Story highlights Crew makes one run, makes long turn and does another pass

All the while, Australian technicians listen for pings

Searching a grid is like boxing in a tic-tac-toe board

Forget the needle. Forget the haystack.

Searchers in the south Indian Ocean are narrowing in on a series of pings — weakening 33.3 kHz pings -- that they heard twice over the weekend and two times again Tuesday.

Here's how they are doing it:

As you know by now, the Australian vessel Ocean Shield began the underwater phase of the search late last week, lowering the familiar wing-shaped towed pinger locator (TPL-25) into the sea.

The TPL is connected to the ship by about 4,500 meters (2.8 miles) of cable. The cable is so long that, were the TPL allowed to sink, it might actually touch the deep ocean floor. But as the ship sails, the TPL is pulled at a depth of about 3,000 meters (about 1.9 miles), some 1,000 to 1,500 meters (0.6 to 0.9 miles) above the bottom.

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Photos: The search for MH370 Photos: The search for MH370 Two years after Malaysia Airlines Flight 370 went missing, a relative of one of the passengers burns incense in Beijing on March 8, 2016. Flight 370 vanished on March 8, 2014, as it flew from Kuala Lumpur, Malaysia, to Beijing. There were 239 people on board. Hide Caption 1 of 43 Photos: The search for MH370 On July 29, police carry a piece of debris on Reunion Island, a French territory in the Indian Ocean. A week later, authorities confirmed that the debris was from the missing flight. Hide Caption 2 of 43 Photos: The search for MH370 Staff members with the Australian Transport Safety Bureau examine a piece of aircraft debris at their laboratory in Canberra, Australia, on July 20. The flap was found in June by residents on Pemba Island off the coast of Tanzania, and officials had said it was highly likely to have come from Flight 370. Experts at the Australian Transport Safety Bureau, which is heading up the search for the plane, confirmed that the part was indeed from the missing aircraft. Hide Caption 3 of 43 Photos: The search for MH370 In late February, American tourist Blaine Gibson found a piece of plane debris off Mozambique, a discovery that renewed hope of solving the mystery of the missing flight. The piece measured 35 inches by 22 inches. A U.S. official said it was likely the wreckage came from a Boeing 777, which MH370 was. Hide Caption 4 of 43 Photos: The search for MH370 Relatives of the flight's passengers console each other outside the Malaysia Airlines office in Subang, Malaysia, on February 12, 2015. Protesters had demanded that the airline withdraw the statement that all 239 people aboard the plane were dead. Hide Caption 5 of 43 Photos: The search for MH370 A police officer watches a couple cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The couple's son was on the plane. Hide Caption 6 of 43 Photos: The search for MH370 Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27, 2014. Data from communications between satellites and the missing flight was released the day before, more than two months after relatives of passengers said they requested it be made public. Hide Caption 7 of 43 Photos: The search for MH370 Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14, 2014. Hide Caption 8 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13, 2014. Hide Caption 9 of 43 Photos: The search for MH370 The HMS Echo, a vessel with the British Roya; Navy, moves through the waters of the southern Indian Ocean on April 12, 2014. Hide Caption 10 of 43 Photos: The search for MH370 A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9, 2014. Hide Caption 11 of 43 Photos: The search for MH370 A relative of a missing passenger cries at a vigil in Beijing on April 8, 2014. Hide Caption 12 of 43 Photos: The search for MH370 Australian Defense Force divers scan the water for debris in the southern Indian Ocean on April 7, 2014. Hide Caption 13 of 43 Photos: The search for MH370 A towed pinger locator is readied to be deployed off the deck of the Australian vessel Ocean Shield on April 7, 2014. Hide Caption 14 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4, 2014. Hide Caption 15 of 43 Photos: The search for MH370 On March 30, 2014, a woman in Kuala Lumpur prepares for an event in honor of those aboard Flight 370. Hide Caption 16 of 43 Photos: The search for MH370 The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, 2014, after other relatives left en masse to protest the Malaysian government's response to their questions. Hide Caption 17 of 43 Photos: The search for MH370 A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27, 2014. Hide Caption 18 of 43 Photos: The search for MH370 Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27, 2014. Hide Caption 19 of 43 Photos: The search for MH370 People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27, 2014. Hide Caption 20 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24, 2014. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived." Hide Caption 21 of 43 Photos: The search for MH370 Grieving relatives of missing passengers leave a hotel in Beijing on March 24, 2014. Hide Caption 22 of 43 Photos: The search for MH370 A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22, 2014. Hide Caption 23 of 43 Photos: The search for MH370 A Chinese satellite captured this image, released on March 22, 2014, of a floating object in the Indian Ocean, according to China's State Administration of Science. It was a possible lead in the search for the missing plane. Surveillance planes were looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia. Hide Caption 24 of 43 Photos: The search for MH370 Satellite imagery provided by the Australian Maritime Safety Authority on March 20, 2014, showed debris in the southern Indian Ocean that could have been from Flight 370. The announcement by Australian officials raised hopes of a breakthrough in the frustrating search. Hide Caption 25 of 43 Photos: The search for MH370 Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight. Hide Caption 26 of 43 Photos: The search for MH370 A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19, 2014. Hide Caption 27 of 43 Photos: The search for MH370 On March 18, 2014, a relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet. Hide Caption 28 of 43 Photos: The search for MH370 U.S. Navy crew members assist in search-and-rescue operations in the Indian Ocean on March 16, 2014. Hide Caption 29 of 43 Photos: The search for MH370 Members of the Chinese navy continue search operations on March 13, 2014. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, search efforts expanded west into the Indian Ocean. Hide Caption 30 of 43 Photos: The search for MH370 A Vietnamese military official looks out an aircraft window during search operations March 13, 2014. Hide Caption 31 of 43 Photos: The search for MH370 Malaysian air force members look for debris near Kuala Lumpur on March 13, 2014. Hide Caption 32 of 43 Photos: The search for MH370 Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12, 2014. Hide Caption 33 of 43 Photos: The search for MH370 A member of the Vietnamese air force checks a map while searching for the missing plane on March 11, 2014. Hide Caption 34 of 43 Photos: The search for MH370 A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported on March 8, 2014. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10, 2014. Hide Caption 35 of 43 Photos: The search for MH370 A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews on March 9, 2014, before returning to search for the missing plane in the Gulf of Thailand. Hide Caption 36 of 43 Photos: The search for MH370 Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9, 2014. Hide Caption 37 of 43 Photos: The search for MH370 Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9, 2014. Hide Caption 38 of 43 Photos: The search for MH370 The rescue vessel sets out from Sanya in the South China Sea on March 9, 2014. Hide Caption 39 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8, 2014. Hide Caption 40 of 43 Photos: The search for MH370 A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8, 2014. Hide Caption 41 of 43 Photos: The search for MH370 Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8, 2014. Hide Caption 42 of 43 Photos: The search for MH370 Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference at a hotel in Sepang on March 8, 2014. "We deeply regret that we have lost all contacts" with the jet, he said. Hide Caption 43 of 43

