BY JUSTIN RAY

Follow the countdown and launch of the ULA Delta 2 rocket with a GPS navigation spacecraft. Reload this page for the very latest on the mission.

Additional coverage for subscribers:
VIDEO: THE DELTA 2 ROCKET BLASTS OFF WITH GPS 2R-18 PLAY
VIDEO: MOBILE SERVICE TOWER IS ROLLED BACK FOR LAUNCH PLAY
VIDEO: GPS SATELLITE ANIMATION PLAY
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THURSDAY, DECEMBER 20, 2007

A productive year for the Delta rocket program that launched spacecraft to observe Earth, dig up frozen water on Mars and explore uncharted worlds in the asteroid belt was capped with a successful ascent of a modernized GPS navigation satellite on Thursday afternoon from Cape Canaveral.

Read our full launch story.

A gallery of launch photos is posted here.

2330 GMT (6:30 p.m. EST)

This was United Launch Alliance's 13th rocket flight in 2007 and the 8th in just the last 94 days.

"With the launch of GPS 2R-18(M), ULA completes a tremendously successful first year of operation and demonstrates its commitment to 100 percent mission success," said Mark Wilkins, United Launch Alliance vice president of Delta Programs. "As we continue to provide safe, cost-effective, reliable access to space, we are privileged to serve an important role in critical missions, such as GPS, which are force multipliers for our men and women in uniform serving our country throughout the world."

2133 GMT (4:33 p.m. EST)

Controllers have established contact with the GPS 2R-18 satellite, which is headed for the orbiting GPS navigation constellation to enter service in a couple of weeks.

2112 GMT (4:12 p.m. EST)

This is the 79th consecutive successful Delta 2 rocket launch dating back to May 1997. The Delta 2's overall history since debuting in 1989 has achieved 132 successes in 134 flights.

The next Delta 2 rocket launch is planned for mid-March from Cape Canaveral, Florida. The vehicle will deploy GPS 2R-19, the next replacement satellite for the Global Positioning System.

2112 GMT (4:12 p.m. EST)

T+plus 68 minutes, 18 seconds. SPACECRAFT SEPARATION! The U.S. Air Force's Global Positioning System Block 2R-18 spacecraft has been released from the Delta 2 rocket's third stage to complete this afternoon's launch from Cape Canaveral.

2110 GMT (4:10 p.m. EST)

T+plus 66 minutes, 45 seconds. Good burn reported on the third stage.

2110 GMT (4:10 p.m. EST)

T+plus 66 minutes, 27 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-18 spaeccraft. Separation of the payload is about two minutes away.

2109 GMT (4:09 p.m. EST)

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

2108 GMT (4:08 p.m. EST)

T+plus 64 minutes, 50 seconds. Third stage ignition! The Thiokol Star 48B motor is firing to propel the GPS 2R-18 satellite into its targeted orbit for this launch.

2108 GMT (4:08 p.m. EST)

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

As the launch sequence continues with the third stage burn and subsequent deployment of the GPS 2R-18 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.

2107 GMT (4:07 p.m. EST)

T+plus 63 minutes, 21 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.

2106 GMT (4:06 p.m. EST)

T+plus 62 minutes, 35 seconds. Confirmation now received that Delta 2 rocket's second stage burn is in progress to boost the vehicle into a higher orbit. The Guam tracking site is providing the live telemetry relay from the rocket back to Cape Canaveral.

2104 GMT (4:04 p.m. EST)

T+plus 60 minutes. The Delta 2 rocket should be coming within the Guam tracking station's coverage zone in the next two minutes.

2054 GMT (3:54 p.m. EST)

T+plus 50 minutes. The rocket is soaring over the Indian Ocean as it continues in the coast phase of the mission.

2049 GMT (3:49 p.m. EST)

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

2039 GMT (3:39 p.m. EST)

T+plus 35 minutes. Today's official liftoff time was 3:04:00.353 p.m. EST.

2031 GMT (3:31 p.m. EST)

T+plus 27 minutes. 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.

2025 GMT (3:25 p.m. EST)

T+plus 21 minutes. Ascension Island is relaying data from the rocket to engineers at Cape Canaveral. The vehicle has entered the slow rolling motion designed to keep the thermal conditions even across the rocket.

