Assembly of United Launch Alliance's next Atlas 5 rocket is underway in the towering integration facility at Cape Canaveral's Complex 41 pad to deploy an ultra-secure U.S. communications satellite in September.

File image of last Atlas 5 with AEHF satellite. Credit: Pat Corkery/United Launch Alliance

The rocket is taking the shape of the 531 configuration in the Atlas 5 family, which will feature a five-meter-diameter nose cone, three strap-on solid fuel boosters and a single-engine Centaur upper stage.

That power will lift the Air Force's Advanced Extremely High Frequency satellite No. 3 into a supersynchronous transfer orbit from the Florida spaceport.

Liftoff is scheduled for Sept. 25 at 2:36 a.m. EDT (0636 GMT). However, officials are looking at the possibility to move up the launch by few days.

The AEHF satellite series, built by Lockheed Martin, is a growing constellation in space that will ring the globe to link civilan leadership with military forces anywhere on the planet.

"We depend on this satellite system in existential circumstances for the United States. When communications from the President and the National Command Authority has to get through to our forces to execute options and in circumstances that are just the worst imaginable, this is the system we depend on," said Gen. William Shelton, commander of Air Force Space Command.

An artist's concept of the AEHF satellite in space. Credit: U.S. Air Force

ULA Atlas 5 rockets successfully launched AEHF 1 on Aug. 14, 2010 and AEHF 2 on May 4, 2012. A constellation of at least six satellites in the series is planned.

Considered among the Defense Department's most critical spacecraft, the AEHF satellites will ensure a survivable line of contact between the president, military commanders and troops on the battlefield even in nightmarish scenarios of nuclear war.

"This is the satellite that provides the president the capability to communicate with deployed forces when there's a nuclear environment, either nuclear attack in progress or post-nuclear attack when the atmosphere is simulated from the nuclear effects. It is the satellite, again, that just has to be there," Shelton said.

The AEHF constellation is the next-generation replacement to the aging MILSTAR line, offering faster data speeds and expanded capacity for secure communications across the world.

Assembly of the launch vehicle began on Friday, Aug. 2 when the first stage was lifted aboard the mobile platform at the Vertical Integration Facility.

The three solid rocket boosters were attached last week and the Centaur upper stage was hoisted into place today.

File image of Centaur lift for mating to Atlas 5. Credit: NASA-KSC

AEHF 3, which arrived in Florida on July 10 from Lockheed Martin's satellite facility in Sunnyvale, Calif., is proceeding through its own processing at a separate cleanroom facility.

The craft is undergoing final testing, the loading of maneuvering propellants and encapsulation in the two halves of the rocket's nose cone. The payload will be moved to the Atlas assembly building and mated to the launcher to complete the 196-foot-tall rocket for flight.

It will be the sixth Atlas of the year, the 40th overall since 2002, the 15th in service to the Defense Department and United Launch Alliance's 75th flight since its formation in 2006.