CONELRAD : Conelrad (CONtrol of ELectronic RADiation) was set up in 1951 to provide warnings to the public during the Cold War. Upon alert, most stations would go off the air. Each remaining station was to move to either 640 or 1240 kHz, and alternate with other such local stations, supposedly so no enemy Direction Finding equipment could lock onto locations in the US. Or course, most stations were not really that far apart, in air miles, so it was not a very useful system. Actual activations were apparently very few.