Continuing 20 years of building and maintaining the Global Positioning System, the workhorse Delta 2 rocket this morning successfully launched another satellite for the navigation network known the world over.

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"Congratulations to the Air Force and all of our mission partners on today's successful GPS satellite launch," said Jim Sponnick, United Launch Alliance's vice president of the Delta Product Line. "One third of the 140 successful Delta 2 launches have been dedicated to GPS satellites. The ULA Delta team is extremely proud of the role we've played in launching this incredible satellite constellation. During the past two decades, GPS has changed how people navigate around their neighborhoods and around the globe. GPS has also greatly improved military operations as well as numerous maritime, aircraft and business operations worldwide."

This is the 87th consecutive successful Delta 2 rocket launch dating back to May 1997. The Delta 2's overall history since debuting in 1989 has achieved 140 successes in 142 flights.

The next Delta 2 rocket launch will occur from Vandenberg Air Force Base in California on May 5 carrying a payload known as the Space Tracking and Surveillance System Advanced Technology Risk Reduction, or STSS-ATRR, a research and development mission for the Missile Defense Agency.

Delta has delivered the GPS 2R-20 spacecraft into a highly elliptical transfer orbit. The satellite will fire its own kick motor Thursday to circularize the orbit. Deployment of the solar panels and antenna appendages is planned for Friday. The satellite should be checked out and ready to join the navigation network in a couple of weeks to replace an aging craft more than 12 years old.

T+plus 68 minutes, 16 seconds. SPACECRAFT SEPARATION! The U.S. Air Force's Global Positioning System Block 2R-20 spacecraft has been released from the Delta 2 rocket's third stage to complete this early morning launch from Cape Canaveral.

T+plus 66 minutes, 11 seconds. The third stage has burned out of its solid fuel, ending the Delta 2 rocket's powered flight for the launch of the GPS 2R-20 spacecraft. Separation of the payload is about two minutes away.

T+plus 65 minutes, 40 seconds. Good chamber pressure on the third stage.

T+plus 64 minutes, 57 seconds. Third stage ignition! The Thiokol Star 48B motor is firing to propel the GPS 2R-20 satellite into its targeted orbit for this launch.

T+plus 64 minutes, 20 seconds. The solid-fueled third stage has spun up.

As the launch sequence continues with the third stage burn and subsequent deployment of the GPS 2R-20 satellite, the second stage will conduct one additional engine firing to deplete its remaining fuel supply and lower its orbital inclination. This is designed to minimize the amount of land that the stage flies over during its natural fall from orbit and future reentry.

T+plus 63 minutes, 15 seconds. The second stage has completed its second burn of this launch. In the next minute, tiny thrusters on the side of the rocket will be fired to spin up the vehicle in preparation for jettison of the second stage.

T+plus 62 minutes, 40 seconds. The Delta 2 rocket's second stage is firing again. The burn is in progress to boost the vehicle into a higher orbit.

T+plus 62 minutes, 22 seconds. The Guam tracking site is receiving data from the rocket and relaying that telemetry back to Cape Canaveral.

T+plus 60 minutes. The rocket is soaring over the western Pacific Ocean, and restart of the second stage engine is just a couple of minutes away.

T+plus 53 minutes. The Delta 2 rocket should be coming within the Guam tracking station's coverage zone in about 10 minutes, restoring a live telemetry link from the vehicle.

T+plus 45 minutes. According to the launch sequence, the thermal conditioning roll of the rocket will end in about 11 minutes. Ignition of the second stage is coming up in 17 minutes.

T+plus 35 minutes. Restart of second stage engine is expected at about T+plus 62 minutes, 29 seconds. The stage will fire for approximately 42 seconds to raise the orbit's high point, or apogee. That will be followed by separation between the second and third stages. The upper stage will burn to inject GPS into the intended transfer orbit. Deployment of the payload to complete the launch is expected at T+plus 68 minutes.

T+plus 26 minutes, 10 seconds. The vehicle just passed out of range from the Ascension Island tracking site in the central Atlantic Ocean. The next station to acquire the rocket's signal will be Guam a half-hour from now.

As this coast phase of the launch continues, you can see a map of the rocket's planned track here.

