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CAPE CANAVERAL — Beginning its fourth launch campaign of the year, the Atlas 5 rocket program has started stacking the vehicle that will boost a satellite 11,000 miles high for the Global Positioning System.

United Launch Alliance crews commenced the assembly work today at Cape Canaveral’s Vertical Integration Facility for the upcoming mission carrying the Air Force’s GPS 2F-10 navigation spacecraft.

Liftoff is targeted for July 15 during a morning window of 11:36 to 11:55 a.m. EDT (1536-1555 GMT).

The 189-foot-tall rocket will be flying in the basic, 401-variant that features an RD-180 main engine powering the first stage and an RL10 on the Centaur upper stage. The satellite will be enclosed in a 39-foot-long, 14-foot-diameter aluminum nose cone for atmospheric ascent.

The GPS 2F-10 is destined for the constellation 11,000 nautical miles high, inclined 55 degrees to either side of the equator. The network serves the U.S. military and civilian users around the globe for precision navigation and timing services.

This new satellite will take the place of the 11-year-old GPS 2R-11 craft launched aboard Delta 303 in March 2004 into Plane C, Slot 3 of the constellation. It moves to a backup role in the network once GPS 2F-10 enters service about a month after launch.

To make numerical room for the new addition to the constellation, the GPS 2A-26 launched aboard Delta 237 in July 1996 will be decommissioned following the GPS 2F-10 launch.

It will be the second GPS launch this year, following a Delta 4 deployment flight successfully carried out March 25. One more in 2015 is planned by Atlas in October.

This year has seen three Atlas 5 rockets fly so far, launching its heaviest payload ever — the Navy’s MUOS 3 communications satellite — in January, NASA’s MMS magnetospheric science experiment in March, valued at $1.1 billion, and the Air Force’s X-37B spaceplane in May on another secretive voyage.

(See our Atlas archive.)

The rocket has six more missions on the manifest for 2015, including launches of the GPS 2F-10 satellite, the MUOS 4 spacecraft, a classified NRO payload from Vandenberg, Mexico’s Morelos 3 communications satellite, GPS 2F-11 and Orbital ATK’s Cygnus cargo-delivery craft for the International Space Station.

The stacking work for AV-055 began this morning as the bronze-colored, 106.6-foot-long, 12.5-foot-diameter first stage was pulled to the VIF doorway, erected upright and hoisted inside the building to be put aboard the mobile launcher platform.

Known as the Common Core Booster, the stage produces 860,000 pounds of thrust to lift the rocket off the ground. It burns kerosene fuel and supercold liquid oxygen during the initial minutes of flight.

In the coming days, the barrel-like interstage adapter will be added and then the Centaur upper stage will be lifted in place. Centaur is 41.5 feet in length, 10 feet in diameter and is fueled by liquid hydrogen and liquid oxygen.

The payload is undergoing its own processing at a nearby Air Force facility. It will be delivered to the VIF and mated to the Atlas-Centaur in early July.

GPS 2F-10, a 3,400-pound, modernized navigation satellite made by Boeing, features improved accuracy, enhanced internal atomic clocks, better anti-jam resistance and a civil signal for commercial aviation. It is the 10th of 12 Block 2F satellites to launch.

The launch will be the 55th Atlas 5 rocket since 2002 and the 21st for the Air Force since 2007. It also marks the 70th GPS satellite launch since 1978, the 15th to use an Atlas rocket and the fourth Block 2F on Atlas. For United Launch Alliance, it is the company’s 97th launch overall since 2006 and the sixth this year.