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HFGCS

The High Frequency Global Communication System (HFGCS) is an HF network run by the United States Air Force (USAF). It is also usable by the United States Navy and the Department of State.

Ground stations

The HFGCS ground stations are situated at U.S. air bases around the world. The voice callsigns listed are used when the base is identified directly. The 2G ALE callsigns are usually derived from the bases' corresponding IATA or ICAO airport codes. All ground stations are remotely controlled from Andrews.

Station Voice 2G ALE
Andersen Air Force Base, Guam Andersen GUA
Andrews Air Force Base, MD, USA Andrews ADW
RAF Ascension Island Ascension HAW
RAF Croughton, UK Croughton CRO
Naval Support Facility Diego Garcia Diego Garcia JDG
Elmendorf Air Force Base, AK, USA Elmendorf AED
Hickam Air Force Base, HI, USA Hickam HIK
Lajes Field, Portugal Lajes PLA
Offutt Air Force Base, NE, USA Offutt OFF
Naval Radio Transmitting Facility Isabela Puerto Rico JNR
Naval Air Station Sigonella, Italy Sigonella ICZ
Beale Air Force Base, CA, USA West Coast MCC
Yokota Air Base, Japan Yokota JTY

Voice

The voice activity takes place on the following channels, all on the upper sideband:

Apart from two-way air-to-ground traffic, these channels also feature one-way broadcasts of messages from the Joint Chiefs of Staff. When addressed to all stations, those are transmitted simultaneously on all four channels by all ground stations; imperfect synchronization between stations results in the noticeable delay effect.

The network control station changes its callsign once a day at 00:00 UTC. The static call Mainsail may also be used to refer to any ground station.

Air-to-ground traffic

The main channel for air-to-ground traffic is 11175 kHz. The secondary channels are 8992 kHz (24/7), 15016 kHz (daytime only), and 4724 kHz (nighttime only).

Apart from frequently changing tactical callsigns, the following static calls may also be heard:

  • Any Station: Any station currently present on the network, including aircraft.
  • Last Caller: The station that had just called on the given channel.
  • Reach ###: Air Mobility Command transport aircraft.
  • Doom ##, Rogue ##Skull ##: Boeing B-52 strategic bombers.

Aircraft regularly enter (or check into) and exit (or check out of) the network. To do either, the aircraft usually asks the ground station, which then authenticates the operation by giving a pre-defined three letter code and expecting a single letter in return. Less often, the aircraft may enter in the blind, giving a two-letter authentication code directly.

Unclassified phone patches may take place over HFGCS channels. To initiate one, the aircraft would first call the ground station, requesting a phone patch. Afterwards, the aircraft would give either a 7-digit DSN number or a 10-digit public number to call.

Emergency Action Messages

Emergency Action Messages (EAM) are the main type of one-way broadcasts on HFGCS. While most EAMs are addressed to all stations, some may be regionalized and/or transmitted from aircraft.

EAMs consist of a string of alphanumeric characters, using all letters of the English alphabet, and all digits except 0, 1, 8, and 9. The first two characters remain constant for some period of time, ranging from a few days to a few weeks. The standard message length is 30 characters.

On the air, the messages are spelled using the NATO phonetic alphabet according to the following procedure:

  1. "All stations, all stations, this is <callsign>, <callsign>."
  2. Optionally, when a message is addressed to somebody in particular: "For," followed by the individual recipient callsigns, repeated together thrice.
  3. The first six characters of the message, followed by "Stand by," repeated altogether thrice.
  4. "Message follows."; Optionally, when a message is not 30 characters long, "of <number> characters" may be inserted after "message."
  5. The message in full.
  6. "I say again."
  7. The message in full.
  8. Optionally, when a message is not 30 characters long, "This concludes a message of <number> characters."
  9. Depending on whether there is another message to follow: "This is <callsign>, more to follow, stand by." or "This is <callsign>, out."; <callsign> is the same as in step 1.

In some cases, the operator may instruct recipients to disregard the message.

Skyking messages

Skyking messages (also known as Foxtrot messages) are a less common type of one-way broadcasts, believed to be associated with strategic operations.

Skyking messages consist of a code word followed by the time (in minutes of the current hour) and a two-letter authentication code. The code words are names of popular rock bands and movies.

On the air, the messages are broadcast according to the following procedure:

  1. "Skyking, skyking, do not answer."
  2. The code word, repeated twice.
  3. "Time" followed by the minutes of the current hour, as two digits, each spelled separately.
  4. "Authentication" followed by two letters, spelled using the NATO phonetic alphabet.
  5. "I say again", followed by the steps 1 to 4 repeated.
  6. "This is <callsign>, out."

In some cases, the operator may instruct recipients to disregard the message.

Test counts

Test counts are made multiple times a day using the following format: "This is <callsign> with a test count; testing, 1 2 3 4 5, 5 4 3 2 1; this is <callsign>, out." The ground station of origin usually identifies itself directly, but Mainsail or today's network control station callsign may also be used.

Data

The HFGCS ground stations may be reached using 2G ALE (MIL-STD-188-141A) on the following channels, all on the upper sideband:

  • 3137 kHz
  • 4721 kHz
  • 5708 kHz
  • 6721 kHz
  • 9025 kHz
  • 11226 kHz
  • 13215 kHz
  • 15043 kHz
  • 18003 kHz
  • 23337 kHz

The ground stations and the aircraft exchange text messages using the 2G ALE Automatic Message Display (AMD) capability, containing either clear chatter or encrypted data. During idle periods, the ground stations emit soundings.

SIPRNet

The SIPRNet is a computer network used to exchange classified information within the Department of Defense and the Department of State. The HFGCS ground stations provide an HF radio gateway to this network.

The following channels, all on the upper sideband, are used for SIPRNet over HF. Connectivity is established using 2G ALE; the ground stations use their usual 2G ALE callsigns, but with the suffix "SPR." During idle periods, the ground stations emit soundings.

  • 3113 kHz
  • 5702 kHz
  • 6715 kHz
  • 8968 kHz
  • 11181 kHz
  • 15091 kHz
  • 17976 kHz
  • 27870 kHz

NIPRNet

The NIPRNet is a computer network used to exchange unclassified information within the Department of Defense. The HFGCS ground stations provide an HF radio gateway to this network.

The following channels, all on the upper sideband, are used for NIPRNet over HF. Connectivity is established using 2G ALE; the ground stations use their usual 2G ALE callsigns, but with the suffix "NPR." During idle periods, the ground stations emit soundings.

  • 3068 kHz
  • 4745 kHz
  • 5684 kHz
  • 8965 kHz
  • 11199 kHz
  • 13242 kHz
  • 17973 kHz
  • 20631 kHz
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