The Post-Loss Radio Signals

This paper was originally published in the on line edition of the Eighth Edition of the Earhart Project Book in October 2000.

The TIGHAR research data base contains many known post-loss signals that were thought by contemporary observers to possibly have originated from NR16020. Many were weak carrier signals with no modulation and no accompanying identifying characteristics other than frequency. A few signals, however, are well-documented and 4 such cases were selected for analysis in this edition. A volunteer TIGHAR Team is working on assessing and cataloging the remaining signals to identify candidates for future detailed analysis.

The 4 cases analyzed below are:

  1. The Nauru intercept on July 3rd (GMT).
  2. The HMS Achilles intercept on July 3rd (GMT).
  3. Responses to the Honolulu radio station KGMB broadcast on July 5th (GMT).
  4. The “281” message intercepted in Honolulu on July 5th (GMT).

The details of each case are discussed elsewhere and are not repeated here except where essential to this analysis.

In each case, the hypothesis “The signal of interest could have originated from NR16020 on Gardner Island” was tested on the basis of the SNR computed by the model. Since the Itasca departed the vicinity of Howland Island late on July 2nd to search for Earhart, the Itasca's actual position, taken from the ship's deck log, was used in calculating SNR when a question arose as to whether the Itasca could have heard a signal from Earhart.

Acceptance of the hypothesis supports, but does not prove, the theory that Earhart and Noonan reached Gardner. Such a proof would require showing that the signal could not have originated from any other place, a task which is beyond the scope of this edition.

The Nauru Intercept

At 1030 GMT on July 3rd, Itasca intercepted a message [1] from KPH (Radiomarine Corporation of America) at Bolinas, CA to Coast Guard radio San Francisco, reporting the following message from Nauru and requesting that it be relayed to Itasca: “Voice heard fairly strong signals strength to S3 0843 0854 GMT 48.31 meters speech not interpreted owing bad modulation or speaker shouting into microphone but voice similar to that emitted from plane in flight last night with exception no hum of plane in back ground.” 48.31 meters is the wavelength corresponding to a frequency of 6210 kHz.

At 1200 GMT on July 3rd, a message was sent from Sydney, Australia, presumably by the U.S. Consulate, to the Secretary of State [2] in Washington, relaying information from Amalgamated Wireless that Nauru radio had sent the following message to Bolinas radio: “At 6:31, 6:43, and 6:54 PM Sydney time today on 48.31 meters, fairly strong signals, speech not intelligible, no hum of airplane in background but voice similar to that emitted from plane in flight last night between 4:30 and 9:30 PM. Message from plane when at least 60 miles south of Nauru received 8:30 PM Sydney time July 2nd saying “a ship in sight ahead.” Since identified as steamer Myrtle Bank which arrived Nauru daybreak today. Reported no contact between Itasca and Nauru radio. Continuous watch being maintained by Nauru radio.” The three times given in the message for hearing the “fairly strong” signals correspond to 0831 GMT, 0843 GMT, and 0854 GMT. The times given for signal intercepts the previous night correspond to the interval between 0630 GMT and 1130 GMT. The time given for the “ship sighted” signal corresponds to 1030 GMT.

These two messages illustrate the problems inherent in the relatively crude methods of radio communication used in connection with the Earhart flight and post-loss search. Messages were sent in Morse Code and message drafters typically tried to use the fewest possible words. The results were often vague or misleading, producing the sort of confusion known in military circles as the “fog of war.” The messages above provide a good example of the "fog" that permeated the Earhart episode. Both messages purport to describe the same report from Nauru to Bolinas Radio, but neither includes all of the information in the report. Only a reader privy to both messages would have the full context.