T+plus 21 minutes, 30 seconds. Ascension Island is relaying data from the rocket to engineers at Cape Canaveral.

The orbit is right on the pre-planned parameters, the telemetry manager says.

T+plus 19 minutes. As the rocket coasts in this parking orbit, it performs a "BBQ roll" maneuver to keep the thermal conditions on the vehicle equal. This maneuver was scheduled to start at about T+plus 17 minutes, 20 seconds and conclude at T+plus 56 minutes, 25 seconds.

T+plus 16 minutes. The official liftoff time was 4:34:00.244 a.m. EDT.

T+plus 13 minutes. The rocket is flying out of range from the Antigua tracking station. The next site to acquire signal will be Ascension Island in about seven minutes.

T+plus 11 minutes, 18 seconds. The vehicle is 97.4 miles in altitude, 1,686 miles downrange from the launch pad and traveling at 16,685 mph.

T+plus 10 minutes, 54 seconds. SECO 1. The second stage engine cutoff has occurred, completing the motor's first firing of the day. The Delta 2 rocket with GPS 2R-20 has arrived in a preliminary orbit around Earth following launch today from Cape Canaveral. The vehicle will coast for about 52 minutes before the second stage is re-ignited.

T+plus 10 minutes, 20 seconds. The vehicle is 97 miles in altitude, 1,466 miles downrange from the launch pad and traveling at 16,206 mph.

T+plus 10 minutes. About a minute left in this initial firing of the second stage.

T+plus 9 minutes, 20 seconds. The vehicle is 97 miles in altitude, 1,250 miles downrange from the launch pad and traveling at 15,403 mph.

T+plus 9 minutes. The second stage engine is still firing, consuming a hydrazine propellant mixture and nitrogen tetroxide oxidizer. This burn is needed to achieve a parking orbit around Earth.

T+plus 8 minutes, 30 seconds. The vehicle is 95.9 miles in altitude, 1,074 miles downrange from the launch pad and traveling at 14,776 mph.

T+plus 7 minutes, 30 seconds. The vehicle is 92 miles in altitude, 864 miles downrange from the launch pad and traveling 14,074 mph.

T+plus 6 minutes, 45 seconds. The vehicle is 87 miles in altitude, 712 miles downrange from the launch pad and traveling 13,600 mph.

T+plus 6 minutes, 30 seconds. Second stage engine chamber pressure continues to look good.

T+plus 5 minutes, 50 seconds. The vehicle is 78 miles in altitude, 549 miles downrange from the launch pad and traveling 13,173 mph.

T+plus 5 minutes, 10 seconds. The vehicle is 69 miles in altitude, 426 miles downrange from the launch pad and traveling 12,857 mph.

T+plus 4 minutes, 56 seconds. The rocket's nose cone enclosing the spacecraft has been jettisoned.

T+plus 4 minutes, 40 seconds. The spent first stage has been jettisoned and the Delta's second stage engine has ignited!

T+plus 4 minutes, 26 seconds. MECO. The first stage main engine cutoff is confirmed.

T+plus 4 minutes. A half-minute remaining in the first stage burn. Everything is reported normal aboard the vehicle.

T+plus 3 minutes, 35 seconds. The vehicle is 44 nautical miles in altitude, 177 miles downrange from the launch pad, traveling at 8,479 mph.

T+plus 3 minutes, 20 seconds. The main engine is still firing normally, burning a mixture of highly refined kerosene fuel and supercold liquid oxygen.

T+plus 3 minutes. The vehicle is 37 nautical miles in altitude, 117 miles downrange from the launch pad, traveling at 6,617 mph.

T+plus 2 minutes, 30 seconds. The vehicle is 28.8 nautical miles in altitude, 75.1 miles downrange from the launch pad, traveling at 5,366 mph.

T+plus 2 minutes, 15 seconds. The three air-ignited solid rocket boosters have burned out and separated. The rocket is now flying solely on the power generated by the liquid-fueled first stage main engine.

T+plus 1 minute, 40 seconds. The vehicle is 14.8 nautical miles in altitude, 32.7 miles downrange from the launch pad, traveling at 3,243 mph.

