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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.

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.  The message usually ends with “Mainsail Out”, but can change based off where it is being sent from ex. (Offutt out).

Force Direction Messages
(FDMs) are also 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.

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.  A Skyking message begins with the reader stating “Skyking Skyking do not answer”, and then a 3 letter trigraph is read, then two numbers for the time of the hour, and ends with a 2 letter authentication string.  The message is also 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

 

Articles

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

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

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

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