A declassified picture shows the two DSCS satellites stacked together inside space shuttle Atlantis' cargo bay before deployment in 1985. Photo: U.S. Air Force/NASA

A new U.S. national security communications satellite is set for launch Thursday as the military shuffles it orbiting fleet and retires an aging craft deployed from a classified space shuttle mission 15 years ago.

A Lockheed Martin Atlas 2A rocket will carry the Defense Satellite Communications System B11 spacecraft in a $280 million mission scheduled to start with an evening blastoff from Cape Canaveral Air Force Station in Florida. Thursday's launch window extends from 7:36 to 8:55 p.m. EDT (2336-0055 GMT).

Known by its acronym DSCS, the constellation of five primary and five reserve satellites orbit 22,300 miles above Earth in a worldwide constellation to provide jam-resistant, high-data rate communications.

"DSCS is an important part of the communications infrastructure for the Department of Defense," said Lt. Col. Terry Peterson of the DSCS program office at Los Angeles Air Force Base in California.

From their orbital perch, the satellites beam secure and uninterrupted communications between military users, the White House, every U.S. embassy around the globe and the intelligence community, providing telephone, facsimile, video, e-mail and Internet connection.

Military communications on DSCS allow leaders at the Pentagon to talk directly to commanders in the battlefield, in the air or at sea.

The DSCS B11 satellite to be launched Thursday will undergo orbital maneuvering and substantial testing once its reaches space. Peterson says it should enter service in mid-March to cover the Eastern Atlantic region of the DSCS constellation.

A DSCS 3 satellite during pre-flight testing at manufacturer facilities. Photo: Lockheed Martin

The addition of DSCS B11 is part of an elaborate plan that will see one of today's primary satellites transition to the reserve fleet, making room for the new satellite in the main constellation. Once in the backup fleet, that middle-aged craft will replace the 15-year old B4 satellite, which will be boosted out of its operational orbit.

"The B11 satellite will meet our on-orbit requirements," Peterson said.

The soon-to-be retired DSCS B4 satellite was one of two launched in October 1985 on the maiden flight of space shuttle Atlantis. It was a hush-hush military mission of the shuttle in the days when Department of Defense payloads were kept classified. But it is now known that an Inertial Upper Stage carried the two craft from the shuttle to geostationary orbit and even pictures have been released showing the satellites in Atlantis' cargo bay (one seen above).

There have been 11 previous DSCS satellites launched in this current series over the past 18 years and one has been retired already. Initial launches were performed by Titan 34D rockets and the single space shuttle mission. The program later began using Atlas 2A rockets. Two more satellites are awaiting launches in May 2002 and May 2003, respectively, aboard Boeing Delta 4 rockets.

The final four craft, with Thursday's launch being the second, were built with improvements in the Service Life Enhancement Program to increase tactical communications capability by 200 percent, the Air Force says.

Spaceflight Now is your source for unrivaled live coverage of the launch beginning with a first-of-its-kind Webcast of countdown video and launch team communications starting at 4:15 p.m. EDT (2015 GMT) Thursday from Cape Canaveral.

Our Mission Status Center will also have continuous running updates to provide a play-by-play call of the countdown and 30-minute flight of the Atlas rocket.