Authorities say submarines searching for MH370 will only be deployed when another signal is detected.
SEARCH crews have relocated signals, hoped to be from MH370's black box, another two times.
Search co-ordinator and retired Air Chief Marshal Angus Houston has told reporters in Perth the discovery by Australian ship Ocean Shield is "further encouraging".
"The detection yesterday afternoon was held for approximately five minutes and 32 seconds," he said.
"The detection late last night was held for approximately 7 minutes."
It takes the number of transmissions detected by Ocean Shield to four.
"Yesterday's signals will assist in better defining a reduced and much more manageable search area on the ocean floor," Air Chief Marshal Houston added.
But he said authorities are not yet at the point of deploying the autonomous underwater vehicle 'Blue Fin 21'.
"The better Ocean Shield can define the area, the easier it will be for the autonomous underwater vehicle to subsequently search for aircraft wreckage."
"I believe we are searching in the right area, but we need to visually identify aircraft wreckage before we can confirm with certainty that this is the final resting place of MH370," he argued.
Late last week Ocean Shield detected two signals consistent with transmissions from a flight data recorder and a cockpit voice recorder. The first signal continued for two hours, while the second was just 13 minutes.
Based on Perth time, the first was found on Saturday, April 5 at 4.45pm, the second was heard on Saturday, April 5 at 9.27pm, the third was detected on Tuesday, April 8 at 4.27pm and the fourth was heard on Tuesday, April 8 at 10.17pm.
Searching for clues ... a fast response craft from Australian Defence Vessel Ocean Shield towing Able Seaman Clearance Diver Michael Arnold as he searches the ocean for debris Picture: Lt Ryan Davis/Australian Defence Source: AFP
The Air Chief Marshal said data analysis of the first two detections found they were consistent with "the specification and description" of a flight data recorder.
According to work conducted by the Australian Joint Acoustic Analysis Centre at HMAS Albatross in Nowra NSW, it was not of natural origin and was "likely sourced" from specific electronic equipment.
"The analysis determined that a very stable distinct and clear signal was detected at 33.331 kilohertz and that it consistently pulsed at a 1.106 second interval," he said.
The search effort is also being boosted by a sonar buoy search, with a modified RAAF P3 aircraft to coordinate with Ocean Shield in the same area.
But he warned there was a lot of silt, "that could complicate the search".
He said it was important for Ocean Shield to collect as much data as possible ahead of deploying 'Blue Fin 21'.
"The reason we want to do that is there's no second chances.
"It looks like the signals we've picked up recently have been much weaker than the original signals we picked up, so that means probably we're either a long way away from it, or in my view more likely, the batteries are starting to fade and as a consequence, the signal is becoming weaker," he warned.
"So we need to as we say in Australia, make hay while the sun shines."
Looking ... An Australian Orion aircraft arrives back at Pearce Airbase in Bullsbrook, 35 kms north of Perth after assisting in the search for missing Malaysia Airlines Flight MH370. Picture: AFP Source: AFP
But he expressed his confidence the underwater device would be deployed soon and something will be found.
"I'm now optimistic that we will find the aircraft, or what is left of the aircraft, in the not too distant future," the Air Chief Marshal said.
"But we haven't found it yet because this is a very challenging business. We're relying on transmissions that have come and gone and I'd just like to have that hard evidence, a photograph evidence that there's pieces of aircraft down there to know that actually this is the final resting place of MH370."
He and Defence Minister David Johnston remain confident that there is life in the black box despite the days that have passed.
"May I say the battery life in the black box transponder has a long way to go in our opinion, certainly several days into the future," Senator Johnston told reporters in Canberra.
"We are just as aggressive as we were last week in pursuing and trying to isolate that signal," he said.
Divers brought in ... Able Seaman Clearance Diver Matthew Johnston is towed by the navy's Ocean Shield fast response craft as he scans the water for debris. Picture: Lt Ryan Davis/Australian Defence Source: AFP
Australian officials maintain the signals are still their most "positive lead".
Air Chief Marshal Houston today said a towed pinger is still trying to relocate the signals and will do so for "several days".
COMMENT: MH370 — the world's greatest detective story
When the 'Blue Fin 21' is deployed it will do 20-hour sweeps to collect data and pictures, he said, but can't do both at the same time.
He admitted it was a race against time.
"There is a chance that the locator beacon is about to cease transmission or has ceased transmission."
Officials have spelled out the challenges of the mission — among them ships picking up their own signals as they search for traces of the missing Malaysia Airlines jet.
FALSE LEADS AND FRUSTRATION
Air Chief Marshal admitted there have been "false leads" in the past and Ocean Shield needs to be allowed to steadily continue its work trying to find another signal.
"Some … have been actually transmissions from the ship that was actually searching and it got its own transmissions back again," he said.
"So funny things happen in that environment and you can't assume things."
While much of the searching is at a hi-tech level, the human eye is invaluable.
Spotters on planes and ships have now been joined by divers, shown in photographs released yesterday by defence bosses.
The divers are taken out to look at any suspect pieces of debris seen from the vessels.
Race against time ... Royal New Zealand Air Force P-3K2 Orion crew member, Sergeant Sean Donaldson, preparing to drop a marker buoy. Picture: Cpl Jessica De Rouw/Australian Defence Source: AFP
DETAILS OF TODAY'S SEARCH
Crews will continue trying to relocate the signals today, hoped to be from MH370's black box.
Australian ship Ocean Shield is in the northern end of the search zone, with Chinese ship Haixun 01 and HMS Echo searching to the south.
Up to 15 aircraft and 14 ships will be part of today's effort.
The centre of the search area is approximately 2261 kilometres north west of Perth.
Scattered showers are expected, as a weak front moves in from the south east.
Anguish. Feng Zhishang cries as family members mark the birthday of his son Feng Dong, a passenger on board the Malaysia Airlines Flight 370. Picture: AFP Source: AP
TEARFUL VIGIL IN BEIJING
In Beijing, family members of passengers on board Malaysia Airlines flight MH370 have held a tearful vigil in Beijing to mark one month since contact with the plane was lost.
"We've been waiting and holding on here for already 31 days," said Steve Wang, one of the relatives.
"Don't cry anymore. Don't hurt anymore. Don't despair. Don't feel lost," he counselled others who gathered for the vigil.
