Ocean Shield: on the trail of deep sea pings

Underwater “pings” are the strongest clues yet to the whereabouts of the missing Malaysian plane, flight MH370. Two batches of these ultrasound signals have now been detected by an Australian ship 1600 kilometres north-west of Perth.

“I believe we are now searching in the right area,” said Angus Houston, head of the Joint Agency Coordination Centre that is managing the multinational search operation from Perth, at a press conference this morning.

Planes carry two flight recorders – the cockpit voice recorder (CVR) and the flight data recorder (FDR). Both are equipped with an underwater location device that emits regular ultrasound pulses, aka pings, for 30 days after they are submerged.


The Australian ship Ocean Shield first detected two ping streams on 6 April along an arc in the south Indian Ocean that satellite signals had recently pinpointed as flight MH370’s final, fatal route. One lasted 140 minutes, one 13.

This morning, Houston announced that the ship had detected two more ping streams (view their locations on this map). The two new streams “will assist in defining a better and much more manageable search area on the ocean floor”, he said. The ocean is 4500 metres deep in this region.

However, it was the second stream in the first batch of signals that was particularly encouraging, said Houston, because there were “two distinct pinger returns” sounding simultaneously – indicating that both the CVR and FDR have been located. “Clearly, this is a most promising lead,” he added.

Crucially, detailed signal analysis of the ultrasound pings has established that they are coming from electronic equipment rather than, say, a whale pod or gas bubbling noisily from the seabed. A flight recorder pinger is designed to sound unnatural and distinctive: its pings are centred on a frequency of 37.5 kilohertz and chirp for 9 milliseconds once every second.

Bluefin rescue

However, only sonar or visual evidence from a robotic submarine can confirm that the signals are from the missing Boeing 777. So if the position of any debris field can be refined, an autonomous submarine, Bluefin-21, will be sent down to 50 metres above the seabed to scan for objects of interest with its sonar, which has a range of 1000 metres.

A good position fix for Bluefin-21 is key since it cannot home in on the flight recorder pingers. “It does not have pinger location functionality,” says Bluefin Robotics president David Kelly. Instead, it uses sonar to create “acoustic images” of the seabed and moves at a walking pace.

If the robot finds possible seabed targets using its sonar, it would then be fitted with lights and video cameras to provide imagery. A separate, heavy-duty submersible equipped with powerful remote-controlled robotic grippers would then begin retrieving the flight recorders and wreckage.

First though, Ocean Shield is looking for more pings from the flight recorders to home in on the precise position of any wreckage before the pingers’ lithium batteries run down. With a lifetime of 30 days the batteries are now into their 34th day – but it is possible they can last up to 40 days in some cases.

The Royal Australian Air Force will also be dropping 84 buoys equipped with sonar and radio into the ping field that Ocean Shield has identified. Each “sonobuoy” will lower a microphone to a depth of 300 metres and radio back any pings they hear via a surface antenna.

Chinese misunderstanding

Meanwhile, a possible signal reported on 5 April by a Chinese team using the rescue ship Haoxin 01, which deployed a dinghy and handheld underwater microphone, was probably a red herring. The signal came some 600 kilometres away from Ocean Shield’s and is more likely to be a mistake caused by a sample pinger activated by damp air, say, or noise from their own ship, than the same signal skipping over large distances.

“At lower frequencies below 1 kilohertz you can get refraction of the sound signal that does give large skip distances. But it is most unusual at these very high frequencies,” says John Powis, head of the NATO Rescue Submarine Service in Faslane, UK. “I strongly suspect the Chinese team have misunderstood some other signal.”

The Malaysian Airlines Boeing 777 with 239 passengers and crew on board took off for Beijing on 8 March. After an hour in the air its radar was silenced and its datalink with the airline severed. In radio silence, it then diverted off course, rounding Indonesia and heading south towards Antarctica. No reason for this diversion has yet been confirmed.

Only advanced Doppler effect analysis of a vestigial hourly signal the plane sent to a British satellite has allowed the rough path of the plane to be tracked. It is thought to have flown on until its fuel ran out some 1600 kilometres north-west of Perth, where a final partial satellite signal is thought to have signified the failure of its on-board electronics after its fuel ran out. The underwater pings have indeed been heard near this estimated location.