HFGCS

The High Frequency Global Communications System (HFGCS) is the HF communication system of the United States Air Force.

The HFGCS has 13 ground stations near U.S. air bases around the world, all of which are remotely controlled from the Network Control Station at the Andrews Air Force Base.

Voice

The voice channels carry two-way communications with aircraft using many different callsigns. The main frequency is 11175 during the day, or 4724 during the night.

The aircraft regularly enter and exit 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.

The HFGCS provides a phone patch service, allowing aircraft to make unclassified phone calls via the DSN. Phone patches are usually requested on the main frequency, but then moved elsewhere by the ground station to free up the channel.

In addition to two-way communications, there are several types of one-way traffic, usually transmitted on all frequencies simultaneously by all ground stations:

• Test count: These are made multiple times a day, and are most commonly associated with shift changes at the Network Control Station. A typical test count is broadcast as follows: "This is <callsign> with a test count; testing, 1 2 3 4 5, 5 4 3 2 1; this is <callsign>, out."
• Emergency Action Message (EAM): A very common broadcast type which consists of an encrypted string of alphanumeric characters, usually 30 characters long. A typical EAM is broadcast according to the following procedure:
  1. "All stations, all stations, this is <callsign>, <callsign>."
  2. Optionally, if a message is addressed to a particular station: "For <callsign>," repeated twice.
  3. The first six characters of the message, followed by "Stand by," repeated altogether thrice.
  4. "Message follows."; alternatively, if a message is not 30 characters long: "Message of <number> characters follows."
  5. The message in full.
  6. "I say again."
  7. The message in full.
  8. Optionally, if a message is not 30 characters long: "This complete message of <number> characters."
  9. "This is <callsign>, out."; alternatively, if there is another message to follow: "This is <callsign>, more to follow, stand by."
• Skyking (or Foxtrot): An uncommon broadcast type which consists of a codeword, the time, and an authentication code. The codewords are names of popular rock bands and movie titles. A typical Skyking is broadcast according to the following procedure:
  1. "Skyking, skyking, do not answer."
  2. The codeword, repeated twice.
  3. "Time" followed by the minutes of the current hour, as two digits, each spelled separately.
  4. "Authentication" followed by two letters.
  5. "I say again," followed by the steps 1 to 4 repeated.
  6. "This is <callsign>, out."

From time to time, the operator may inform stations to disregard the last message after broadcasting it.

EAM-Register (EAM-Watch) (Discord)

ALE

The ALE channels are used primarily for text messaging with the 2G ALE Automatic Message Display (AMD) feature, although radio checks using plain voice are common. Each active ground station emits an ALE sounding every 3 hours.

SIPRNet

The HFGCS provides an HF gateway into the SIPRNet, a U.S. Department of Defense computer network for classified information. Links are established using 2G ALE, while the data mode used is MIL-STD-188-110A.

References

1. List of ALE identifiers used by US military (Army, Navy, Marine Corps) on HF nets (September 30, 2019)