T+plus 1 minute, 30 seconds. The Delta 2 rocket now weighs half of what it did at liftoff 90 seconds ago.

T+plus 1 minute, 10 seconds. All six ground-start solid rocket boosters have burned out of propellant and separated from the Delta 2's first stage. A moment before the jettison occurred, the three remaining motors strapped to rocket ignited to continue assisting the rocket's RS-27A main engine on the push to space.

T+plus 56 seconds. The Delta 2 rocket has passed through the region of maximum aerodynamic pressure during its atmospheric ascent.

T+plus 35 seconds. The vehicle is now breaking through the sound barrier.

T+plus 30 seconds. The six solid rocket motors ignited on the launch pad have surpassed their period of maximum thrust. Each of the Alliant Techsystems-made boosters generate roughly 100,000 pounds of thrust.

T+plus 20 seconds. Delta is maneuvering on course to deliver the GPS 2R-20 spacecraft into the world's premier navigation network.

LIFTOFF! Liftoff of the Delta 2 rocket, continuing 20 years of building and maintaining the Global Positioning System. And the vehicle has cleared the tower!

T-minus 30 seconds. SRB ignitors will be armed at T-minus 11 seconds.

The launch ignition sequence will begin in the final two seconds of the countdown when a ULA engineer pushes the engine start switch. The process begins with ignition of the two vernier engines and first stage main engine start. The six ground-lit solid rocket motors then light at T-0 for liftoff.

T-minus 1 minute. Sixty seconds from launch. The vehicle's second stage hydraulic pump has gone to internal power after its pressures were verified acceptable.

T-minus 100 seconds. First stage LOX topping to 100 percent is underway.

T-minus 2 minutes. Pressurization of the first stage liquid oxygen is now beginning. Puffs of vapor from a relief valve on the rocket will be seen in the remainder of the countdown as the tank pressure stabilizes.

T-minus 2 minutes, 30 seconds. The GPS spacecraft has been declared "go" for launch.

T-minus 3 minutes and counting. The rocket's third stage safe and arm devices are being armed.

T-minus 3 minutes, 45 seconds. The Delta 2 rocket's systems are now transferring to internal power for launch.

T-minus 4 minutes and counting! The Delta 2 rocket is ready to launch the GPS 2R-20 military navigation satellite at 4:34 a.m. EDT from pad 17A at Cape Canaveral Air Force Station in Florida.

Standing by to resume the countdown in one minute. A "go" to continue the count has been announced. Clocks will be ticking down from the T-minus 4 minute, targeting a launch at 4:34 a.m. EDT.

At liftoff, the six ground-lit solid boosters and the first stage main engine will fire to power the Delta away from pad 17A and begin the 68-minute ascent to deploy the GPS 2R-20 satellite.

Two minutes remain in the built-in hold. The Air Force status check was just conducted.

Final instructions are being given the launch team before the countdown resumes.

No problems were reported during the readiness poll of the launch team.

Launch team readiness polling is beginning.

Liftoff of the newest Global Positioning System satellite is just ten minutes away.

"GPS 2R-20, built by Lockheed Martin, is a continuation of the modernization program that provides new signals and enhanced capabilities to ensure improved accuracy and signal availability to our users," said Mike Dunn, technical director of the Space and Missile Systems Center's Global Positioning Systems Wing.

"GPS contributes vital capabilities to our nation's military operations, global information infrastructure, emergency response, transportation and telecommunication industries, the international economy and everyday life.

"Our commitment is to ensure this emerging capability continues to deliver precise positioning, navigation and timing service to users around the globe."

T-minus 4 minutes and holding. The countdown has entered the final planned hold point for today's launch attempt. During this planned 10-minute hold, officials will poll the team members in the various control centers to ensure all systems are "go" to proceed with the countdown for liftoff at 4:34 a.m. EDT.

The launch weather officer confirms the current conditions are acceptable for liftoff of the Delta 2 rocket this morning.

The first stage kerosene fuel tank is being pressurized for flight.

The Lockheed Martin-built GPS spacecraft payload is now confirmed to be on internal battery power for flight.

