On August 17, 1976, an intelligence intercept was delivered to analysts working for the NSA at its facility in Fort Meade, Maryland. The intercept was from a Soviet military source, variously identified as Colonel [Redacted] of the Strategic Missile Forces or Major [Redacted] of the Eighth Chief Directorate of the KGB. The intercept details a recurring problem that had been noticed over the previous several weeks with the new Duga over-the-horizon radar system.

The contents of the intercept, which is only available as a summary of a translation, go into detail about radio interference caused by the Duga system—well-known by amateur radio operators at the time, who nicknamed Duga the “Russian Woodpecker” for its adverse effects on ham radio transmissions—but in an altogether novel way. Something was causing some of the signals to change in between their transmission in Chernihiv and their reception in Chernobyl. The 10hz signal would be returned as an 8hz signal, but only on certain transmissions, and only for certain signals within those transmissions.

Colonel [Redacted] (or Major [Redacted]) apparently noted that, since the Duga system used a consistent and repeating pattern of 10hz tones (the “woodpecker” of the “Russian Woodpecker”), it was possible to find a pattern in the altered tones. He apparently did so, reporting the following pattern: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71. The available information helpfully notes for non-mathematicians that these are the first 20 prime numbers.

Apparently, Colonel [Redacted] (or Major [Redacted]) requested official access to the Duga array at Cherniv in order to analyze and/or respond to the signal; this was apparently denied. A subsequent request to triangulate the source of the interference was apparently granted, but there is no information about what was discovered. Analysis of Soviet records following the collapse of the USSR have shown no trace of a Colonel [Redacted] or Major [Redacted]. The Duga arrays at Chernihiv and Chernobyl have both been inspected following the collapse of the USSR, but have offered no clues as to the signal or its origin.

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