2024 GMT (3:24 p.m. EST)

T+plus 20 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.

2018 GMT (3:18 p.m. EST)

T+plus 14 minutes. The rocket has flown out of range from the Antigua tracking station. The next site to acquire signal will be Ascension Island in about six minutes.

2017 GMT (3:17 p.m. EST)

T+plus 13 minutes, 30 seconds. The rocket has successfully achieved a parking orbit with an apogee of 109.88 miles, perigee of 93.97 miles and inclination of 37.5 degrees. That is right on the pre-planned orbit parameters.

2014 GMT (3:14 p.m. EST)

T+plus 10 minutes, 56 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-18 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.

2014 GMT (3:14 p.m. EST)

T+plus 10 minutes. About a minute left in this initial firing of the second stage. The rocket is 97 miles in altitude, 1,390 miles downrange from the launch pad and traveling at 16,000 mph.

2013 GMT (3:13 p.m. EST)

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.

2012 GMT (3:12 p.m. EST)

T+plus 8 minutes, 15 seconds. Altitude now 95 miles, downrange distance 1,020 miles and speed 14,600 mph.

2011 GMT (3:11 p.m. EST)

T+plus 7 minutes, 45 seconds. Second stage engine chamber pressure remains solid.

2010 GMT (3:10 p.m. EST)

T+plus 6 minutes, 50 seconds. The Air Force's downrange tracking station on Antigua Island has acquired the rocket's signal.

2010 GMT (3:10 p.m. EST)

T+plus 6 minutes, 30 seconds. The rocket is 84 miles in altitude, 659 miles downrange from the launch pad and traveling over 13,000 mph.

2009 GMT (3:09 p.m. EST)

T+plus 5 minutes, 30 seconds. Good chamber pressure and fuel-flow rates reported on the second stage engine.

2009 GMT (3:09 p.m. EST)

T+plus 5 minutes, 3 seconds. The rocket's nose cone enclosing the spacecraft has been jettisoned.

2008 GMT (3:08 p.m. EST)

T+plus 4 minutes, 42 seconds. The spent first stage has been jettisoned and the Delta's second stage engine has ignited!

2008 GMT (3:08 p.m. EST)

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

2007 GMT (3:07 p.m. EST)

T+plus 3 minutes, 55 seconds. The vehicle is 49 miles in altitude, 218 miles downrange from the launch pad and traveling at 9,700 mph.

2007 GMT (3:07 p.m. EST)

T+plus 3 minutes, 30 seconds. The main engine is still firing normally, burning a mixture of highly refined kerosene fuel and supercold liquid oxygen.

2007 GMT (3:07 p.m. EST)

T+plus 3 minutes. The Delta 2 rocket is 35 miles in altitude, 114 miles downrange from the launch pad and traveling at 6,500 mph.

2006 GMT (3:06 p.m. EST)

T+plus 2 minutes, 14 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.

2005 GMT (3:05 p.m. EST)

T+plus 1 minute, 45 seconds. Delta continues its steep climb. All looks normal with the vehicle.

2005 GMT (3:05 p.m. EST)

T+plus 1 minute, 25 seconds. The vehicle is 10 miles in altitude, 24 miles downrange from the launch pad and traveling at 2,300 mph.

2005 GMT (3:05 p.m. EST)

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.

2004 GMT (3:04 p.m. EST)

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

2004 GMT (3:04 p.m. EST)

T+plus 40 seconds. The vehicle is now breaking through the sound barrier.

2004 GMT (3:04 p.m. EST)

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.

2004 GMT (3:04 p.m. EST)

T+plus 20 seconds. All six ground-lit solid boosters and the first stage main engine are firing to power the Delta away from pad 17A on this 68-minute ascent to deploy the GPS 2R-18 satellite.

2004 GMT (3:04 p.m. EST)

LIFTOFF! Liftoff of the NAVSTAR GPS 2R-18 satellite aboard a Delta 2 rocket, continuing to modernize the Global Positioning System for users around the world. And the tower is clear!

2003 GMT (3:03 p.m. EST)

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.

2003 GMT (3:03 p.m. EST)

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.

