Search teams have begun using a towed pinger locator to hunt for the black box of missing Malaysia Airlines flight MH370.
Two ships with locator capabilities are searching a 240km (150 mile) underwater path, in the hope of recovering the plane's data recorder.
Up to 14 planes and nine ships were due to take part in Friday's search.
The plane disappeared on 8 March en route from Kuala Lumpur to Beijing. It was carrying 239 people.
It is believed to have crashed in the southern Indian Ocean, although no confirmed debris has been found from the plane.
The search is being co-ordinated from the city of Perth in Western Australia.
Angus Houston, head of the Joint Agencies Coordination Centre (JACC) leading the search, said that two ships had "commenced the sub-surface search for emissions from [the] black box pinger".
Australia naval vessel Ocean Shield was using a towed pinger locator from the US Navy, while HMS Echo, which had similar capabilities, was also searching.
"The two ships will search a single 240km track converging on each other," Air Chief Marshal Houston, who is retired, said.
ACM Houston said that the area had been picked on the basis of analysis of the satellite data.
It was based on work regarding "how the aircraft might have performed and how it might have been flown", to choose the "area of highest probability as to where it might have entered the water".
He pointed out that this data was continuing to be refined, but the current search was based on the "best data that is available".
Given the progress in data evaluation and calculation, "there is some hope we will find the aircraft in the area we are searching", he added.
In a statement, JACC said up to 10 military planes, four civilian planes and nine ships would be deployed in Friday's search efforts.
The focus is on a search area of about 217,000 sq km (84,000 sq miles), 1,700 km (1,000 miles) north west of Perth.
Fair weather was forecast for Friday, with visibility of around 10km (six miles), JACC said.
Meeting staff involved in the search on Friday, Australian Prime Minister Tony Abbott said: "It is probably the most difficult search that's ever been mounted."
"A large aircraft seems like something that would be easy enough to locate - but a large aircraft that all but disappeared and disappeared into inaccessible oceans is an extraordinary, extraordinary challenge that you're faced with."