"In addition to this being our seventh modernized GPS satellite, it will also carry a new research and development demonstration payload for transmitting a third civil signal known as L5. The signal is in a protected band of frequencies, which will provide a second signal to aviation users," said Mike Dunn, technical director of the Space and Missile Systems Center's Global Positioning Systems Wing.

T-minus 15 minutes and counting. Countdown clocks are running again following the planned 20-minute hold. The count will continue to the T-minus 4 minute mark where another planned hold is scheduled. Launch remains set to occur at 4:34 a.m. EDT (0834 GMT).

The GPS 2R-20 spacecraft cargo atop the Delta 2 rocket is now switching to internal power for launch.

The launch team is being polled for "ready" status to resume the countdown at the end of the hold as scheduled.

GPS 2R-20 will become the 216th primary payload put into space by the venerable Delta 2 rocket over the past 20 years. Here's a look at some other stats about today's mission. This will be:

• The 340th Delta rocket launch since 1960
• The fourth Delta of 2009
• The 142nd Delta 2 rocket mission since 1989
• The 68th Delta 2 rocket fly in the 7925 configuration
• The 59th GPS satellite to launch
• The 48th time a Delta 2 has launched with a GPS satellite
• The 20th GPS Block 2R satellite and 7th modernized
• The 107th Delta 2 rocket to fly from Cape Canaveral
• The 61st Delta 2 launch from pad 17A

T-minus 15 minutes and holding. Clocks have entered the first of two planned hold periods during the final portion of the Terminal Countdown. This pause will last 20 minutes in duration.

These holds are designed to give the launch team a chance to deal with any problems and catch up on work that could be running behind schedule.

The first stage engine steering checks just finished. Technicians will take the next few minutes to review the data from the tests.

The second stage engine slews are complete. First stage tests have begun.

The launch team is beginning the "slew" or steering checks of the first and second stage engines. These are gimbal tests of the nozzles on the first stage main engine and twin vernier engines and second stage engine to ensure the rocket will be able to steer itself during launch.

The Delta 2 rocket is basking in the glow of the powerful spotlights at Cape Canaveral's Complex 17 as the countdown enters the final 60 minutes until launch.

Over the past two decades, Delta 2 rockets have successfully placed 46 satellites into orbit for the Global Positioning System. But teams had to overcome a major hurdle in preparing for this launch.

"While it may seem that after a while these launches become routine, let me assure you that each mission has its own unique set of challenges to overcome. This mission was no different. Originally scheduled for launch in June 2008, a problem with a newly delivered lot of Delta 2 ordnance timers was discovered that required a redesign, a requalification program," said John Wagner, the mission director and chief technical director at the Space and Missile System Center's Launch and Range Systems Wing.

The solid-fuel third stages on the Delta 2 rockets spin up and separate from the second stage during launch. That second stage carries the guidance system, so the third stage is pointed in the right direction and simply released to perform its brief mission. Fuse-like ordnance timers control the critical events on the third stage, such as ignition and release of the payload.

A test failure in the factory led to the extensive effort to build new timers and rigorously test them, delaying Delta missions that needed third stages like the GPS 2R-20 and 2R-21 launches.

"This issue was resolved and we're now back on track to launch the final two Air Force Delta 2/GPS missions," Wagner said.

NASA's Kepler spacecraft successfully launched with the new timers earlier this month.

The checks of the rocket's safety systems are underway. That will be followed a short time from now by engine steering tests.

Countdown clocks are continuing to the T-minus 15 minute mark where a 20-minute built-in hold is planned. A final 10-minute hold at T-minus 4 minutes will lead to the target liftoff time of 4:34 a.m. EDT.

Today's launch window was shortened from 15 minutes to 7 minutes. This change avoids the rocket's trajectory causing any interference with the international space station. The revised period extends from 4:34 to 4:41 a.m. EDT.

Loading of the Delta 2 rocket's first stage liquid oxygen tank has been accomplished. The filling process took 26 minutes and 35 seconds, ending at 3:17:03 a.m. The tank will be replenished through the countdown to replace the super-cold liquid oxygen that naturally boils away.

