. “It is most unlikely that both pingers from the aircraft will emit at the same frequency, but the JACC have not made it clear which detections correspond to which frequencies.”

This is important, as it is possible that the aircraft broke up and the two recorders are in different locations. The wide spread – around 30 km diameter of the detections highlights the difficulty of localising any contacts.

“It shows the problems of talking about detection “range” in the deep ocean,” says Winter. “It is not necessarily the case that the closer you get to a source, the stronger its signal will get. The propagation effects are such that signals can travel for many miles and have wildly varying levels of attenuation, an effect known as “ducting” and “convergence zones”.”

The time for which a contact is held is however highly significant in terms of distance from the source. On that basis, Winter says it is likely that the aircraft is located closer to where the initial two-hour contact was made on April 5. “The TPL provides no directional information: all it indicates are the presence, frequency and strength of the detected signal, so at this stage, the search effort can only really be confident that the source of the signals is somewhere in a 15-20 km radius.”

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