An artist's concept of a Lockheed Martin Space Systems-built GPS Block 2R satellite orbiting Earth like the one to be launched this week. Photo: Lockheed Martin

A new Global Positioning System military navigation satellite will be launched into space Friday night to replace the very first spacecraft deployed in the U.S. Air Force constellation 11 years ago.

A Boeing Delta 2 rocket is slated for liftoff at 11:05 p.m. EDT (0305 GMT Saturday) from pad 17A at Cape Canaveral Air Force Station in Florida. A 30-minute launch window will extend to 11:35 p.m. EDT.

Follow the countdown and launch in our Mission Status Center.

Air Force officials quickly changed the seemingly routine launch of another NAVSTAR GPS satellite into a full-fledged replacement mission after the GPS 2-1 craft reached the end of its life on March 26. It was decommissioned on April 14, well exceeding its seven-year design life.

The veteran satellite served as the cornerstone of the Air Force's highly successful GPS constellation of 24 spacecraft that circle 10,900 miles above Earth, providing precision location, speed and timing information to guide U.S. military troops, aircraft, submarines, ships, weapons and civilians around the world.

Lofted on February 14, 1989, GPS 2-1 was built by Rockwell International. Since then, two advanced generations of GPS satellites have been developed and flown. Known as Blocks 2A and 2R, these satellite models have allowed the military to enhance the capabilities of GPS.

Sitting atop the $50 million Delta 2 for this week's launch is the fourth GPS Block 2R satellite manufactured by Lockheed Martin Space Systems, valued at $42 million. Block 2R craft, the latest generation, were built with improvements over early satellites including the ability to operate longer when out of contact with ground controllers, better accuracy by communicating with other GPS satellites, larger fuel supplies and additional radiation protection.

Officials were originally aiming the new satellite for another part of the GPS constellation. But when the GPS 2-1 spacecraft showed signs it was about to stop working, the launch was replanned.

For those of you keeping score, GPS 2-1 is officially known in Air Force jargon as Space Vehicle No. 14 and the new GPS 2R-4 is SV No. 51.

The loss of one satellite, however, has not threatened the GPS network, said Capt. Gerald Anderson, chief of launch operations for the Air Force's GPS program.

"Right now the constellation is healthy," he said in an interview this week.

The GPS 2R-4 satellite is attached to the Delta 2 rocket's third stage in a processing facility at Cape Canaveral earlier this month. Photo: Lockheed Martin/Russ Underwood

The GPS system is separated into six orbital planes, all of which still have the minimum of four satellites needed to operate. In fact, three of the planes currently have an addition satellite, giving the constellation a total of 27 working spacecraft today. The Air Force had launched "replenishment" satellites over the past couple of years after the primary 24-spacecraft constellation was finished.

Friday's launch will insert the GPS 2R-4 satellite into the so-called E1 slot of the constellation, previously occupied by GPS 2-1.

"This launch will directly replace a satellite that has reached the end of its life," Anderson said.

After the new satellite arrives in space, it will be maneuvered into the GPS network and tested. The Air Force expects the craft to enter service on May 15, said Maj. David Keller, chief of aquisition operations for the GPS Joint Program Office.

Users of GPS rely on handheld receivers that pick up signals from four satellites passing overhead at any given time. The receiver measures the time delay for the signal to reach itself from the satellite, which ultimately shows the distance to the orbiting satellite.

Military users can then determine their exact location to within feet, speed within a fraction of a mile per hour and time to within a millionth of a second whether it be on the ground, in the air or at sea. GPS was used extensively in the featureless desert battlefield of the Gulf War.

Civilian receivers have less accuracy but are popular nonetheless for a wide-range of users like hikers, boaters, pilots and mapmakers.

Preparations on track for liftoff
Boeing and Air Force officials completed the Flight Readiness Review on Tuesday and gave technicians approval to fuel the Delta 2 rocket's second stage with storable nitrogen tetroxide and Aerozine 50 fuels on Wednesday.

The satellite was transported to the launch pad last week and the protective nose cone of the rocket was installed over the weekend.

Like the previous 31 GPS launches, Boeing will use its three-stage Delta 2 rocket featuring nine strap-on solid rocket motors and a 9.5-foot diameter payload fairing.

The early weather forecast is calling for favorable conditions Friday evening with an 80 percent chance of acceptable conditions. The main threat will come in the form of thunderstorm anvil clouds that might develop near a weather front expected to approach the Cape late Friday.

At liftoff time, Air Force Launch Weather Officer Joel Tumbiolo predicts a few clouds at 3,000 feet and scattered clouds at 25,000 feet, visibility of 10 miles or better, west-southwesterly winds 12 gusting to 20 knots, a temperature of 74 to 76 degrees F and relative humidity of 50 percent.

If the launch is delayed until Saturday night for some reason, there is a 90 percent chance of acceptable conditions with the only concern being gusty ground winds.

This will be Boeing's third Delta 2 rocket launch of 2000 and the second to occur from the Cape.