2002 GMT (3:02 p.m. EST)

T-minus 75 seconds. The Air Force's Eastern Range has given the all-clear to launch.

2002 GMT (3:02 p.m. EST)

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

2002 GMT (3:02 p.m. EST)

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.

2001 GMT (3:01 p.m. EST)

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

2001 GMT (3:01 p.m. EST)

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

2000 GMT (3:00 p.m. EST)

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

2000 GMT (3:00 p.m. EST)

T-minus 4 minutes and counting! The Delta 2 rocket is ready to launch the GPS 2R-18 military navigation satellite at 3:04 p.m. EST from pad 17A at Cape Canaveral Air Force Station in Florida.

1959 GMT (2:59 p.m. EST)

Standing by to resume the countdown in one minute. All systems are "go" for launch.

1958 GMT (2:58 p.m. EST)

Two minutes remain in the built-in hold.

1957 GMT (2:57 p.m. EST)

The Air Force launch poll has been conducted with no problems reported.

1955 GMT (2:55 p.m. EST)

The entire launch team has given a "go" for liftoff at 3:04 p.m. EST.

1955 GMT (2:55 p.m. EST)

Launch team readiness polling is underway.

1949 GMT (2:49 p.m. EST)

Liftoff is 15 minutes away. There are no reports of any technical constraints and weather conditions are excellent today.

1945 GMT (2:45 p.m. EST)

T-minus 4 minutes and holding. The countdown has entered the final planned hold point for this afternoon's launch attempt. During this planned 15-minute hold, officials will poll the various team members in the soft blockhouse, Range Operations Control Center and Mission Directors Center. If all systems are "go," the countdown will resume for liftoff at 3:04 p.m. EST.

1943 GMT (2:43 p.m. EST)

The launch weather officer confirms conditions remain "go" for liftoff.

1940 GMT (2:40 p.m. EST)

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

1938 GMT (2:38 p.m. EST)

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

1934 GMT (2:34 p.m. EST)

T-minus 15 minutes and counting. The planned hold in the countdown has been released. Clocks will now tick down to T-minus 4 minutes where the final hold is scheduled leading to liftoff at 3:04 p.m. EST.

1933 GMT (2:33 p.m. EST)

The GPS 2R-18 spacecraft cargo atop the Delta 2 rocket is now switching to internal power for launch.

1930 GMT (2:30 p.m. EST)

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

1924 GMT (2:24 p.m. EST)

Now half-way through this built-in hold at T-minus 15 minutes. Once the countdown resumes, clocks will tick down to the T-minus 4 minute mark where a 15-minute hold is planned. Activitites are going smoothly for this afternoon's 3:04 p.m. liftoff of the Delta 2 rocket with the GPS 2R-18 spacecraft.

Today's launch will be:

• The 331st Delta rocket launch since 1960
• The 9th to fly this year (8 Delta 2s, 1 Delta 4-Heavy)
• The 134th Delta 2 rocket mission since 1989
• The 57th GPS satellite to launch
• The 46th time a Delta 2 has launched a GPS satellite
• The 18th GPS Block 2R satellite and 5th modernized

1917 GMT (2:17 p.m. EST)

Second stage pressurization has been completed.

1915 GMT (2:15 p.m. EST)

NEW TIME CONFIRMED. The launch team was just instructed to set up for the new liftoff time of 3:04 p.m. EST (2004 GMT). To sync up with the new time, the planned hold at T-minus 4 minutes will be lengthened from its normal 10-minute duration to 15 minutes.

The five-minute slip in the launch was ordered to ensure the rocket doesn't lift off at a time that would take the trajectory too close to another object already in orbit.

1914 GMT (2:14 p.m. EST)

T-minus 15 minutes and holding. The countdown has entered the 20-minute built-in hold. The pause is designed to give the launch team a chance to work any problems or catch up on activities that might be running behind schedule. Engineers will also have time to examine all the data from the just-completed steering tests.

1910 GMT (2:10 p.m. EST)

Mission managers are looking to adjust the target launch time to 3:04 p.m. EST, five minutes later than planned. This is due to a COLA, or Collision Avoidance, period that prohibits liftoff in the initial minutes of today's window, ensuring the rocket isn't launched on a trajectory that would pass too close to an object already in space.

