Going for its fifth flight since December in an unprecedented tempo, United Launch Alliance workers have begun stacking an Atlas 5 rocket for blastoff May 15 carrying a vital satellite to replenish the Global Positioning System.

File image of first stage stacking. Credit: NASA-KSC

The GPS 2F-4 spacecraft will ascend into a constellation of satellites circling 11,000 nautical miles above Earth that provides precision navigation and timing to U.S. military forces and civilian users worldwide.

The May 15 liftoff is scheduled during a window stretching from 5:39 to 5:58 p.m. EDT (2139-2158 GMT).

It marks the first time Atlas and GPS have been paired together for a launch since 1985, when the last craft was carried aloft in the original batch of research and development satellites for the navigation system. Atlas boosters conducted 11 launches of the Block 1 series from February 1978 through October 1985 from Vandenberg Air Force Base in California.

Since then, the Delta family of rockets deployed and serviced the operational network of GPS satellites that grew from a military system into a global phenomenon that touches everyday life for millions.

Fast forward 28 years from that Vandenberg pad to Wednesday morning at Cape Canaveral where assembly started on the powerful Atlas 5 to launch the 64th GPS satellite, a modernized Block 2F bird to bolster the orbiting utility.

Putting the two-stage rocket together got underway this morning as ULA workers brought the bronze-colored first stage to the Vertical Integration Facility for stacking operations.

By mid-day, the booster was secured aboard its mobile launcher, anchored on small supports that protrude from the platform. At liftoff, explosive bolts free the rocket and those supports will retract into the platform walls as the vehicle powers its way off the pad on 860,000 pounds of thrust.

Known as the Common Core Booster, the stage was pulled by a semi-truck up the road from the Atlas Spaceflight Operations Center high bay to the 30-story VIF building where workers attached lifting cranes. The 106.6-foot-long stage was rotated vertical, then maneuvered into the building and stood upright on the mobile platform.

The stage is equipped with a dual-nozzle RD-180 main engine that will burn kerosene fuel and supercold liquid oxygen during the initial minutes of flight.

Upcoming will be installation of the interstage adapter that tappers the 12.5-foot diameter first stage to the 10-foot-wide Centaur upper stage.

The cryogenic upper stage will be hoisted atop the interstage Friday to complete the basic buildup of the Atlas 5.

Centaur's single RL10 engine, fueled by liquid hydrogen and liquid oxygen, will perform the necessary burns to achieve orbital velocity and then shape the orbit for deployment of GPS 2F-4.

The stage is 41.5 feet in length and also houses the navigation unit that serves as the rocket's guidance brain.

The rocket is known as the 401 configuration of the multi-varient Atlas 5 family, which is tailored with strap-on solid boosters and different sized nose cones to match the cargo's mass and size.

The spacecraft is undergoing processing at its own cleanroom facility for final testing, fueling and encapsulation in the rocket's nose cone. GPS 2F-4 will be brought to the Atlas assembly building and mated to the rocket in early May.

The flight-ready rocket will be rolled to the Complex 41 pad aboard the mobile platform on May 14. The launch day countdown begins a little after 10 a.m. EDT (1400 GMT).

Civil users around the globe rely on GPS for navigation assistance in transportation to the less obvious role in providing accurate timing stamps on banking transactions. The system developed to support U.S. military forces and their guided munitions.

The Air Force is in the midst of further advancing the GPS network by deploying this new Block 2F breed of satellite, produced by Boeing, that features improved accuracy, enhanced internal atomic clocks, better anti-jam resistance, a civil signal for commercial aviation, a longer design life and reprogrammable onboard processors to evolve with future needs.