What North Korean intelligence listens to
Officially, the U.S. may be backing down from its well-planned series of information operations designed to show North Korea that it is serious about deterrence and power, lest there be misperceptions that could magnify misunderstandings. This is not a silly telegraphed retreat: The U.S. had no idea, or would not believe, how truly scared Soviet leaders were of a U.S. first strike in 1983, and went ahead with a war games exercise that nearly precipitated World War III.
That said, I doubt that the military deception/special technical operations part of this campaign is over. A group of radio monitoring enthusiasts dutifully log all of the broadcasts on the Air Force's High Frequency Global Communications System. The HFGCS, stood up in 1992, is a reliable, redundant worldwide communications network that allows deployed aircraft to talk with fixed and flying command and control centers. There are 13 base stations across the world, ensuring virtual global coverage with plenty of overlap. The frequencies are published openly; the broadcasts are analog (although a digital transition is coming) and in the clear because secure telephones and secure HF radio networks don't work well together yet. The HFGCS is one of several Department of Defense communication systems that broadcast Force Direction Messages (FDMs), which consist of priority coded orders, and emergency action messages, or EAMs, which generally consisted of groups of letters and numbers, coded instructions to special units, deployed strategic (nuclear) fighters, and authentication messages between command posts (like the National Airborne Operations Center) and the TACAMO planes that communicate with submarines. Often, these units will use the HFGCS to request information, ingest e-mail and tactical data, contact other units on different communication systems, and to practice for the day that the end of the world comes.
The HFCGS is also monitored by the bad guys, including, of course, North Korea. Their intelligence agencies are on the alert for anything that indicates a different pattern of communication, even if they aren't able to decipher the specific EAMs. But they're also listening in case, say, a U-2 spy plane accidentally gives away its location, or bomber pilots say things like "weapons are authorized."
Actually, according to several of those hobbyists, the U.S Air Force and the U.S. Strategic Command have started to use a new format for their EAMs, just as the crisis with North Korea heated up. Generally, EAMs to nuclear forces are short, consisting of 30 characters often divided into four or five blocks. Thrice daily, test EAMs are sent out. Every day, the call-sign for the airborne command posts change. (Today, one of the TACAMO planes was "ESTIMATE.") In recent days, the EAMs have included a preamble, as well as a "character count," a way for those receiving the messages to know for sure exactly how many characters they're supposed to receive. In theory, the preambles refer to a specific action or mechanism.
This seems complicated, and I don't fully understand it all, but that may be the point. If you're North Korea, you'll notice that your main adversary is no longer communicating with its nuclear forces in the same way. This suggests that something is up. What that something is may well be nothing — or it might mean that the U.S. has ratcheted up an alert status, or has changed the way it exercises nuclear command and control forces. The unknown-ing-ness is the point.
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