1909 GMT (2:09 p.m. EST)

The first stage steering checks are finished.

1905 GMT (2:05 p.m. EST)

The second stage engine slews have been completed.

1904 GMT (2:04 p.m. EST)

Technicians are 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.

1855 GMT (1:55 p.m. EST)

Range Safety is performing inhibited checks of the command destruct receivers. The CRDs would be used in destroying the Delta rocket should the vehicle veer off course or malfunction during the launch.

1843 GMT (1:43 p.m. EST)

Loading of the Delta 2 rocket's first stage liquid oxygen tank just concluded. The filling process took 27 minutes and 28 seconds, ending at 1:43:16 p.m. The tank will be replenished through the countdown to replace the supercold liquid oxygen that naturally boils away.

The rocket now stands fully fueled for liftoff at 2:59 p.m. 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 earlier this week. The nine strap-on booster rockets and third stage use solid propellants.

1840 GMT (1:40 p.m. EST)

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.

1829 GMT (1:29 p.m. EST)

Launch is exactly 90 minutes away. All systems are appear to be "go" for liftoff at 2:59 p.m. EST this afternoon from Cape Canaveral.

1825 GMT (1:25 p.m. EST)

Now 10 minutes into this approximate 25-minute process to fill the first stage liquid oxygen tank. The outer skin of the rocket is beginning to ice over as the supercold oxidizer pumps into the vehicle.

1815 GMT (1:15 p.m. EST)

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 ULA 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.

1811 GMT (1:11 p.m. EST)

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

1809 GMT (1:09 p.m. EST)

Preps for liquid oxygen loading are starting.

1759 GMT (12:59 p.m. EST)

Two hours and counting.

1757 GMT (12:57 p.m. EST)

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

1755 GMT (12:55 p.m. EST)

The latest weather forecast is predicting an 80 percent chance of conditions being acceptable for a liftoff during this afternoon's 2:59 to 3:13 p.m. EST window. All of the weather rules are "go" right now.

The outlook predicts just a few clouds at 3,000 feet, seven miles of visibility, easterly winds from 120 degrees at 10 to 12 knots and a temperature around 76 degrees F. The only concern is additional cloud cover developing that could violate the launch rules.

Should the launch be delayed to Friday for some reason, the weather odds worsen to a 60 percent chance of acceptable conditions. The predicted weather for tomorrow's 2:54 to 3:09 p.m. EST window would include scattered-to-broken clouds at 3,000 feet, a broken deck at 10,000 feet and another broken level at 25,000 feet, isolated showers in the area, seven miles of visibility, westerly winds from 270 degrees at 15 gusting to 20 knots and a temperature around 73 degrees F.

1747 GMT (12:47 p.m. EST)

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

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.

1745 GMT (12:45 p.m. EST)

The first stage fuel tank of the Delta 2 rocket has been fully loaded for today's planned 2:59 p.m. EST launch. The tank was filled with a highly refined kerosene, called RP-1, during a 20-minute, 27-second process that concluded at 12:44:52 p.m.

1741 GMT (12:41 p.m. EST)

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

1738 GMT (12:38 p.m. EST)

The launch team has computed that the full load for the first stage fuel tank is 10,054 gallons.

Once the tank is filled to 98 percent or 9,800 gallons, the "rapid load" valve will be closed and the slower "fine load" phase will continue to top off the tank.

1731 GMT (12:31 p.m. EST)

First stage propellant loading has passed the 4,000-gallon mark. This process to load the kerosene fuel takes about 20 minutes.

1729 GMT (12:29 p.m. EST)

The countdown is entering the final two-and-a-half hours to liftoff of the GPS 2R-18 spacecraft aboard the Delta 2 rocket. Fueling is underway, the launch team is not reporting any significant technical issues with the vehicle and the weather forecast looks good for a liftoff today.

1724 GMT (12:24 p.m. EST)

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.

1721 GMT (12:21 p.m. EST)

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.

1719 GMT (12:19 p.m. EST)

First stage helium and nitrogen pressurization is complete. And the "go" has been given for the start of fueling.

