On July 15 North Korea activated, for the first time since 2000, a “numbers station.” This is a special radio station that uses a specific shortwave radio frequency that periodically (on pre-arranged dates and times) broadcasts sequences of numbers. These broadcasts often last a few minutes then that channel goes silent.
Numbers stations were a popular 20th century technique for sending secret messages to agents in foreign nations or combat zones. It began during World War I, with Morse code used instead of a spoken voice. Given the frequency used and the strength of the signal the audience for these new North Korean messages was in South Korea, China or Japan. Analysis of the audio indicated that the North Koreans were using Cold War era Russian (Soviet) broadcast equipment. Apparently this gear was put in storage in 2000 and revived for the new broadcasts.
Since the Cold War ended few nations still use numbers stations, most notably Cuba, China, Taiwan, Israel and, until 2000, North Korea. For nations with international Internet access (and this does not include North Korea) there are more effective Internet based methods for sending secret messages. North Korea is known to have used these Internet based method so it is a mystery as to why they have revived their numbers station broadcasts. Then again it could just be another ploy to frighten their enemies (which is just about everyone, especially South Korea, the United States and Japan.) These three nations do not appear alarmed by the broadcasts, but that could be said for most North Korean threats.