The Radio Riddle

The Radio Riddle comprises two questions:

(1) What was Earhart’s closest point of approach (CPA) to Howland Island?

(2) Why was 2013 GMT the last time that the Itasca heard a radio signal from Earhart on July 2nd?

Computer modeling was used to solve the riddle. The computer model, technical background, methods, assumptions and detailed results are discussed in Post-Loss Radio Signals Technical Analysis.

The signal-to-noise ratio (SNR) was computed for Earhart signals on 3105 kHz and 6210 kHz, at hourly intervals from 1900 GMT through 2300 GMT, versus distance in 50-mile increments to a maximum of 350 nautical miles along the 157°/ 337° line of position (LOP) through Howland Island.

Earhart’s estimated CPA to Howland Island was derived by finding the distance at which the computed SNR corresponded to the most conservative interpretation of the signal strength estimate given in the Itasca’s radio log. That distance was found to be a maximum of 80 nautical miles. It was concluded that Earhart reached the CPA at 1912 GMT.

The question of why the Itasca didn’t hear any radio signals from Earhart after 2013 GMT was answered by noting that the computer model results showed a rapid deterioration of propagation conditions along the Howland Island LOP, as a function of time, frequency, and distance. On 3105 kHz, the deterioration was so rapid that by the time of her next scheduled transmission at 2115 GMT, Earhart was beyond the maximum range at which signals could be heard by the Itasca. Things were marginally better on 6210 kHz. If Earhart had shifted to 6210 kHz following her 2013 GMT signal, the Itasca would have heard her scheduled 2115 GMT signal. But by the time of her scheduled 2215 GMT signal, she was beyond the maximum range at which the Itasca could hear her on 6210 kHz.

Therefore, we conclude that the Itasca heard no signals from Earhart after 2013 GMT because: (a) she did not transmit on 6210 kHz following her 2013 GMT signal; (b) she was too far from the Itasca at 2115 GMT for her signal to be heard on 3105 kHz; and (c) she was too far from the Itasca at 2215 GMT for her signal to be heard on either frequency. It is worth noting that Earhart could have reached Gardner Island before her scheduled 2215 GMT transmission time. So the possibility of safe arrival at Gardner cannot be ruled out on the basis of these findings. On the other hand, these findings do not rule out the possibility that Earhart simply decided to stop transmitting after 2013 GMT, or that she encountered a sudden catastrophic event causing loss of the aircraft before she could send a signal.

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