The rocket now stands fully fueled for liftoff. The vehicle's first stage was successfully loaded with RP-1 kerosene fuel along with the liquid oxygen over the past hour-and-a-half. The second stage was filled with its storable nitrogen tetroxide and Aerozine 50 fuels last week. The nine strap-on booster rockets and third stage use solid propellants.

The first stage liquid oxygen tank just reached the 95 percent full level. The "rapid load" valve was closed, with the slower "fine load" phase continuing to fill the tank.

The launch time forecast has been adjusted slightly to increase the temperature to 66-68 degrees F.

The launch weather officer says the reconnaissance aircraft won't be needed to check the clouds this morning. The weather conditions are looking just fine for the Delta's liftoff.

A bright white plume of vapors have begun streaming from a vent on the rocket and the bottom of the vehicle is icing over as the super-cold liquid oxygen continues to flow into the first stage.

The countdown is progressing smoothly and on schedule for liftoff of the Delta rocket some 90 minutes from now at 4:34 a.m. EDT.

Now 10 minutes into this approximate 25-minute process to fill the first stage liquid oxygen tank.

LOX loading begins! Cryogenic liquid oxygen, chilled to Minus-298 degrees F, has started flowing from the storage reservoir at Complex 17, through plumbing and into the bottom of the Delta 2 rocket. The LOX will be consumed by the first stage main engine during the first four-and-a-half minutes of flight along with the 10,000 gallons of RP-1 kerosene already loaded aboard the vehicle.

The official "go" has been given by the launch director to start filling the rocket's first stage with liquid oxygen.

The launch team has received approval to begin preparations for loading the rocket's first stage liquid oxygen tank as planned.

All weather rules are "go" for launch right now. Some clouds are moving in from the northwest. The launch meteorologists have the option of deploying the weather reconnaissance aircraft, if needed, for interrogating the clouds in the final hour of the countdown to ensure there's no problem for the rocket's ascent. That decision is still to come.

The forecast for this morning's 4:34 a.m. EDT launch time predicts scattered clouds, easterly winds 10-15 knots and a temperature of 62-63 degrees F.

Two hours and counting.

Today's launch of the GPS 2R-20 satellite aboard a Delta 2 rocket from pad 17A comes just a couple of weeks after another Delta 2 lifted off from neighboring pad 17B with NASA's Kepler planet-finder spacecraft.

Lt. Col. Erik Bowman, commander of the Air Force's 1st Space Launch Squadron that oversees Delta 2 operations at the Cape, offers these thoughts about the busy month at Complex 17:

"For this launch, it's been a fairly challenging time period for us between the Kepler and now, particularly as the 1st SLS and ULA teams have been drawing down as we get closer to flyout. The team has risen to the occasion to support despite the reduced manpower we have associated with the impending flyout later this year."

The Air Force will launch its final Delta 2 rocket in August.

"Although we've been busy preparing the GPS 2R-20 mission since the Kepler launch, processing both of these vehicle actually began in earnest in early January. Since that time, the team has accomplished almost 400 major launch processing tasks to be able to launch both vehicles within this 18-day period, at times performing as many as six major operations in a day over multiple locations. By the time we actually get to the day of launch, most of the really hard work is done. A launch day is simply the final step of execution."

The launch team has completed work to turn on and configure the Delta's onboard guidance computer.

The next milestone in the count will be loading super-cold cryogenic liquid oxygen into the first stage starting about 30 minutes from now.

The kerosene and liquid oxygen will be consumed by the stage's Pratt & Whitney Rocketdyne RS-27A main engine and twin vernier steering thrusters during the initial four-and-a-half minutes of flight.

The first stage fuel tank of the Delta 2 rocket has been fully loaded for today's planned 4:34 a.m. EDT launch. The tank was filled with a highly refined kerosene, called RP-1, during an 18-minute, 19-second process that concluded at 2:17:09 a.m.

Rapid-loading of the RP-1 tank has concluded with 9,800 gallons having been pumped into the rocket. Fine load is continuing.

Now 9,000 gallons aboard the rocket.

The launch team has computed that the full load for the first stage fuel tank will be 9,975 gallons.

Once the tank is filled to the 98 percent level, the "rapid load" valve will be closed and the slower "fine load" phase will continue to top off the tank.

