The logging of a fifth signal spurred Friday's hunt for missing Malaysian airliner MH370 as search crews work round-the-clock to find elusive wreckage in the southern Indian Ocean.
The Australian-led operation out of a Perth airbase is racing to gather as many signals as possible to determine an exact resting place for the Boeing 777 before sending down a submersible to plumb the depths.
The ping-emitting beacons on Flight MH370's data and cockpit voice recorders are expected to fade, more than a month after the plane vanished with 239 people on board.
With analysis of the latest ping underway, the Perth-based Joint Agency Coordination Centre (JACC) said Friday the search area had been further reduced to two zones totalling 46,713 square kilometres (18,06 square miles).
The core of the search is now 2,312 kilometres (1,436 miles) northwest of Perth.
An Australian air force P-3C Orion surveillance plane, which had been dropping dozens of sonar buoys into the remote waters of the search zone, captured the new signal on Thursday afternoon.
"The acoustic data will require further analysis overnight but shows potential of being from a made-made source," JACC chief Angus Houston said in a statement late Thursday.
The Orion was flying close to the area where two signals were detected at the weekend and two more on Tuesday by Australian navy ship Ocean Shield.
The vessel is dragging a US Navy "towed pinger locator" to listen for emissions from the black boxes.
Friday's weather forecast in the search zone was for 10-15 knot southerly winds with isolated showers, seas swells of one to 1.5 metres (three to five feet) and visibility of five kilometres during the showers.
US Seventh fleet spokesman Commander William Marks had earlier voiced optimism that the first two sets of signals showed the hunt was getting "closer and closer".
"When you put those two (sets of pings) together, it makes us very optimistic," Marks told CNN on Thursday.
"This is not something you find with commercial shipping, not something just found in nature -- this is definitely something that is man-made, consistent with what you would find with these black boxes."
Marks said he expected the pings to last "maybe another day or two" as the batteries powering the black box beacons fade after their normal lifespan of about 30 days.
- Still no debris spotted -
No floating debris from the Malaysia Airlines aircraft has yet been found, JACC said, despite the massive multinational air and sea operation.
JACC says the high-tech underwater surveillance is intended to define a reduced and more manageable search area in depths of around four kilometres (2.5 miles).
Houston has stressed the need to find the wreckage and urged repeatedly against unduly inflating hopes, for the sake of the families of missing passengers and crew who have endured a month-long nightmare punctuated by a number of false leads.
But he has voiced renewed optimism as day by day the search edges forward with new information.
No other ships are being allowed to sail near the Ocean Shield as it must work in an environment as free of noise as possible.
But JACC announced that up to 12 military aircraft, three civil aircraft and 13 ships will join Friday's hunt.
JACC says says it should not be long before a US-made autonomous underwater vehicle called a Bluefin-21 will be sent down to investigate, but has cautioned that it will have to operate at the very limits of its capability given the vast depths involved.
In Malaysia, Home Minister Zahid Hamidi said there was "no conclusive evidence yet" from the continuing investigation into what caused the plane to divert from its Kuala Lumpur-Beijing route.
Zahid said 180 people had been interviewed, including relatives of passengers and crew as well as airline ground staff and engineers.
Numerous theories have been put forward to explain MH370's baffling disappearance.
They include a hijacking or terrorist attack, a pilot gone rogue or a sudden catastrophic event that incapacitated the crew and left the plane to fly for hours until it ran out of fuel.