The nation's newest Global Positioning System satellite has completed its post-launch checkout and entered service in the orbiting constellation.

The Air Force's GPS 2F-2 spacecraft was boosted into orbit July 16 from Cape Canaveral atop a United Launch Alliance Delta 4 rocket.

Controllers have spent the past month getting the Boeing-built satellite prepped to transmit precision navigation and timing information to users across the planet.

"This next-generation GPS 2F satellite has been set healthy and is ready to begin providing a strong, clear and secure signal," said Air Force Col. Bernard Gruber, director of the GPS Directorate. "The Air Force and allied military forces around the world use GPS devices in virtually every system to improve their capabilities and effectiveness while reducing risk to the warfighter."

The 50th Space Wing's 2nd Space Operations Squadron at Schriever Air Force Base in Colorado took command of the satellite on August 19 to add GPS 2F-2 into the operational network.

"We're extremely honored to accept command and control of the second GPS 2F satellite," said Lt. Col. Jennifer Grant, 2 SOPS commander. "We look forward to continuing to provide our warfighters and civil users with the most accurate position, navigation, and timing signal available in the history of GPS. We are proud to be the world's gold standard."

This is the second satellite in the Block 2F series of GPS spacecraft with improved accuracy, enhanced internal atomic clocks, better anti-jam resistance, a civil signal for commercial aviation and a longer design life. Boeing is building a dozen craft to upgrade the constellation's foundation over the coming years.

"GPS is deeply woven into everyday life and is the foundation of global, civil, commercial and defense applications for more than 1 billion users worldwide," said Craig Cooning, vice president and general manager of Boeing Space & Intelligence Systems. "Boeing has a long legacy of support on the GPS program to the U.S. Air Force, and the delivery of this new 2F satellite augments the constellation's ability to provide highly accurate, three-dimensional position, velocity and timing information 24 hours a day in all weather conditions."

Continuing a prolific partnership that has benefited billions of users around the world, the Delta rocket family today successfully launched its 50th satellite for the Global Positioning System.

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Check out this collection of beautiful liftoff photos taken by Pat Corkery for United Launch Alliance.

Check back later this morning for a launch wrapup story and photos.

"Congratulations to the entire team on the successful launch of the GPS 2F-2 satellite," said Jim Sponnick, United Launch Alliance's vice president of mission operations. "Today's one-at-a-time launch represents the 50th successful GPS launch on a Delta launch vehicle. We are proud to have served alongside the government and contractor teams over the last two decades in successfully launching GPS missions for the U.S. Air Force and providing important capabilities for our national defense and for millions of civilian and commercial users around the world."

"I am extremely proud of the tremendous efforts that hundreds of people on the launch team have expended for today's launch. I would like to thank United Launch Alliance, Boeing, my launch vehicle team from SMC's Launch and Range Systems Directorate and a host of others for their dedication to mission success," Col. Bob Hodgkiss, the GPS 2F-2 mission director, said in a post-launch press release.

"I'm extremely pleased with today's successful launch; the GPS system's overall navigational accuracy will improve as more GPS 2F space vehicles are put into service," Col. Bernie Gruber, director of SMC's Global Positioning Systems Directorate, said in a post-launch press release. "The improved accuracy, reliability and security of the GPS system ensure that the Air Force will continue to meet its navigation and timing commitments to GPS users around the world."

T+plus 3 hours, 33 minutes, 11 seconds. SPACECRAFT SEPARATION! The newest satellite for the Global Positioning System has been deployed into orbit from the United Launch Alliance Delta 4 rocket's upper stage, completing today's successful ascent.

The 3,400-pound spacecraft, built by The Boeing Company, is a next generation navigation satellite that will be bring new advancements to the orbiting constellation. It should be ready for use in a month.

T+plus 3 hours, 31 minutes. Just two minutes away from casting GPS 2F-2 free from its launcher.

T+plus 3 hours, 30 minutes. The rocket has achieved the correct roll rate as planned.

T+plus 3 hours, 29 minutes. Spin-up of the rocket stage has started in preparation for releasing GPS 2F-2 about four minutes from now.

T+plus 3 hours, 27 minutes. The upper stage is reorienting itself into the proper attitude for release of the satellite.

T+plus 3 hours, 22 minutes, 29 seconds. Final burn complete! The RL10B-2 motor fired back to life for the third of three burns to send the GPS 2F-2 spacecraft into the proper orbit for the navigation network.

