Many thanks to SRAA contributor, John Lutz, who notes:

Emergency Action Message (EAM) transmissions are associated with the High Frequency Global Communications System (HFGCS). The HFGCS is a network of single sideband transmitters used by the United States Air Force to transmit encrypted messages to in-flight military aircraft, ground stations and in some cases, US Navy surface vessels. The purpose and format of the messages are subject to wide-ranging speculation, but are generally thought to be for directing nuclear-capable forces regarding the execution of specific attack options.Starting time: 04:52 UTCFrequency: 8.992 MHzLocation: Shorewood, IL EN51vmReceiver and antenna: FT-920 transceiver and ground-mounted 23-foot vertical antenna