1703 GMT (12:03 p.m. EST)

The launch team has started activation of the rocket's guidance system, as well as the pressurization steps for the first and second stage helium and nitrogen systems and second stage fuel tanks.

1659 GMT (11:59 a.m. EST)

BEGIN COUNT. The Terminal Countdown has been initiated for today's launch of the Global Positioning System 2R-18 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 2:59 p.m. EST from pad 17A at Cape Canaveral Air Force Station in Florida.

The weather forecast remains favorable with an 80 percent chance of acceptable conditions during today's 14-minute launch window.

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.

1650 GMT (11:50 a.m. EST)

The launch team has been polled to ensure all stations are manned and systems are prepared to proceed with the countdown. Everyone reported "ready."

Liftoff of the Delta 2 rocket remains targeted to occur ontime at 2:59 p.m. EST.

1559 GMT (10:59 a.m. EST)

T-minus 150 minutes and holding. Clocks are entering a planned 60-minute built-in hold in the countdown. Over the next hour, all launch team members and management officials will be seated at their consoles. A series of polls will be conducted to verify everyone is ready to enter Terminal Count at end of the built-in hold.

1435 GMT (9:35 a.m. EST)

A collection of photos from this morning's sunrise rollback of the mobile service tower is posted here.

1355 GMT (8:55 a.m. EST)

Retraction of the service gantry from around the Delta 2 rocket at pad 17A was completed earlier this morning. Pre-launch chores are underway at the complex to set the stage for the Terminal Count to begin in a couple of hours.

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. The tower also provided the primary weather protection and worker access to the rocket during its stay on the oceanside complex.

Liftoff remains targeted for 2:59 p.m. today.

WEDNESDAY, DECEMBER 19, 2007

A modernized replacement satellite for the Global Positioning System awaits launch Thursday afternoon aboard a Delta 2 rocket from the Florida spaceport at Cape Canaveral.

Liftoff of the United Launch Alliance booster is scheduled for 2:59 p.m. EST (1959 GMT) from pad 17A. The day's available launch window extends 14 minutes to 3:13 p.m. EST (2013 GMT).

The weather outlook calls for an 80 percent chance of favorable conditions. Cloud cover will be the only concern.

Meteorologists are predicting scattered cumulus clouds at 2,500 and scattered cirrus clouds at 25,000 feet, good visibility, easterly winds 10 to 12 knots and a temperature around 73 degrees F.

The GPS 2R-18 spacecraft is the fifth in a series of eight with enhanced features designed to rejuvenate the GPS constellation.

The modernized satellites transmit additional signals and feature improvements aimed at greater accuracy, tougher resistance to interference and enhanced performance for users around the world.

The new civilian signal removes navigation errors caused by the Earth's ionosphere. The military advancements will provide a more robust jam-resistant signal and enable better targeting of GPS-guided weapons in hostile environments.

Launch readiness reviews held Wednesday gave an "all systems go" to enter into the countdown early Thursday. Rollback of the mobile service tower is expected sunrise as ground crews button up the rocket and launch pad for the final Cape liftoff of the year.

The Terminal Countdown commences at 11:59 a.m. EST, followed shortly thereafter by loading kerosene fuel and then supercold liquid oxygen into the first stage.

The 12-story Delta 2 rocket is 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 4,500-pound 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.

"Built by a dedicated, experienced team, this advanced satellite will provide another step forward in our customer's goal to modernize the GPS constellation," said Don DeGryse, Lockheed Martin's vice president of Navigation Systems. "We are extremely proud of our partnership with the Air Force to maintain and enhance the vitally important GPS mission and we look forward to achieving mission success for our customer."

Controllers will maneuver the $75 million craft into the Plane C, Slot 1 position of the constellation to take the place of GPS 2A-24. That satellite then moves into another role replacing the ailing GPS 2A-20 satellite, which was launched in May 1993 with a seven-year design life.

The orbiting network emits continuous navigation signals that allow users to find their position in latitude, longitude and altitude and determine time.

"The Air Force is committed to providing improved capabilities to ensure users around the globe receive the maximum benefit of the positioning, navigation and timing service provided by GPS," said Col. David Madden, commander of the Space and Missile Systems Center's Global Positioning Systems Wing.

Watch this page for live updates throughout the countdown and launch!