T+plus 3 hours, 21 minutes, 40 seconds. Good burn in progress as the vehicle flies at 10,000 feet per second.

T+plus 3 hours, 21 minutes, 10 seconds. Engine chamber pressure is right on target.

T+plus 3 hours, 20 minutes, 53 seconds. Ignition! The upper stage if firing once again. This burn will last approximately 99 seconds in duration.

T+plus 3 hours, 19 minutes. The upper stage of the Delta 4 rocket is nearing the end of its long coast upward away from Earth. The motor is preparing to burn its engine one more time to propel GPS 2F-2 the rest of the way into orbit.

T+plus 3 hours, 17 minutes. The vehicle is 11,037 miles in altitude within view of the Guam and Diego Garcia tracking stations.

T+plus 3 hours. Now 20 minutes away from engine re-start on the upper stage. In preparation for that event, the rocket will complete the BBQ thermal roll and begin maneuvering itself into the proper orientation for ignition. Then, the stage will start a system chilldown with cryogenic fuel and use its tiny thrusters to settle the propellants in the tanks before firing the engine.

Ignition occurs at T+plus 3 hours, 20 minutes for a 99-second firing. Spacecraft separation happens at T+plus 3 hours, 33 minutes.

T+plus 2 hours, 30 minutes. The rocket is soaring away from Earth in its current egg-shaped orbit. The upcoming engine firing at 6:01 a.m. EDT will occur as the rocket nears apogee. The burn lasts 99 seconds and puts the spacecraft into an 11,045 nautical mile circular orbit inclined 55 degrees to the equator.

Deployment of GPS 2F-2 from rocket to complete the launch is expected at 6:14 a.m. EDT.

T+plus 2 hours. The upper stage has completed one passive roll program and just started another. This one will continue until it's almost time for the next burn. The maneuvers ensure the thermal conditions are distributed across the vehicle.

T+plus 100 minutes. Unlike the previous generations that rode on smaller rocket Delta 2 rockets, the dozen next-generation GPS 2F craft will be delivered into orbit atop the Delta 4 and Atlas 5 fleets of boosters in the Air Force's Evolved Expendable Launch Vehicle program.

The Delta 2's, although highly reliable, weren't powerful enough to inject the GPS satellites directly into the orbiting constellation. The three-stage vehicles released the craft into a highly elliptical orbit stretching from 100 miles at its closest point with Earth to some 11,000 miles at its highest point, which is the altitude where the GPS network resides. The newly-launched satellites were themselves fitted with a solid-fuel kick motor that ignited a few days into flight and finished the job of propelling the craft into a circular orbit.

United Launch Alliance's powerful Delta 4 and Atlas 5 rockets will haul the GPS 2F satellites directly to their desired destinations, bypassing the circuitous route of the past. So instead of taking days to reach the GPS orbit, the new 2F-2 satellite will get there in three-and-a-half hours today.

Check out this photo gallery of the launch as seen from the NASA Causeway.

T+plus 65 minutes. To recap, the Delta 4 rocket carrying the GPS 2F-2 navigation satellite launched from Cape Canaveral's Complex 37 at 2:41 a.m. EDT after a flawless countdown and some welcomed cooperation by the weather.

All appeared to go smoothly during the ascent and no problems were reported by the telemetry folks as the vehicle flew to its measured waypoints en route to orbit.

The rocket is now coasting toward a point about 11,000 nautical miles above the planet where the third and final burn is planned at T+plus 3 hours and 20 minutes.

T+plus 30 minutes. The rocket has just entered a two-hour, 56-minute coast period before the final engine burn to deliver GPS 2F-2 into the desired circular orbit to join the navigation network.

T+plus 26 minutes. Delta has flown into the transfer orbit with an apogee of 11,025 nautical miles, perigee of 129 nautical miles and inclination of 43.3 degrees.

T+plus 24 minutes, 43 seconds. SECO 2. The second of three firings by the upper stage during tonight's launch has been completed.

T+plus 24 minutes. This burn injects the rocket into a highly elliptical orbit stretching out to the GPS constellation at its furthest point.

T+plus 23 minutes, 30 seconds. About one minute left in this burn by the Delta 4 rocket's upper stage.

T+plus 22 minutes, 45 seconds. Engine chamber pressure, pitch and yaw control, all looking good.

T+plus 22 minutes. This burn will last just over three minutes in duration.

