Enigma ID N/A

Name High Frequency Global Communications System
Frequencies 4724, 6739, 8992, 11175, 13200, 15016. Croughton AFB also on 6712.
Status Active
Voice Live male and female operators
Emission mode USB
Location Worldwide
Activity pages

Known counterpart stations

High Frequency Global Communications System (HFGCS) is a worldwide network of transmitters used by United States Air Force (USAF) to transmit coded messages to aircrafts, operation codes called skykings and for air-to-ground communication. Incorrectly all of the coded traffic on HFGCS is referred to as Emergency Action Messages (EAM) - very rarely if ever a coded message on HFGCS is sent in case of national emergency.

Coded traffic

HFGCS coded traffic occurs very frequently, at least twice an hour, and consist of long strings of both letters and numbers. These begin with the operator reciting first six characters of the message thrice, followed by "Stand by". Then the message is given in full, followed by "I say again" and repeat of the message. The traffic ends with the name of the air base where the message originated, followed by "out".

From July 2013 to April 2015, the ending was always "Mainsail out", regardless of originating air base. Mainsail was a collective call sign for all the ground stations in the network.


These emanate from Army Material Command and Army Contracting Command of the United States Strategic Command, and are the highest priority messages that interrupt all other traffic. Skyking is a collective call sign for all aircraft and missile crews committed to Single Integrated Operation Plan, and all of those copy these messages. Skykings consist of "Skyking, Skyking, do not answer", a three-character ID, timestamp in minutes into the hour, a two-character authentication string. These are repeated twice, with "I say again" between the repeats. Occasionally the operator may ask to disregard their latest skyking.

HFGCS ground stations

All these stations transmit at once, allowing worldwide reception of messages, and creating distinctive echo when multiple sites are being received in one location. HFGCS is mainly controlled from Andrews AFB.

  • Andersen AB, Guam
  • Andrews AFB, Maryland
  • Ascension Island
  • Croughton AB, United Kingdom
  • Diego Garcia Naval Station, Indian Ocean
  • Elmendorf AFB, Alaska
  • Hickam AB, Hawaii
  • Keflavik NAS, Iceland
  • Lajes AB, Azores
  • McClellan, California
  • Offutt AFB, Nebraska
  • Salinas, Puerto Rico
  • Sigonella Naval Station, Italy
  • Yokota AB, Japan
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