Dozens of previously dismissed radio signals were actually credible transmissions from Amelia Earhart, according to a new study of the alleged post-loss signals from Earhart's plane.
The transmissions started riding the air waves just hours after Earhart sent her last inflight message.
The study, presented on Friday at a three day conference by researchers of The International Group for Historic Aircraft Recovery (TIGHAR), sheds new light on what may have happened to the legendary aviator 75 years ago. The researchers plan to start a high-tech underwater search for pieces of her aircraft next July.
"Amelia Earhart did not simply vanish on July 2, 1937. Radio distress calls believed to have been sent from the missing plane dominated the headlines and drove much of the U.S. Coast Guard and Navy search," Ric Gillespie, executive director of TIGHAR, told Discovery News.
"When the search failed, all of the reported post-loss radio signals were categorically dismissed as bogus and have been largely ignored ever since," he added.
Using digitized information management systems, antenna modeling software, and radio wave propagation analysis programs, TIGHAR re-examined all the 120 known reports of radio signals suspected or alleged to have been sent from the Earhart aircraft after local noon on July 2, 1937 through July 18, 1937, when the official search ended.
They concluded that 57 out of the 120 reported signals are credible.
"The results of the study suggest that the aircraft was on land and on its wheels for several days following the disappearance," Gillespie said.
Earhart used radio transmissions on her last flight on July 2, 1937, during her record attempt to fly around the world at the equator.
At 07:42 local time, as she flew toward the target destination, Howland Island in the Pacific, with her navigator Fred Noonan, Earhart called the Coast Guard cutter Itasca, stationed at Howland Island to support her flight.
“We must be on you, but cannot see you -- but gas is running low. Have been unable to reach you by radio. We are flying at 1,000 feet,” she said.
Earhart's final inflight radio message occurred a hour later, at 08:43.
“We are on the line 157 337. We will repeat this message. We will repeat this on 6210 kilocycles. Wait,” she said.
According to TIGHAR, the numbers 157 and 337 refer to compass headings -- 157 degrees and 337 degrees -- and describe a navigation line that passed not only Howland Island, the target destination, but also Gardner Island, now called Nikumaroro.
This uninhabited atoll in the southwestern Pacific republic of Kiribati is where TIGHAR believes Earhart and Noonan landed safely and ultimately died as castaways.
According to TIGHAR's hypothesis, Earhart would have used the aircraft's radio to make distress calls for several days until the plane was washed over the reef and disappeared before Navy searchers flew over the area.
TIGHAR built a detailed catalog and analysis of all the reported post-loss radio signals, and selected the credible ones based on their frequencies.
Transmissions from Earhart's Electra (NR16020) were possible on three primary frequencies: 3105 kHz, 6210 kHz and 500 kHz. For the latter, however, there were no reported post loss signals.
On her world flight, Earhart transmitted on 3105 kHz at night, and 6210 kHz during daylight, using her 50-watt WE-13C transmitter.
The Itasca transmitted on 3105 kHz, but did not have voice capability on 6210 kHz.
Under favorable propagation conditions, it was possible for aircraft operating on the U.S. west coast at night to be heard on 3105 kHz in the central Pacific. Indeed, the Itasca reported hearing such signals on one occasion.
There were three 50-watt Morse code radio stations in Nicaragua which could be heard on a receiver tuned to 3105 kHz, but the stations sent only code, not voice.
Moreover, all transport aircraft in the area used assigned route frequencies, instead of 3105 kHz.
"Therefore, other than Itasca, Earhart’s Electra was the only plausible central Pacific source of voice signals on 3105 kHz," said Gillespie.
Although several of the analyzed post-loss signal reports were determined to be hoaxes, Gillespie ruled out the hypothesis of an illegal transmitter "given the numerous constraints militating against successfully perpetrating a signal transmission hoax."
"We do not really have hoax transmissions but rather reports from people who, for whatever reason, claimed to have heard something they did not hear," Gillespie said.
To make multiple transmissions, the Electra plane needed to run the right-hand, generator-equipped engine to recharge the batteries.
"The safest procedure is to transmit only when the engine is running, and battery power is required to start the engine," said Gillespie. "To run the engine, the propeller must be clear of obstructions, and water level must never reach the transmitter."
To verify the hypothesis that the plane landed on Nikumaroro's reef, TIGHAR researchers analyzed tidal condition on the island from 2 to 9 July 1937, the week following Earhart disappearance.
It emerged that transmission of credible signals occurred in periods during which the water level on the reef was low enough to permit engine operation.
According to Gillespie, at least four radio signals are of particular interest, as they were simultaneously heard by more than one station.
The first signal, made when the pilot had been officially missing for just 5 hours, was received by the Itasca, and two other ships, the HMS Achilles, and the SS New Zealand Star.
The Itasca logged “We hear her on 3105 now - very weak and unreadable/ fone” and asked Earhart to send Morse code dashes.
The Achilles did not hear “very weak and unreadable” voice, but heard Itasca’s request and heard dashes in response. The SS New Zealand only heard the response dashes.
In other cases, credible sources in widely separated locations in the U.S., Canada, and the central Pacific, reported hearing a woman requesting help. She spoke English, and in some cases said she was Amelia Earhart.
In one case, on July 5, the U.S. Navy Radio at Wailupe, Honolulu heard a garbled Moorse code: “281 north Howland - call KHAQQ - beyond north -- won’t hold with us much longer -- above water -- shut off.”
At the same time, an amateur radio operator in Melbourne, Australia, reported having heard a "strange” code which included KHAQQ, Amelia's call sign.
According to Gillespie, the re-analysis of the credible post loss signals supports the hypothesis that they were sent by Earhart’s Electra from a point on the reef at Nikumaroro, about ¼ mile north of the shipwreck of the British freighter SS Norwich City.
"The results of the study show a body of evidence which might be the forgotten key to the mystery. It is the elephant in the room that has gone unacknowledged for nearly seventy-five years," said Gillespie.