CONELRAD
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CONELRAD (Control of Electromagnetic Radiation) was a planned method of emergency broadcasting to the public of the United States in the event of World War III. It served two purposes; to prevent Soviet bombers from homing-in on American cities by using broadcast stations as beacons, and to provide essential civil defense information. U.S. President Harry S. Truman established CONELRAD in 1951.
All broadcast stations would monitor a designated station in an area. In the event of an emergency, all United States television and FM radio stations were to leave the air. Selected Mediumwave AM stations would broadcast on one of two frequencies, 640 kHz or 1240 kHz, at low power. Radio sets manufactured after 1953 had these frequencies marked by the triangle-in-circle symbol of Civil Defense.
CONELRAD had a simple system for alerting the public and other "downstream" stations that consisted of a sequence of shutting the station off for five seconds, returning to the air for five seconds, again shutting down for five seconds, and then transmitting a tone for 15 seconds. Beginning in 1957, operating U.S. amateur radio stations were required to verify at least once every 10 minutes that a normal broadcast station was on the air. If not, the amateurs were required to stop transmitting.
CONELRAD was succeeded by the Emergency Broadcast System in 1963, and the Emergency Alert System in 1997, both administered by the FCC.
Fictional treatments of how CONELRAD would work can be found in the novel Alas, Babylon by Pat Frank, and the 1962 movie Panic in Year Zero, as well as the CBS television network's 1958 made-for-TV movie A Day Called 'X'.