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.
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