This week's planned launch of the next Global Positioning System satellite aboard a Boeing Delta 2 rocket has been delayed a few days so workers can replace a suspect pump.

A Delta 2 rocket will launch GPS 2R-12. Photo: Carleton Bailie/Boeing

"Boeing has a flightworthiness concern with the Delta 2 first stage hydraulic pump," Air Force launch officials said.

"A new pump, with improved inspection and testing procedures, will be installed on the Delta 2/GPS 2R-12 launch vehicle."

The rocket and its $45 million GPS 2R-12 payload are assembled atop pad 17B at Cape Canaveral Air Force Station, Florida. Liftoff had been targeted for 8:11 p.m. EDT Friday, June 4.

A new date has not been established for liftoff, but launch could occur on or about Wednesday, June 9. The day's launch window would extend from 7:51 to 8:18 p.m. EDT.

"A June 9 launch date assumes successful execution of a green light schedule," military officials said. "The Air Force considers the current launch date TBD."

The pump swap out was ordered after problems were uncovered during launch preparations for a different Delta 2 rocket.

"Boeing discovered this issue as a result of a pump failure that occurred during Delta 2 testing at Vandenberg Air Force Base, California," the Air Force said.

That Delta 2 rocket is undergoing its launch campaign at Vandenberg for the June 19 liftoff of NASA's $785 million Aura atmospheric observatory mission. Boeing says the Aura launch remains on schedule.

An artist's concept of a GPS Block 2R satellite orbiting Earth. Credit: Lockheed Martin

Once the GPS 2R-12 spacecraft is deployed in orbit, it will replace an ailing satellite in the U.S. military's global navigation constellation.

The newest craft is destined for Plane F, Slot 4, taking over the position held by the GPS 2A-16 satellite that has long surpassed its design life. It was launched on November 22, 1992.

Air Force program officials say GPS 2A-16 "is aging and clock performance is degrading."

Controllers will reposition GPS 2A-16 in the same orbital plane, making it a backup satellite for the remainder of its life, officials said.

The fresh GPS 2R-12 craft is expected to be operational by the end of June, depending on the new launch date selected.

GPS 2R-12 is also known as SVN-60; GPS 2A-16 is SVN-32.

The orbiting network currently has 28 working satellites that are relied upon by U.S. military forces to guide aircraft, ships, troops and precision bombs.

GPS satellites send continuous navigation signals that allow users around the world to find their position in latitude, longitude and altitude and determine time. The signals are so accurate that time can be figured to less than a millionth of a second, velocity to within a fraction of a mile per hour and location to within a few feet.

The mission patch for launch of GPS 2R-12. Photo: Boeing/Air Force

Launch of GPS 2R-12 originally was targeted for July 20. But it was moved up to early June to accommodate the postponement of NASA's Mercury orbiter, called MESSENGER, from May to late-July aboard a Delta 2 booster from the same launch pad.

"The Delta 2/GPS team works closely together to carefully plan future launches and identify any opportunities that will benefit the overall Delta 2 launch manifest," said Capt. Ivan Acosta of the 1st Space Launch Squadron at Cape Canaveral.

"It was all about synchronizing actions -- planning ahead and having the resources available, just in case -- and it paid off," added Lt. Col. Brad Broemmel, the Air Force launch director.

MESSENGER has narrow launch opportunities lasting a matter of days every few months to begin its lengthy trek to Mercury. Assembly of its Delta 2 launcher begins within days of the GPS 2R-12 liftoff from pad 17B.

This will be the second of three routine GPS replacement launches scheduled in 2004. The GPS 2R-11 craft was successfully lofted into orbit on March 20, and the next mission is slated for flight from Cape Canaveral on September 22.

The first "modernized" GPS 2R satellite featuring two new military signals and a second civil signal to improve capabilities for users will launch early next year.