The High Frequency Global Communications System (HF-GCS) is a specialized network of shortwave transmitting stations operated by the United States Air Force. It serves to communicate with aircraft in the air, ground stations, and certain surface resources of the United States Navy. The entire HF-GCS infrastructure is centrally managed from Andrews Air Force Base. Originally known as the Global High Frequency System, it was renamed before October 1, 2002. The HF-GCS stations primarily use frequencies in the range of 5, 8, and 11/12 MHz, although other frequencies are also utilized. The primary voice frequencies include 4724.0 kHz, 6739.0 kHz, 8992.0 kHz, 11175.0 kHz, 13200.0 kHz, and 15016.0 kHz. In addition to the HF-GCS, U.S. aircraft frequently use HF stations from the Military Affiliate Radio System as well as HF stations from the Canadian Armed Forces to transmit messages. A common application of the HF-GCS is to conduct phone calls from the air to inquire about local weather conditions, organize refueling, and relay information about passengers and crew back to the base.