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The HF-GCS

Name: High Frequency Global Communications System
Country of Origin: USA (See Below)
Voice Summary: Male/Female Live
Frequencies: 4724, 6712 (Croughton), 6739, 8992, 11175, 13200, and 15016
Mode: USB

Detailed Profile: EAMs and HF-GCS

Station Summary

The HF-GCS is used by the United States Air Force to send instructions for their operations through messages, and most commonly send Emergency Action Messages (EAMs).  The HF-GCS is not exclusive to the USAF, and is used by other countries too, but not as often.  They also send higher priority messages known as “Skyking Messages” which will even be read over-top and interrupt an EAM to be read.  Both of these messages are time sensitive and are read live in NATO Phonetic letters.  The primary HF-GCS frequency is 11175 kHz, and broadcasts 24/7 on that frequency as well as 8992 kHz.  The HF-GCS also broadcasts on 4724, 6712 (Croughton), 6739, 13200, and 15016 during their scheduled times, which is still most of the day.

The High Frequency (HF) Global Communications System (HFGCS) supports war plans and operational requirements for the following organizations: White House Communications Agency (WHCA), Joint Chiefs of Staff (JCS), Air Mobility Command (AMC), Air Combat Command (ACC), AF Air Intelligence Agency (AIA), Air Force Materiel Command (AFMC), Air Force Space Command (AFSPC), United States Air Forces in Europe (USAFE), Pacific Air Forces (PACAF), and Air Weather Service (AWS).

Test counts are also normally sent over the HF-GCS:
“This is mainsail with a test count… testing 1, 2, 3, 4, 5… 5, 4, 3, 2, 1”

Sometimes you will notice an echo of the voices in the background. These echos are a result of propagation delays because of slow landlines and the satellites going to the multiple widely spread transmission sites.

 

Emergency Action Messages

EAMs are frequently read on the HF-GCS frequencies and usually won’t take you long to hear one.  They begin with a 6 letter header with a purpose that has not been publicly released for information and this header is repeated 3 times.  Then the message continues afterward and is repeated.  A typical EAM message is 30 characters long but can be different.  There have been EAMs over 200 characters long before.  The message usually ends with “Mainsail Out”, but can change based off where it is being sent from ex. (Offutt out).  Mainsail is the collective callsign for all ground stations in the network.

Sometimes messages are intended for specific recipients, and tactical callsigns are read at the end of a message.  For example “For WAITER” and “For TRINITY” have been used as tactical callsigns before.

Force Direction Messages
(FDMs) are another type of message sent through the HF-GCS, however there is no way to tell if an FDM is being sent or if it’s just another EAM.

The Single Channel Transponder System (SCTS) provides Emergency Action Message (EAM) and Force Direction Message (FDM) dissemination capability to command centers (USSTRATCOM, EUCOM, SHAPE, NMCC, USSPACECOM, and PACOM) and force elements for the control of strategic and non-strategic nuclear forces.

 

Skyking Messages

Also known as “Foxtrot Broadcasts” are Skyking messages.  These are a higher priority message and are sent in a different format from EAMs.  Skyking messages will sometimes even interrupt an ongoing EAM since it’s the highest priority.  “Skyking” is the collective callsign for sending messages to all Single Integrated Operation Plan (SIOP) aircraft and missile Ops which are also responsible for deploying strategic bombers, reconnaissance aircraft, and various support aircraft.

A Skyking message begins with the reader speaking “Skyking Skyking do not answer”, followed by a 3 letter trigraph, then two numbers for the time of the hour, and ends with a 2 letter authentication string.  The message is then repeated.  Skyking messages have the same ending as a regular EAM and can also change depending on where it is sent from.

Example of a Skyking Message

Example of the operator telling the receiver to disregard his last message

 

Locations
The HF-GCS has multiple transmission sites in different countries to ensure they can transmit worldwide, thus the name “Global Communications System”.

HF-GCS Sites

Andersen Air Base, Guam
Andrews AFB, Maryland
Ascension Island
Croughton AB, United Kingdom
Diego Garcia Naval Station, Indian Ocean
Elmendorf AFB, Alaska
Hickam AFB, Hawaii
Keflavik NAS, Iceland
Lajes AB, Azores
McClellan, California
Offutt AFB, Nebraska
Salinas, Puerto Rico
Sigonella Naval Station, Sicily, Italy
Yokota AB, Japan
Air Force Eastern Test Range, Florida
“MPA”, Unknown

Each of the sites also has an ALE callsign.  ALE (Automatic Link Establishment) is an HF system used to connect to an HF station or a network of stations. These are the callsigns used for the stations.

Articles

http://mt-milcom.blogspot.com/p/what-is-emergency-action-message-or-eam.html

https://theaviationist.com/tag/emergency-action-message/

https://fas.org/nuke/guide/usa/c3i/saccs.htm

https://www.nps.gov/mimi/learn/historyculture/emergency-action-messages.htm

http://www.globalsecurity.org/military/systems/aircraft/systems/hfgcs.htm

https://mt-milcom.blogspot.com/2015/06/usaf-hf-global-communications-system.html

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