This question in the title has popped up more frequently on various radio newsgroups now that EAMs transmitted on the HF-GCS are signed by using the call sign "Mainsail" instead of the Global station name as has been done in the past. This change was first noted back on July 26, 2013, by Ron Perron in Maryland.
So who or what is Mainsail? I will let the USAF answer that question directly in the article below from the Joint Base Andrews website.
Andrews radio operators assist crewmembers worldwide
by Margo Turner, 89th Airlift Wing Public Affairs
5/2/2006 - ANDREWS AIR FORCE BASE, Md. -- As an Air Force aircraft approaches Yokota Air Base, Japan, the pilot sets his radio to a specified frequency and says, "Main Sail, Main Sail" -- the call sign for any global radio station.
Within seconds, the pilot hears, "This is the Yokota operator." That operator is actually with the 789th Communications Squadron at the Centralized Network Control Station, or CNCS, here.
Many pilots and other aircrew members think they are talking with someone at the location they contacted on the radio, but in reality, the operator sits in front of a console in a building here at Andrews, said Capt. Kenneth Black, 789th CS operations flight commander.
The CNCS has 14 high-frequency ground stations located around the world, the captain said. The transmitter and receiver for each station, however, is controlled from the CNCS.
"We provide air-to-ground communications for command and control of aircraft as they fly around the world," Captain Black said.
The CNCS also provides phone-patch and message-relay services, ship-to-shore communications and emergency assistance for Department of Defense users; and high-frequency communications for the president, vice president, cabinet members and other senior officials while aboard special air mission aircraft. Phone patches are a connection between a telephone line and another communications device, such as a radio.
Additionally, Spanish-speaking radio operators handle radio requests from pilots and other aircrew members from Central and South American countries.
Each station in the CNCS has a console and a small speaker, which crackles with high-frequency static all the time, said Airman 1st Class Corina Arangure, 789th CS radio operator.
Radio operators spend 12-hour shifts monitoring designated stations, Airman Arangure said. They listen to military aircraft, ships and other DOD users requesting assistance, such as a phone patch or to have a message relayed to a ground location.
The radio operators have no way of knowing what to expect when they hear someone using the "Main Sail" call sign, Captain Black said.
A person calling in may want to talk to the command post at the base where his or her aircraft is heading, he said. The caller may not know the phone number of the command post or doesn't have a phone on board the aircraft. The CNCS radio operator can call the command post and then connect the aircraft operator to a person at the command post, he said.
Another CNCS radio operator may receive a phone call from someone trying to contact an aircraft, he said.
"A lot of aircraft don't have telephones," Captain Black said. "We're given the call sign the aircrew uses and (we) broadcast it over the high-frequency global communications system."
People call in seeking assistance for various reasons, such as engine trouble or an onboard emergency, said Staff Sgt. Latoya Edwards, 789th CS radio operator.
The 789th CS operations flight has 89 radio operators, who are all enlisted Airmen, said Senior Master Sgt. Jeff Haynes, 789th CS operations flight chief. Airmen manned the 14 high-frequency stations until six years ago, when the Air Force upgraded its equipment and had the transmitter and receiver for each station remotely controlled from the CNCS here.
Computer software is being upgraded in the CNCS, which will allow operators to sit at a console, log in and monitor any of the other consoles, Captain Black said.
The software upgrade will also enable the Air Force to have another CNCS at Offutt Air Force Base, Neb., which will serve such as backup to the Andrews unit, Sergeant Haynes said.
Captain Black said the radio operators at Andrews provide a valuable service to servicemembers, national leaders and the Department of Defense.
"They maintain a listening watch to ensure global mission success," he said.