The rocket's RP-1 tank is half-full already.

First stage propellant loading has passed the 3,000-gallon mark. This process to load the kerosene fuel takes about 20 minutes.

Fueling begins! About 10,000 gallons of the kerosene propellant, called RP-1, are pumping into the base of the rocket from storage tanks at pad 17A as fueling of the Delta 2's first stage begins for today's launch.

Preparations for loading the Delta 2 rocket's first stage RP-1 fuel tank are beginning. After verifying valves, sensors, flow meters and equipment are ready, the highly refined kerosene fuel will start flowing into the vehicle a few minutes from now.

The first stage helium and nitrogen systems have been pressurized. And the "go" has been given for the start of fueling operations.

The rocket's guidance system is being turned on.

And the launch team is starting the steps to pressurize the first and second stage helium and nitrogen systems and the second stage fuel and oxidizer tanks.

BEGIN COUNT. The Terminal Countdown has been initiated for today's launch of the Global Positioning System 2R-20 satellite aboard the Delta 2 rocket.

The next three hours will be spent fueling the rocket, activating systems and performing final testing before liftoff at 4:34 a.m. EDT from pad 17A at Cape Canaveral Air Force Station in Florida.

Workers have cleared the Complex 17 area in advance of the hazardous portion in today's launch operation. But a warning horn is now being sounded three times at the seaside complex as a precaution to alert any remaining personnel in the vicinity that they should leave immediately.

The pad clear status will allow the start of activities such as pressurizing the helium and nitrogen storage tanks inside the rocket's first and second stages, along with the second stage fuel and oxidizer tanks.

The countdown clocks currently stand at T-minus 150 minutes and counting. Two planned holds -- at the T-minus 15 minute and the T-minus 4 minute points -- will give the launch team some time to catch up on any work running behind. The first hold will last 20 minutes in duration, the second extends 10 minutes.

Today's launch window has been shortened from 15 minutes to 7 minutes. This change will avoid the rocket's trajectory causing any interference with the international space station.

The revised period extends from 4:34 to 4:41 a.m. EDT.

The launch team has been polled to ensure all stations are manned and systems are set for the Terminal Countdown. Each team member voiced a "ready" to begin the count.

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Weather conditions are looking great. There's just a few clouds around but no rain showers. The official odds of acceptable launch weather have improved to 90 percent.

T-minus 150 minutes and holding. The countdown is entering the first of three planned holds that will occur over the course of the night. This first pause is 60 minutes long.

The two later holds -- at the T-minus 15 minute and the T-minus 4 minute points -- will give the launch team some time to deal with issues and catch up on any work running behind. Those two holds combined add up to a half-hour in duration.

In the top-left corner of this page, you will see the Air Force team patch for this launch that features Mighty Mouse. Here's a description from the Air Force of the logo's symbolism:

"Mighty Mouse characterizes heritage and was an important icon in American history and American culture - just like GPS. Additionally, the Delta 2 is the smallest ULA launch vehicle, but always comes through to 'save the day' as the most reliable spacelift vehicle in the Air Force's inventory.

"The arrow in the background signifies the booster as it lifts off from Earth. The NAVSTAR represents the GPS satellite atop our 'mighty' Delta 2. The additional 19 stars in the background illustrate the previous 19 GPS 2R missions. The one small yellow star touching the surface of the planet represents GPS 2R-1, which was lost."

The Delta 2 rocket stands poised on the launch pad with the Global Positioning System 2R-20 satellite tucked inside its nose cone for liftoff at 4:34 a.m. EDT.

"We are looking forward to yet another successful launch. Since the first GPS launch in 1978, we have incorporated measures to ensure improved accuracy and signal availability to users worldwide. This launch is a testament to the new signals and improved capabilities that are being implemented for users to continue to receive uninterrupted positioning, navigation and timing services. I would like to commend the 45th Space Wing, the 50th Space Wing, United Launch Alliance, our industry partners, the Delta 2 and 2R-M launch team for their diligence and hard work for making this launch possible," said Col. Dave Madden, commander of the Space and Missile Systems Center's Global Positioning Systems Wing.

