HFGCS on 11175 kHz: the US Air Force global HF network

What you hear

Most of the time, nothing; then an American voice, clear and unhurried: "Mainsail, Mainsail, this is Reach three one five, radio check." A ground station answers with its own name, Andrews, Offutt, Croughton, Lajes, and the exchange is over in ten seconds. Then, at any hour, the thing the frequency is famous for: "All stations, all stations, this is Andrews. Stand by for a message." A pause, and a string of letters and numbers read in the phonetic alphabet, "Foxtrot Six Alpha Tango Kilo Romeo Whiskey", twenty or thirty characters, sometimes far more, read twice, with the first few characters repeated at the end as the message's name, and then "Andrews out". That is an Emergency Action Message, an EAM. Its content is encrypted: what you hear is the code, not the meaning.

The rest of the traffic: "Skyking" broadcasts, which begin "Skyking, Skyking, do not answer" and carry a short code word, a time and an authentication; phone patches, in which a ground station connects an aircraft's radio to a telephone line and you hear one side of a conversation about fuel, parking or a change of destination.

Who transmits and why

The High Frequency Global Communications System is the US Air Force's worldwide HF network: a ring of ground stations on US soil and at bases abroad, Andrews near Washington, Offutt in Nebraska, Croughton in England, Lajes in the Azores, Ascension Island, Diego Garcia, Sigonella in Sicily, Guam, Hawaii, Alaska and Japan among them, keyed from a central point and transmitting the same traffic on the same frequencies at once. Any US military aircraft anywhere in the world can call "Mainsail", the collective callsign, and whichever station hears it best answers. The system carries routine traffic for transport and tanker crews and, in the EAMs, the command traffic of the nuclear forces, which is why it never goes off the air and why the messages are read on every frequency at the same time.

The primary frequencies are 4724, 6739, 8992, 11175, 13200 and 15016 kHz, all upper sideband; 11175 kHz is the one most listeners sit on: it works for most of the day over most of the world.

Listening to it is not forbidden. The network transmits in the clear, for anyone with a receiver, and ShackDeck only receives; in most countries receiving an open transmission is legal, though some places limit what you may do with its content, such as retransmitting it. This is how things generally stand, not legal advice, and the rules where you live are yours to know.

When and where to listen

11175 kHz is a daytime and evening frequency: in Europe it is open from mid-morning until a few hours after sunset, and the ground stations in England and the Azores are heard loudly across the continent, with the American stations arriving by skywave in the afternoon and evening. Aircraft are heard too, usually more weakly. After dark in Europe the lower frequencies take over: 8992 kHz in the evening and 6739 kHz through the night, each with its own row in the catalogue.

Because the EAMs come at any time, the only method is patience: leave the receiver on in the background and turn it up when the preamble starts.

In ShackDeck

Station catalogue → Military → HFGCS 11175. The program picks a public receiver that covers 11175 kHz, tunes it and sets USB; you need no receiver, antenna or account of your own. If the frequency is dead at your hour, open the 8992 or 6739 kHz rows.