One of those networks is associated with High Frequency (HF) radio communications. Here's a summary of that network, pasted below from here.
United States Air Force High Frequency Global Communications System (HFGCS)
(Updated 2014) HFGCS History HFGCS used to be GHFS. GHFS (Global High Frequency System) began on 1 June 1992, when it was created out of two earlier HF networks, the GIANT TALK used by the Strategic Air Command, and the Global Command and Control System (GCCS), used by the rest of the Air Force. This change reflected the reorganization of the Air Force following the end of the cold war, with SAC becoming part of United States Strategic Command (STRATCOM), TAC becoming the Air Combat Command (ACC), and MAC becoming the Air Mobility Command (AMC; call sign "Reach"). GHFS became HF-GCS in 2003, on completion of the multiyear equipment upgrade called SCOPE COMMAND, and nearly all control was moved to Andrews Air Force Base in Maryland. Current HFGCS stations consist of a receiver site, a transmitter site, and a control system. These are extremely configurable and flexible, and they can be operated remotely, locally, or any combinations of both. Emission can be USB voice, Automatic Link Establishment (ALE), or data modem. Most traffic on the main channels is voice, though a large number of other channels exist for ALE autoscan. At some point HF-GCS lost its hyphen, and now it's just HFGCS. The station list, schedules, and procedures used to appear in an unclassified flight manual, but this document has since become secret. More recently, in 2013, the ground stations for the most part have stopped identifying broadcasts with their locations. This reflects the full "lights out" automation of the system, and its extreme flexibility. Usually, the identity of the specific transmitter is no longer particularly relevant. Currently, broadcasts are identified with "MAINSAIL." This is the long-time group call sign for all ground stations. Similarly, mobiles calling MAINSAIL can be regarded as a CQ to any ground station. Other HFGCS Facts At one time, HFGCS was listed as providing the following services: Unclassified phone patches Message relay Mission following EAM broadcasts Data modem services Emergency assistance MIJI reports (Meaconing, Intrusion, Jamming, Interference) RTTY has not been supported for several years now. Phone patch traffic is down on this net, though a lot of it has moved to MARS channel 13927 kHz USB. An ALE autodialer used to be provided on 9025 kHz. Its status is unknown. Remaining HFGCS Patches or long conversations can still be moved to discrete frequencies, to free up the net. Most traffic is EAM broadcasts. This stands for Emergency Action Message, though very few are really national emergencies. EAMs have the distinctive preamble of their first six characters and "Stand By." The latter prosign reflects the high traffic precedence of these coded military orders or bulletins, which exceeds most other non-override traffic except for in-flight emergencies. Other users are expected to stand by when one is in progress. This also explains why EAMs don't listen first. They just start up. SKYKING messages are special EAM, Emergency Action Messages, presumably the nuclear go/no-go codes. Urgent ones are repeated three or four times, instead of the usual two. SKYKING is a group callsign for units in the Single Integrated Operation Plan (SIOP) for strategic operations." "Do not answer" is the standard procedure for self-authenticating broadcasts, meaning that recipients don't need to give away their positions or existence by challenging the traffic. The distinctive touch-tone beeps (DTMF tones) are still on occasion heard at the end of transmissions. These are left over from Giant Talk, and they may eventually be replaced by another system. These beeps key and unkey remote transmitters. The echoes heard are both propagation delays and/or the result of slow landlines or satellites linking the distant sites. Usually, it's the second one, because the delays are too long for even the longest terrestrial paths. One Giant Talk holdover is 6761 kHz USB, the old SAC night primary. It is now used as an air-to-air channel. Though it's not technically part of HFGCS, it is still used for coordinating in-flight refueling. The status of 10780 kHz, the onetime backup at Cape Canaveral Air Force Station, is unknown. There are reports that this radio asset has been moved or dismantled. From the start, HFGCS was intended for multi-service use. It replaced US Navy's Fleet HICOM (High Command), which phased out and is now dead, though its frequencies are occasionally used for other purposes. One also hears US Army vessels (yes, the Army has a "navy"). HFGCS services are also available to any allies or other units authorized by the US Department of Defense (DoD). Finally, this leaves 6761, the former SAC night primary, as an air-to-air channel. Though it's not technically part of HFGCS, it is still used for coordinating in-flight refueling. At one time, the full information on HFGCS was available to the public in the DoD FLIP (Flight Information Publications). These publications have since been classified as sensitive, and at present they are available in digital files to authorized users only. Therefore, anything you see on the Internet is probably dated. The full schedule has been removed from this web site, in order to prevent error propagation. HFGCS Lexicon: A--- US Army callsign, usually a troop boat or terminal AR Aerial Refueling, usually with a track number AUTODIN Automated Digital Network Block time Final arrival time, when plane is in the blocks Class A, B Refers to explosive on board CNCS Centralized Network Control Station (Andrews) DSN Defense Switched Network, old "Autovon" DV Distinguished Visitor FL Flight level (altitude when higher up) Hazmat Hazardous cargo IFE Inflight emergency, as declared by pilot IR Instrument Route; aerial training routes M&W Morale and Welfare (patch) MAINSAIL General call: Any ground station this net METRO Base weather office NIPRNET Non-Secret Internet Protocol Routing Network Offload Fuel, in pounds Pallets Cargo Pax Passengers PIREP Pilot Report; weather observation with standard items REACH Standing callsign: any AMC flight. Followed by tail or mission number, or a special code. RON Remain Over Night (crew needs place to sleep) RTB Return To Base, mission aborted SAM Special Air Mission (VIP) SCOPE System Capable Of Planned Expansion SIPRNET Secure Internet Protocol Routing Network SKYBIRD Group callsign: all STRATCOM assets on net SKYKING Group callsign: all SIOP assets on net ALL FOLLOWING INFORMATION REFLECTS THE LAST PUBLIC PUBLICATION. NO CLASSIFIED MATERIAL WAS DIVULGED IN PREPARING THIS DOCUMENT! PRIMARY FREQUENCIES (kHz USB) 4724.0 6739.0 8992.0 11175.0 13200.0 15016.0 8992 and 11175 are 24-hour. Lower ones are used for traffic at night, higher ones in the day time. EAM can be simulcast on any of these. USAF HFGCS STATION LIST (Some may have closed) Andersen Air Base Guam (ALE: GUA) Andrews Air Force Base MD (ALE: ADW) Ascension Island AF S Atlantic (ALE: HAW) Croughton AB UK (ALE: CRO) Diego Garcia NS Indian Ocean (ALE: JDG) Elmendorf AFB AK (ALE: AED) Falklands? S. Atlantic Ocean (ALE: MPA) Hickam AFB HI (ALE: HIK) Keflavik NAS Iceland (ALE: IKF) Lajes AB Azores (ALE: PLA) McClellan AF CA (ALE: MCC) Offutt AFB NE (ALE: OFF) Salinas PR (ALE: JNR; Voice: "Puerto Rico") Sigonella NAS Sicily (ALE: ICZ) Yokota AB Japan (ALE: JTY) McClellan was known as "West Coast" for a time, but right before everything went to MAINSAIL, it had gone back to the McClellan name. The old base was closed under BRAC, but some air ops remain at the site. The SCOPE COMMAND radios are a separate activity anyway.