The gantry is arriving at its launch position where the tower will be secured to allow the Delta 2 rocket's liftoff carrying the Global Positioning System 2R-20 satellite. Now, workers will turn their attention to configuring the pad and putting the final touches on equipment and the rocket before clearing Complex 17 later tonight.

The Terminal Countdown commences at 1:34 a.m. EDT, followed shortly thereafter by loading kerosene fuel and then super-cold liquid oxygen into the first stage.

Liftoff remains scheduled for 4:34 a.m. EDT.

As the sun sets at pad 17A, the mobile service gantry is slowly rolling away from the Delta 2 rocket for the upcoming middle-of-the-night countdown and launch.

The tower was used to stack the multi-stage vehicle atop the pad's launch mount, attach the nine strap-on solid motors and hoist the payload aboard the rocket. This cocoon-like structure wraps around the Delta to offer weather protection and full access for workers during the pre-launch flow.

Work has been underway this afternoon and evening at Complex 17, as ground crews retracted access platforms and stowed gear in the tower in advance of the structure wheeling away from the rocket.

The sliding doors on the backside of the tower were then opened. The doors enclose the cleanroom area where Delta payloads get attached to the rockets during the campaigns leading up to launch.

The GPS spacecraft launching overnight was delivered to the pad on March 11, then hoisted into the cleanroom and bolted to its Delta for the ride to orbit. The rocket's two-piece nose cone to shroud the satellite during ascent through the atmosphere was installed around the GPS craft last week.

After a rainy and overcast morning, the clouds have given way to partly sunny skies at Cape Canaveral this afternoon. For tonight's countdown and the predawn launch, Air Force meteorologists still predict an 80 percent chance of acceptable conditions.

"Main concern during the countdown and launch window will be possibility of showers moving onshore near the launch site," the forecasters said today.

The launch time outlook calls for scattered clouds at 3,000 feet and a broken deck at 6,000 feet, showers in the vicinity, 10 miles of visibility, easterly winds from 090 degrees at 15 peaking to 22 knots and a temperature of 62-64 degrees F.

The orchestrated sequence of final pre-launch preparations will get underway at Cape Canaveral's Complex 17 later today, leading to rollback of the mobile service gantry and starting the countdown for an overnight liftoff at 4:34 a.m. EDT.

"After more than a year since our last Delta 2/GPS launch, the Delta 2 and GPS programs are eager to get back in the business of launching GPS satellites," said John Wagner, the mission director and chief technical director at the Space and Missile System Center's Launch and Range Systems Wing. "We are ready for the first of two missions this year."

The 12-story Delta 2 rocket will be propelled off the launch pad with its main engine and six strap-on solid-propellant boosters firing to generate over 700,000 pounds of thrust.

A minute into the ascent, the rocket will already be 10 miles up as those six solid motors extinguish and separate. Three remaining solids are ignited for their minute-long firing.

The slender blue first stage keeps its RS-27A engine roaring through the initial four-and-a-half-minutes, climbing about 60 miles over the Atlantic Ocean before being jettisoned. That allows the hypergolic second stage engine to ignite and push the vehicle into a preliminary orbit stretching 111 miles at its high point and 94 miles at its lowest.

The second stage will settle into the parking orbit about 11 minutes after liftoff and begin a quiet coast to the other side of the planet where the launch sequence resumes about 50 minutes later over the western Pacific Ocean.

Another firing of the second stage, this time lasting just 42 seconds, begins the process of shaping the orbit for the GPS satellite cargo. The end result of this burn will be an orbit 670 miles at apogee and 103 miles at perigee.

Tiny thrusters then ignite to rapidly "spin up" the third stage and attached satellite to 55 rpm. The second stage separates from the spinning duo some 64 minutes after liftoff.

The solid-fuel third stage is lit moments later, delivering a minute-and-a-half of thrust to propel the satellite into a highly elliptical orbit reaching 11,000 miles at the high point, 104 miles at the lowest and inclined 40 degrees to the equator.

The Lockheed Martin-built satellite is cast free from the spent rocket 68 minutes into the ascent, completing the Delta 2 vehicle's job.

