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President Truman established the CONELRAD [CONtrol of ELectronic RADiation] system in 1951. to provide emergency alert to the public. Under this first national alerting system in the event of a Soviet attack on the United States, all commercial radio stations would cease normal operation, in order to prevent Soviet bombers from homing in on their targets by using specific radio commercial radio stations as navigation beacons. Instead, selected CONELRAD stations would broadcast on either 604kHz or 1240kHz to inform the public about emergency measures. As part of the system it was obligatory for all radios sold after 1953 to have the CONELRAD frequencies 640/1240 kHz marked with small triangles on the dial. The triangles were referred to as CD marks, for Civil Defense. The marks on the radio dial were to make finding the frequencies easy. This requirement was dropped, when the CONELRAD system was replaced by the Emergency Broadcast System in 1963. By the early 1960's the development of Soviet missiles had made the CONELRAD system obsolete.