Interesting story from Radio Free Europe: "Random Numbers, Persian Code: A Mysterious Signal Transfixes Radio Sleuths -- And Intelligence Experts." The article begins:
The radio signal first started broadcasting on February 28, about 12 hours after the United States and Israel began bombing Iran.
On a scratchy shortwave signal almost twice a day -- in the early morning and early evening on Coordinated Universal Time -- a man's voice can be heard speaking Persian, counting out a series of apparently random numbers. The numbers are read out for varying stretches of time, followed by a pause in which the word tavajjoh -- which translates as "attention" -- is spoken three times.
[snip]
Five days later, it got more interesting.
Beginning on March 4, the signal started to be jammed, with a cacophonous screech of electronic noise that made it all but impossible to hear the numbers. The original transmission paused for a period of time, then moved to another shortwave frequency.
"It's interesting because it started to be jammed on the initial frequency," said Akin Fernandez, who is widely considered an authority on the decades-old encoded radio technology known as a numbers station. "Someone doesn't want the recipient [of the signal] to hear the numbers."
"It's an adversarial situation, two groups acting against one another. The question [is] who has the technical means to jam a station," Fernandez said. "The United States has the means, which means this is being transmitted by Iran. Or then it could be Iran, which means the United States is the transmission source."
"More likely this is an operation against Iran," he said.
The article indicates that a British-based group called Enigma2000 had triangulated the origin of the signal's transmitter: "somewhere in an area encompassing northern Italy, Switzerland, western Germany, eastern France, Belgium, and the Netherlands." Even the jamming--a type of "bubble jammer"--suggests Iran as the source of the jamming with the article noting that this "is exactly the same kind of bubble jammer that is used against Radio Farda, VOA Farsi, Iran International TV shortwave relay, and BBC Farsi[.]"
What they really need to use is a "burst transmission." That's where you take a really long message and speed it up exponentially into a short "burst" of what sounds like white noise. The recipient then records and plays back the burst of noise at a very slow speed...at which point it sounds like normal conversation. I remember hearing this on my shortwave radio back in the 1980's. It's old technology, but sometimes low tech rules.
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