A replacement satellite ascends into space this weekend to strengthen the U.S. military's Global Positioning System -- a constellation of orbiting spacecraft that guides planes, ships, troops and precision weaponry.

A Delta 2 rocket will launch GPS 2R-10. Photo: Carleton Bailie/Boeing SEE GALLERY OF LAST GPS LAUNCH

The NAVSTAR GPS 2R-10 satellite is scheduled for liftoff at 2:50 a.m. EST (0750 GMT) Sunday atop a Boeing Delta 2 rocket from pad 17A at Cape Canaveral Air Force Station, Florida.

The day's available launch window extends a quarter-hour to 3:05 a.m. EST (0805 GMT).

The three-stage Delta 7925-configuration launcher will deploy the satellite into a preliminary orbit around Earth with a high point of 11,000 nautical miles, low point of 100 miles and inclination of 39.0 degrees.

The Lockheed Martin-built satellite will circularize its orbit and raise the inclination to 55 degrees to join the GPS constellation in the days following liftoff.

The $45 million craft will fill the Plane E, Slot 2 position in the GPS network. After on-orbit testing, GPS 2R-10, also known as SVN-47, is expected to enter service in the first-half of January.

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

The new satellite replaces the GPS 2A-10 craft launched on November 26, 1990. Final plans for the aged satellite have not been determined, the Air Force says. "Projected reliability" is the reason it is being replaced, according to the GPS program office.

The GPS network features 24 primary satellites split into six orbital planes with four spacecraft in each. Some planes also have additional satellites to enhance coverage and serve as backups. Today, the orbiting system has 28 craft that are operating, the Air Force says.

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.

Sunday's launch will be the 10th of 21 planned in the Lockheed Martin-built GPS 2R series. Four more are slated in 2004 to continue replacing aging satellites in the network.