The computed SNR is consistent with the Nauru operator's characterization of the signals as "fairly strong." Therefore we accept the hypothesis “The signal of interest could have originated from Gardner Island.” This conclusion is strengthened by the Nauru operator's observation that the voice heard in the signals was similar to that heard from Earhart during the previous night, and by the fact that the speech was unintelligible due to “bad modulation or speaker shouting into microphone” which is consistent with the modulation problem on 6210 kHz cited by the technician who worked on Earhart's radio at Lae.

The HMS Achilles Intercept

On July 3rd, while about 900 nautical miles East-Southeast of Gardner Island, the British cruiser HMS Achilles intercepted an exchange of signals on 3105 kHz, between 0600 GMT and 0620 GMT, in which Achilles heard an unknown station (actually, it was the Itasca) request another unknown station to send dashes, and then heard dashes. Achilles also heard the call letters KHAQQ, Earhart's call sign.

The question is whether the dashes heard by the Achilles could have originated from NR16020 on Gardner Island. Since dashes are essentially CW signals (see Appendix A), the criteria for usability of a CW signal were applied in testing the hypothesis that the dashes heard by Achilles originated from NR16020 on Gardner Island.

The computed SNR at the Achilles was consistent with a signal from Gardner, so the hypothesis “The signal of interest could have originated from Gardner Island” cannot be rejected on the basis of that finding alone.

However, it is instructive to consider the SNR at the Itasca, which had requested Earhart to send dashes on 3105 kHz . . . the request that the Achilles heard. At the time the Achilles heard the dashes, the SNR for a signal from Gardner Island to the Itasca would be very strong and the Itasca clearly would have heard the dashes if they had been sent by Earhart. Since the Itasca did not hear any dashes, we can conclude that the dashes heard by HMS Achilles were not sent from Gardner Island, and we reject the hypothesis.

Responses to the KGMB Broadcast

On July 5th at 0630 GMT, Honolulu radio station KGMB broadcast a request for Earhart to respond on 3105 kHz with dashes. The Pan American Airways (PAA) station at Mokapu Point, a few miles East of Honolulu, reported hearing four distinct dashes on 3105 kHz immediately following the broadcast.

At the time PAA heard the dashes, the SNR at Mokapu Point for a signal from NR16020 at Gardner island was far below the threshold of detection. Therefore, we conclude that the dashes heard by PAA could not have originated from NR16020 at Gardner Island and we reject the hypothesis.

However, the Naval Radio Station at Tutuila, about 620 nautical miles South Southeast of Gardner Island, reported hearing “Four series of dashes” between 0700 and 0704 GMT, and “Four series of dashes” between 0714 and 0716 GMT, and “Eight series of dashes four of which were very strong. Voice indicated but not distinguishable. All on 3105” between 0727 and 0731 GMT.

At the times of these intercepts, the SNR at Tutuila for a signal from NR16020 on Gardner Island was quite strong, and we conclude that the dashes heard by Tutuila could have originated from NR16020 on Gardner Island and we accept the hypothesis.

Therefore, it appears that the signals heard by PAA at Mokapu Point either were spurious or were a hoax.

The “281” Message Intercept

The “281” message was a CW signal heard by three radio operators at the Naval Radio Station at Wailupe, East of Honolulu, between 1130 GMT and 1230 GMT on July 5th.

The SNR for a CW signal from NR16020 on Gardner Island to Wailupe during that period was far below the threshold for either detection or usability, and the hypothesis is rejected. The signal heard at Wailupe could not have originated at Gardner Island.


The signals intercepted by HMS Achilles on July 3rd, and the “281” message intercepted by Navy Radio Wailupe on July 5th, could not have originated from Gardner Island. But the signal heard at Nauru Island on July 3rd and the signals heard by Navy Radio Tutuila on July 5th could have originated from NR16020 at Gardner Island.

Therefore, the possibility that Earhart successfully landed on Gardner Island cannot be ruled out by these results.

1 NARA, RG. 26, Thompson Radio Transcripts. Back.
2 NARA, RG 59, 1930-1939, File 800.79611 Putnam Amel. Earhart/1. Back.
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