The 20-foot(6-metre)tall Atlas 5 rocket, the most powerful launcher in the U.S. inventory, lifted off at 5:15 p.m. EST (2215 GMT), darting through sunny skies as it soared over the Atlantic Ocean to deliver the Navy's first Mobile User Objective System satellite into orbit.

The spacecraft, called MUOS-1, is part of a planned five-satellite network that will operate like cellphone towers in space to significantly boost communications capacity for the U.S. military and partner countries, including, for the first time, UHF satellite communications for mobile users.

"Right now, our units have to be sitting in one place, stationary, with antenna up pointing straight at a satellite to be able to use narrow-band satcom (satellite communications)," program manager Paul Ghyzel told reporters before the launch.

"As we bring MUOS online, we bring up the capability that allows them to move around the battle space and be able to continue communicating beyond line-of-sight (of the satellite)," he said.

The system is "basically like a cellphone tower in the sky," added Mark Pasquale, MUOS program manager and vice president with satellite builder Lockheed Martin.

"You can think of the handheld user terminals as robust, rugged smartphones," he said.

MUOS voice, video and data services will be available to users whether they are in vehicles, ships, submarines, aircraft or simply on foot, though that cell phone-like capability probably won't come until next summer.

General Dynamics C4 Systems, the prime contractor for the handheld user terminals, is still working on the technology needed to communicate with MUOS' 3G-like system. Until then, users can tap the satellite's heritage narrow-band communications system, which is similar to what is currently aboard the Navy's aging UFH Follow-on satellites.

Overall, MUOS is designed to provide military users 10 times more communications capability over existing systems, including simultaneous voice, video and data.

Weighing nearly 15,000 pounds (6,804 kg), MUOS-1 is the heaviest satellite launched to date by an Atlas rocket, which is built and flown by United Launch Alliance, a partnership of Boeing and Lockheed Martin.

The MUOS network, including launch vehicles, will cost the U.S. military more than $5.3 billion.

(Editing by Kevin Gray)

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