If all goes well, another solid-propellant kick motor on the satellite itself will fire in a few days to circularize its orbit at 11,000 miles and increase the inclination to 55 degrees where the GPS constellation flies.

Controllers will maneuver the craft into the Plane B, Slot 2 position of the constellation to take the place of GPS 2A-27. That aging satellite, which was launched in September 1996 with a seven-year design life, will become an in-space backup.

In addition to replacing the old satellite, the modernized GPS 2R-20 also carries a special demonstration payload for a new civil signal, known as L5.

"The entire government-industry team has worked extremely hard to prepare this important spacecraft for flight," said Don DeGryse, Lockheed Martin's vice president of Navigation Systems. "We look forward to successfully demonstrating the L5 signal for our customer and further enhancing the overall performance of the GPS constellation with another sophisticated 2R-M satellite."

A Delta 2 rocket plans to make a thunderous predawn ascent out of Cape Canaveral Air Force Station on Tuesday and hurl a new navigation satellite into the Global Positioning System.

Liftoff from pad 17A will be possible during a 15-minute period extending from 4:34 to 4:49 a.m. EDT (0834-0849 GMT).

Riding atop the three-stage rocket is GPS 2R-20, the latest spacecraft built to sustain the orbiting GPS constellation. It continues a series of modernized satellites equipped to transmit additional signals and provide improvements aimed at greater accuracy, tougher resistance to interference and enhanced performance for users around the world.

"This space vehicle is very clean and healthy. We're excited about the added capability this critical spacecraft will provide our nation," said Lt. Col. John Wagner, the Air Force spacecraft director and the commander of the 45th Launch Support Squadron at the Cape.

The craft will replace the aging GPS 2A-27 satellite launched in September 1996. Now well past its design life, ground controllers plan to move the old satellite into a backup role.

The new GPS 2R-20 spacecraft take over the Plane B, Slot 2 location of the navigation network, considered one of the primary positions in the constellation that is divided into six orbital groupings with multiple satellites flying in each.

The satellite is the 20th built by Lockheed Martin, but the two-ton craft is unique because it carries a special demonstration package to test a new civil signal planned for the aviation community.

"We're very excited about supporting the launch of this specific GPS satellite, which will be the first demonstration of a navigation frequency to be used in aviation applications," said Lt. Col. Erik Bowman, the Air Force launch director and the commander of the 1st Space Launch Squadron at the Cape.

The Air Force needs the satellite in orbit and the new frequency demonstrated before an International Telecommunications Union deadline in late August.

"This is a demonstration of that signal," said Tom Nagle, program manager of civil applications at the Transportation Department. "It is a key part of all aviation planning for the next generation aviation systems."

The Delta 2 rocket is celebrating its 20th anniversary of lofting GPS satellites, having debuted in February 1989 to begin assembling the constellation. The workhorse booster has successfully deployed 46 satellites for the navigation network over the past two decades, in addition to performing dozens of missions for NASA and commercial customers.

"The Delta 2 is ready to launch, and we look forward to completing our 140th successful launch in this storied 20-year history of Delta 2," said John Gerlitz, Air Force Delta 2 program manager at United Launch Alliance.

This is the next-to-last launch of the Delta 2 rocket for the Air Force. The final mission is planned for August to deploy the final GPS 2R-type satellite.

The next series of GPS 2F satellites, slated to begin flying later this year, will rely on the larger Atlas 5 and Delta 4 rockets.

Watch this page for live reports throughout the countdown and launch of GPS 2R-20!

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Air Force weather forecasters are predicting an 80 percent chance of good conditions for Tuesday's predawn liftoff of the Delta 2 rocket from Cape Canaveral, Florida.

"Surface and upper level high pressure will be the dominant weather feature over the southeast U.S. and western Atlantic over the next several days. Persistent and strong onshore flow will result in scattered shower development over the offshore waters. Main concern during the countdown and launch window will be possibility of showers moving onshore near the launch site."

During the 15-minute launch window opening at 4:34 a.m. EDT, the outlook calls for scattered clouds at 3,000 and 8,000 feet, showers in the vicinity, 10 miles of visibility, easterly winds from 090 degrees at 15 peaking to 22 knots and a temperature of 62-64 degrees F.

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