The High Frequency Global Communications System (HFGCS) is a network of single sideband shortwave transmitters of the United States Air Force which is used to communicate with aircraft in flight, ground stations and some United States Navy surface assets.
HFGCS stations tend to operate in the aviation bands clustered around 5, 6, 8 and 11/12 MHz, although other frequencies are in use.
In addition to the HFGCS, U.S. aircraft frequently use Military Auxiliary Radio System (MARS) HF stations (13927.0 kHz) and Canadian Forces HF stations (11232.0 kHz) to relay messages.
One common use for the HFGCS is to place telephone calls from the aircraft in flight by means of the Defense Switched Network (DSN) to an U.S. Air Force base, U.S.
Naval Air Station, U.S. Marine Corps Air Station, U.S. Army Airfield, or Air Force Reserve or Air National Guard installations on civilian airports, or Army Reserve or Army National Guard Aviation Support Facilities on civilian airports, to obtain local weather conditions, to arrange for refueling, and to inform the base of the number of passengers and crew.