"The booster and the spacecraft are fully flight worthy and cleared for launch," Lt. Col. Blaise Kordell, commander of the 1st Space Launch Squadron that launches Air Force Delta 2 rockets, told SPACE.com on Thursday.

Even the sky looks like it will cooperate.

"When we come out of the final hold you can sort of feel your chest tighten up. I never consider it routine because I'm always holding my breath."

Air Force weather forecaster Joel Tumbiolo reports there is an 80 percent chance of acceptable conditions during the 30-minute launch window. His main concern is the possibility that electrically charged clouds blown toward the Cape from an approaching thunderstorm might prevent the liftoff.

If the Boeing-built Delta 2 were to fly through such clouds there is a strong likelihood the rocket would trigger a bolt of lightning that could seriously damage or destroy the mission.

Friday's launch will mark the 31st time a Delta 2 rocket lifts off carrying a GPS satellite. The very first Delta 2 rocket lofted the first Navstar GPS 2 satellite into orbit February 14, 1989. That original 11-year-old satellite failed in March and was retired one week ago, setting the stage for Friday night's mission.

The fact that the aging spacecraft was showing signs of failing made it relatively easy to decide which orbital slot the Air Force should send the already scheduled replacement bird.

"This particular date was pre-planned ahead of time but, depending on the needs of the constellation, we can re-target the rocket by changing the time of day that we launch," said Will Hampton, Boeing's director of Air Force launch programs for the Delta 2.

Hampton said that despite the fact this will be the 31st time a Delta 2 has launched with a Navstar GPS on board, the Boeing and Air Force launch control teams do not consider this "just another routine rocket launch." They are working to remain as vigilant as they can throughout the countdown.

"When we come out of the final hold you can sort of feel your chest tighten up," Hampton said. "I never consider it routine because I'm always holding my breath."

If all goes according to plan, about 25 minutes after launch the Lockheed Martin-built GPS satellite will be released from the Delta 2 rocket's third stage and almost immediately begin a series of checks and tests that will lead to the replacement becoming operational in mid-May.

At that point the GPS constellation of satellites will be back up to its full strength of 27 craft circling the planet every 11 hours. They are spaced far enough apart so that military and civilian users can always receive the navigation signals that make it possible for small hand-held devices to tell you almost exactly where you are on Earth.

For the military, the Navstar system really proved the value of spaceflight when the navigation devices were used extensively during the Persian Gulf War, where a featureless desert made it almost impossible to navigate any other way.

So popular were the GPS receivers then that when the Air Force couldn't meet the demand, soldiers in the field actually ordered the devices from department store catalogs back in the United States and had them shipped overseas, conveniently billed to their personal credit cards.

Today the GPS receiving devices are used everywhere. Commercial boaters, truck drivers and private pilots use GPS to avoid getting lost on the land, over the seas or up in the air.

The system is becoming so mainstream that advertisements for GPS receivers are even showing up in the weekly ads from consumer electronic superstores, with prices that rival other electronic entertainment gadgets, such as DVD players and home stereo equipment.

Public interest in the Navstar system has not gone unnoticed by the men and women responsible for launching the satellites into orbit.

"We all have a sense of pride in that what we are doing has great benefits to the military world, the civil world and the commercial world," Kordell said.