They are the U.S. military's heaviest spacecraft looking for rides to orbit -- the Navy's next-generation satellites providing communications to forces on the move -- and the United Launch Alliance's mightiest version of the Atlas 5 will do the heavy lifting Friday morning to deploy the second bird in the series.

Atlas was rolled to the pad Wednesday. Credit: Walter Scriptunas / Spaceflight Now

Liftoff of the Mobile User Objective System satellite No. 2 is scheduled for 8:48 a.m. EDT (1248 GMT) from Cape Canaveral on Florida's east-central coast.

"At nearly 15,000 pounds, MUOS 2 marks the heaviest satellite launched to date by an Atlas 5," said Jim Sponnick, ULA's vice president of Atlas and Delta programs.

The 206-foot-tall rocket, riding aboard its mobile launch platform, was rolled out from the assembly building to the Complex 41 pad Wednesday morning.

At the same time, other ULA workers just down the road at Complex 37 were conducting a countdown dress rehearsal and fueling test on a Delta 4 rocket for its launch of the Air Force's sixth Wideband Global SATCOM satellite on August 7.

Atlas clocks begin ticking early Friday for the seven-hour countdown. Weather officials will be monitoring isolated showers and thunderstorms coming ashore from the Atlantic, a forecast that gives a 40 percent chance of acceptable conditions during the 44-minute launch window.

With five powerful solid rocket motors mounted to the first stage, the Atlas 5 will thunder skyward in its most energetic configuration, which has been employed only three times before to launch the first MUOS satellite and hurl NASA space probes to Jupiter and Pluto.

The Atlas 5, making its 39th flight in 11 years, is a modular launcher that enables mission designers to mix and match the number of solids and various nose cone selections to meet the needs of a given payload. The highly sophisticated and hefty MUOS series takes all the power.

The spacecraft is packaged atop the rocket for a three-hour climb into its preliminary orbit, a highly elliptical dropoff point that requires three firings by the Centaur upper stage to achieve.

From there, controllers will spend about eight days maneuvering the craft into a circular geosynchronous orbit 22,300 miles above the planet, then three days commanding the craft to spread its power-generating wings and unfurl two giant antennas on the ends of extension booms.

"With the launch of the second MUOS spacecraft, we take the next large step toward providing truly global narrowband communications for our deployed warfighters around the world," said Navy Capt. Paul Ghyzel, manager of the Satellite Communications Program Office.

Atlas was rolled to the pad Wednesday. Credit: Walter Scriptunas / Spaceflight Now

At the heart of the MUOS satellite's communications payload are two gold mesh, umbrella-like antenna reflectors, one to provide the same type of UHF communications like previous Navy spacecraft and the other offering modernized capabilities that the new system will create.

"MUOS will deliver unparalleled high-speed communications access to mobile users for decades," said Iris Bombelyn, vice president of Narrowband Communications at satellite-builder Lockheed Martin.

"With over 20,000 terminals that are capable of connecting to this secure mobile (satellite communications) system, we are at the cusp of our potential. The MUOS satellite provides not only ongoing support for the legacy UHF Follow-On users in the field but a myriad of new services, bandwidth, capacity and applications to mobile users."

Coverage to legacy users will transmit through a 17.7-foot-diameter reflector on the bottom of the MUOS craft and the advanced, multi-beam features of MUOS to significantly increase the transmission capacity over the Navy's previous satellites will use a large 46-foot reflector atop MUOS.

All U.S. military forces rely upon Navy satellites for Ultra High Frequency narrowband communications. UHF offers small, portable units that forces can carry into battle and the frequency enables communications in urban canyons and mountainous terrain, penetrating foliage and transmitting through bad weather.

Each MUOS satellite has 16 times the capacity of the aging UHF satellite constellation.

What's more, the new satellites not only support the current user terminals in operation but also creates a new "rugged smartphone" network to provide 3G-like cellular telephone and data services across the globe.

An artist's concept of MUOS. Credit: Lockheed Martin

"One of the way we frequently describe the new capabilities that MUOS brings is think of a cellphone," Ghyzel said.

"The architecture that we've built with the satellite constellation and with the global ground network, the satellite is the celltower. Anybody that is using a radio that is capable of communicating with MUOS, when they speak their transmission is picked up by the satellite and then routed like a cellular system would route to wherever it needs to be to talk to the guy on the other end.

"So if you are driving down the interstate and you walk to talk to a guy one county over, you may be using the same tower. For Bob to talk to Jim.

"But if Bob is in Florida and wants to talk to his wife in Seattle, he can pick up a cellphone, the tower next to the interstate he is driving on is going to pick up that call, but then it is going to go through a fiber optic network to get to a celltower that is closest to his wife in Seattle and that tower is going to send that call to her cellphone.

"Much like for us in MUOS, if you got somebody that's in Hawaii that needs to talk to a ship that's 200 miles off Hawaii, that traffic is going to go through the satellite that is over the Pacific.

"But if that ship commander needs to talk to somebody that is in Afghanistan, then they are going to transmit over MUOS, the satellite over the Pacific is going to up that transmission, but (it is) then routed through the rest of the MUOS network to the satellite that's going to be over the Indian Ocean, eventually, and then down into Afghanistan.

"You can think of the satellites as the celltowers in the sky. That's a really good way to think of how the system works."

Atlas is set for launch Friday at 8:48 a.m. EDT. Credit: Walter Scriptunas / Spaceflight Now

The MUOS craft were built on Lockheed Martin's A2100 satellite design used by dozens of previous communications spacecraft. All five MUOS craft are clones to each other, with four slated to be operational satellites in the constellation and one considered an on-orbit spare.

MUOS 1 was successfully launched on Feb. 24, 2012. MUOS 3 is eyeing a launch in about 12 months, followed by MUOS 4 in the summer of 2015 and MUOS 5 some time after that.

The Atlas rocket has been the expendable launch vehicle of choice for the Navy's narrowband UHF communications satellites dating back several spacecraft generations to the 1970s, exclusively carrying the entire Fleet Satellite Communications System, Ultra High Frequency Follow On series and now MUOS. A variety of Atlas-Centaur versions have been employed on the previous 20 flights since 1978, all from Cape Canaveral.