From that position, the TPL can detect pings roughly one mile in any direction.

Call it a triumph of science, or incredible luck, but on the very first path, the Ocean Shield, which was following a path suggested by an analysis of Inmarsat satellite data, detected a steady series of pings Saturday afternoon, Perth, Australia time.

As the ship sailed at a leisurely 2 knots (roughly 2.3 miles per hour), the ship continued hearing the pings, monitoring the electronic pulse for 2 hours and 20 minutes as it got stronger, and then, over time, weaker.

Technicians recorded the signals on computers and analyzed their strength, marking the TPL's location at the spot where it detected the peak amplitude, or maximum strength, said Michael Dean, the U.S. Navy's deputy director of ocean engineering.

Two pings and then ....

Once the first "run" was completed, the ship began the time-consuming job of turning around. Because the ship is towing the pinger locator, it can take many hours to turn the vessel around, even when they reel in the TPL to allow a tighter turn. Australian search coordinator Angus Houston said the turning around takes three hours; Dean said it can take as many as eight.

The ship then began a parallel run Saturday night Perth time about 3/4th of a mile away from the original run, close enough that the TPL's ability to detect pings overlaps with the previous pass. The overlap is as much as 50%, Dean said.

Again, the technicians record signals, taking note of the point where the pings are strongest.

In the case of the Ocean Shield, technicians heard two pings on the second run, but for only 13 minutes.

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That only increased the excitement, because Malaysia Airlines Flight 370 has two pingers -- one on the flight data recorder and one on the cockpit voice recorder.

After conducting the two runs, searchers conducted more runs adjacent the earlier sweeps, but they did not detect any pings. That led authorities to express concern that perhaps the batteries had weakened or even died.

But the "runs" continued, and Tuesday, there were two more encounters with the pings. In one run, pings were detected for about 5 minutes and 32 seconds; on the other, about 7 minutes.

Authorities did not say whether the sequence of the runs — which side of the original run they were on. But, Dean said, in traditional searches, runs are made on either side until they are confident they have "boxed in" the strongest signal.

They then begin a series of runs perpendicular to the first set of runs, again trying to "box in" the strongest single.

"Essentially, when you are done you're going to have a tic-tac-toe board and you'll have boxed it in both directions," Dean said.

The box with the strongest signal is where you search.

Detected a different frequency

"I believe we are searching in the right area, but we need to visually identify wreckage before we can confirm that this is the final resting place of MH370," said a cautious Houston. "This is absolutely imperative."

'Hopefully, with lots of (pinger) transmissions, we will have a tight small area" to search, Houston said, saying they may be able to find the wreckage "in a matter of days."

But the TPL appears to have arrived in the nick of time. The fact that pings are being heard for decreasing lengths of time may be due to the batteries dying, Houston said. The fact that only one of the two pingers has been heard during the most recent runs may be evidence that one pinger battery has already died, he said.

Experts said they were not concerned that the pings were detected at a frequency of 33.331 kHz, instead of the design frequency of 37.5.

"We're listening a little bit on either side of that (37.5 kHz) because pinger (frequencies) do drift," Dean said.

The pingers did have the same pulse rate as the MH370 pingers — one ping per second. Searchers "believe the signals to be consistent with the specification and description of a flight data recorder," Houston said.

By using the TPL, searches can greatly reduce the time it takes to locate the missing plane. The TPL can cover terrain six times faster than the Bluefin-21 AUV, the autonomous underwater vehicle that will eventually be deployed to scan and photograph the ocean floor.

If the pingers keep working, the search area can be narrowed to within "a couple of hundred meters of the target," Dean said.

"At this point, as tempting as it is to launch an AUV, you really want to remain rigorous in running lines and seeing if you can reacquire a signal," he said.