The Global HF System (GHFS) was stated by the US Air Force in June of 1992. This system was created by consolidating several other USAF HF networks, including the Strategic Air Command "Giant Talk" system. The goal of the system was to develope a worldwide HF network for providing Command & Control HF communictions to all authorized DOD aircraft and ground stations.
When you hear an aircraft or any stations calling "MAINSAIL" on any of the GHFS frequencies this is a "general" call for any ground station to answer and provided the calling stations with what ever he needs. Any GHFS ground station will respond to the call "MAINSAIL". Depending on the service required, the station bay request the aircraft to change to a discrete frequency for improved and extended service.
EAM's or Emergency Action Message. Another very important aspect of the GHFS is the transmission of EAM's. EAM's are the coded military orders or bulletins that have the highest priority. When EAM's are broadcast, ALL other users of the system are to standby. If you hear a voice giving a long series of letters phonically this is an EAM.
SKYKING messages are special EAM's. These presumably could be the nuclear go/no-go codes. There was a higher than normal number of EAM's and SKYKING messages being transmitted after the attackes of 9/11/01. Urgent SKYKING messages are repeated three or four times instead of the usual two. SKYKING is a group callsign, meaning "Any this net." "Do not answer" is the standard procedure ofr self-authenticating broadcasts, meaning that recipients don't need to give away their positions or existence by challenging the orders. While tuned to any of the GHFS frequncies and hear "SKYKING SKYKING Do not answer" you can bet you're hearing a very important, highest priority, and heavily encrypted message!