- Thursday's search area is scaled back to cover about 22,400 square miles
- Four signals total are detected in the same general area
- Previous signals are "consistent" with that of a flight data recorder
- The signals are getting weaker
In a sea of uncertainty, two bits of good news emerged Wednesday.
Searchers picked up fresh signals that officials hope came from locator beacons attached to the so-called black boxes in the tail of Malaysia Airlines Flight 370, which disappeared more than a month ago while carrying 239 people from Kuala Lumpur to Beijing.
The Australian ship Ocean Shield first picked up two sets of underwater pulses Saturday. It heard nothing more until Tuesday, when it reacquired the signals twice. The four signals were within 17 miles of one another.
"I believe we are searching in the right area, but we need to visually identify wreckage before we can confirm with certainty that this is the final resting place of MH370," said retired Air Chief Marshal Angus Houston, who's coordinating the Australian operation.
The second piece of good news? Authorities analyzed the signals picked up over the weekend and concluded that they probably came from specific electronic equipment rather than from marine life, which can make similar sounds.
"They believe the signals to be consistent with the specification and description of a flight data recorder," Houston said. "I'm now optimistic that we will find the aircraft or what's left of the aircraft in the not too distant future."
Signals getting weaker
Thursday is Day 34 in the search for Flight 370, which went missing March 8. Authorities are pinning their hopes of finding it on the pings.
Time is of the essence: The batteries powering the flight recorders' locator beacons are certified to emit high-pitched signals for 30 days after they get wet.
"The signals are getting weaker," Houston said, "which means we're either moving away from the search area or the pinger batteries are dying."
• The first signal, at 4:45 p.m. Perth time on Saturday, lasted two hours and 20 minutes, he said;
• the second, at 9:27 p.m. Saturday, lasted 13 minutes;
• the third signal was picked up Tuesday at 4:27 p.m. and lasted five minutes and 32 seconds;
• the fourth, at 10:17 p.m. Tuesday, was seven minutes long.
"It's certainly encouraging that more signals have been detected," Pentagon spokesman Adm. John Kirby told CNN. "There is still much work to do, however."
Scouring the ocean for debris
Though plenty of debris has been found, none of it has been linked to the plane, and so the search goes on.
Thursday's effort is set to include up to 10 military planes, four civil aircraft and 13 ships.
Three of them -- the Ocean Shield to the north, and the British HMS Echo and Chinese Haixun 01 to the south -- were focusing underwater.
All told, everyone involved will be scouring a 22,400-square-mile (58,000-square-kilometer) zone centered about 1,400 miles northwest of Perth.
That's roughly the size of West Virginia.
But Thursday's search area is about three quarters of the size of the area teams combed the day before and far smaller than what it was a few weeks ago.
"I think we have got a much clearer picture around the areas that we need to concentrate on," Kevin McEvoy, a New Zealand air force commodore involved in the effort, told CNN's Erin Burnett from Auckland.
Authorities reduced that area after analyzing satellite data and concluding that Flight 370 set off from Kuala Lumpur, Malaysia, toward Beijing, turned back over the Malay Peninsula, then ended up in the southern Indian Ocean.
Why? The answer may reside in the information stored inside the cockpit voice recorder and the flight data recorder.
The ocean to contend with
Hopes were initially raised when a Chinese ship detected pulses last Friday and Saturday that may have been from the plane.
According to McEvoy, "the main focus" now centers on the site of Ocean Shield's discovery. The ship used more advanced detection gear than that aboard the Chinese vessel, whose find was about 375 miles away, leading Houston to believe they are separate signals.
Beyond the dwindling battery life, the ocean also presents challenges: The Ocean Shield signals were in water about 2.6 miles deep, meaning any number of things could literally impede or otherwise disrupt the pulses.
To limit further roiling of the waters, officials are limiting sea traffic in the area. That's one reason that there's no rush to put drones in the water to take photos.
Another reason: Drones are painfully slow. The Ocean Shield towing a pinger locator can search six times the area than can a drone equipped with sonar, Houston said.
"The better the Ocean Shield can define the area, the easier it will be for the autonomous underwater vehicle to subsequently search for aircraft wreckage," he said.
A painstaking process
The more pulses investigators detect, the more they will be able to zero in on the locator beacons, which emit signals for 5 miles in all directions, said Mary Schiavo, a former inspector general for the U.S. Department of Transportation.
Once they lose the signals, authorities will start the painstaking process of using side-scanning sonar to search the ocean floor.
The absence of wreckage near the detected signals leaves some skeptical, worried that the Chinese and Australian ships' finds could mean more false leads in an investigation that's been full of them.
Acknowledging "a very high-speed vertical impact" could explain the lack of aircraft debris, CNN aviation analyst Miles O'Brien said. He said there's reason to be cautious.
"It's either the most extraordinary event, or those pings weren't real," he said. "It's somewhat befuddling."
In Beijing on a 10-day trip to the Asia-Pacific region, U.S. Defense Secretary Chuck Hagel appeared to be hopeful but restrained. "There has been some new evidence here that maybe these new and emerging sounds may lead to something, but it's important we don't lift anyone's hopes -- the families of these passengers -- in an unfair way," he told CNN's Jim Sciutto in an exclusive interview.
Sarah Bajc, the partner of American passenger Philip Wood, told Burnett that she isn't sure about anything.
"All of us pretty well agree that, until there's the bulk of the plane, the bulk of the bodies discovered, and a black box intact, we won't believe that it's final evidence," Bajc said Wednesday from Beijing. "I don't think the authorities have given us much confidence of their investigative skills so far."
The lack of clarity makes it hard to "grieve properly and ... move on," she said.
"I want to fight to find him, in whatever form that ends up being," said Bajc, who is coordinating with other passengers' kin to press for answers. "And I think most of the families feel the same way."
Until he gets answers, Steve Wang, whose mother was on the plane, is clinging to hope while trying to hold himself together. "We're just going through so many kinds of emotion," he said of his position and those of other relatives of passengers. "Desperate, sad and helpless -- something like that. Everything."