Numbers station

A numbers station is a shortwave radio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries. Most identified stations use speech synthesis to vocalize numbers, although digital modes, such as Phase-shift keying and Frequency-shift keying as well as Morse code transmissions are not uncommon. Most stations have set time schedules, or schedule patterns; however, other stations appear to broadcast at random times. Stations may or may not have set frequencies in the HF band.

The first known use of numbers stations was during World War I, and possibly the first listener was Archduke Anton of Austria. The numbers were transmitted in Morse code. Numbers stations operated mainly during the Cold War, yet many continue to operate and some have even switched operators. The Czech Ministry of the Interior and the Swedish Security Service have both acknowledged the use of numbers stations by Czechoslovakia for espionage, with declassified documents proving the same. With a few exceptions, no QSL responses have been received from numbers stations by shortwave listeners who sent reception reports to said stations, which is the expected behavior of a non-clandestine station.

The best-known of the numbers stations was the "Lincolnshire Poacher", which is thought to have been run by the British Secret Intelligence Service.

In 2001 the United States tried the Cuban Five on the charge of spying for Cuba. That group had received and decoded messages that had been broadcast from Cuban numbers stations. Also in 2001, Ana Belen Montes, a senior US Defense Intelligence Agency analyst, was arrested and charged with espionage. The federal prosecutors alleged that Montes was able to communicate with the Cuban Intelligence Directorate through encoded messages, with instructions being received through "encrypted shortwave transmissions from Cuba". In 2006, Carlos Alvarez and his wife, Elsa, were arrested and charged with espionage. The U.S. District Court Florida stated that "defendants would receive assignments via shortwave radio transmissions".

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