T+plus 21 minutes, 45 seconds. Good chamber pressure reported as the engine gets up and running.

T+plus 21 minutes, 24 seconds. Ignition! The RL10B-2 engine, fed by liquid hydrogen liquid oxygen, has been reignited to propel GPS 2F-2 into an intermediate orbit.

T+plus 17 minutes, 20 seconds. The vehicle is now in a coast mode before the next firing of the upper stage engine. Ignition of the RL10 engine is now four minutes away.

T+plus 15 minutes. Delta has flown into a parking orbit with an apogee of 216 nautical miles, perigee of 100 nautical miles and inclination of 41.6 degrees.

T+plus 12 minutes, 45 seconds. Delta is 168 miles in altitude, 1,819 miles downrange from the launch pad, traveling at 25,294 feet per second.

T+plus 12 minutes, 17 seconds. SECO 1. The upper stage engine has shut down after the first of three planned firings to inject the GPS spacecraft into navigation network today.

T+plus 11 minutes, 40 seconds. Delta is 172.1 miles in altitude, 1,576 miles downrange from the launch pad, traveling at 24,400 feet per second.

T+plus 10 minutes, 50 seconds. The upper stage's the RL10 engine working well, still firing as planned to reach a low-altitude parking orbit.

T+plus 10 minutes, 30 seconds. The vehicle's steep climb has leveled off as it gains speed toward orbital velocity.

T+plus 10 minutes. Delta is 170 miles in altitude, 1,241 miles downrange from the launch pad, traveling at 22,581 feet per second.

T+plus 9 minutes, 20 seconds. About three minutes remain in this firing of the upper stage to place the vehicle into an initial parking orbit around Earth.

T+plus 9 minutes, 15 seconds. The RL10 engine is burning a mixture of liquid oxygen and liquid hydrogen cryogenic propellants. Chamber pressures still look good.

T+plus 8 minutes, 30 seconds. Delta is 160 miles in altitude, 951 miles downrange from the launch pad, traveling at 21,072 feet per second.

T+plus 7 minutes, 55 seconds. Good pitch and yaw controls on the upper stage engine.

T+plus 7 minutes, 25 seconds. Delta is 144.5 miles in altitude, 723 miles east of the launch pad, traveling at 19,937 feet per second.

T+plus 7 minutes. First stage flight events occurred very close to planned times.

T+plus 6 minutes, 40 seconds. Rock steady chamber pressure on the upper stage engine.

T+plus 6 minutes, 5 seconds. Delta is 122.8 miles in altitude, 522 miles east of the launch pad.

T+plus 5 minutes, 25 seconds. Good chamber pressure data being reported from the RL10 engine as it thrusts to reach orbit.

T+plus 5 minutes. Delta is 97.1 miles in altitude, 350 miles downrange from the launch pad.

T+plus 4 minutes, 42 seconds. The protective payload fairing enclosing the GPS satellite atop the rocket has separated.

T+plus 4 minutes, 30 seconds. The cryogenic RL10B-2 upper stage engine has ignited!

T+plus 4 minutes, 15 seconds. The Common Booster Core first stage and the attached interstage have been separated in one piece from the Delta 4's upper stage. The upper stage engine's extendible nozzle is dropping into position.

T+plus 4 minutes, 9 seconds. MECO! Main engine cutoff confirmed as the RS-68 powerplant shuts down.

T+plus 4 minutes. The main engine is throttling down to 57 percent thrust in preparation for shutdown.

T+plus 3 minutes, 35 seconds. Delta is 53.7 miles in altitude, 129 miles downrange from the launch pad.

T+plus 3 minutes, 25 seconds. Good pitch and yaw controls on the main engine.

T+plus 3 minutes, 15 seconds. The main engine continues to perform well, consuming its liquid hydrogen and liquid oxygen propellants.

T+plus 2 minutes, 45 seconds. Passing Mach 7.

T+plus 2 minutes, 30 seconds. The RS-68 is consuming nearly a ton of propellants per second as the powerplant pushes the Delta 4 rocket closer to the edge of space.

T+plus 2 minutes, 15 seconds. Delta is 26.5 miles in altitude, 31.9 miles downrange from the launch pad.

T+plus 1 minute, 42 seconds. Solid motor separation! The spent boosters have been shed from the first stage. Delta 4 continues powering its way toward space on the thrust generated by the RS-68 main engine.

T+plus 1 minute, 35 seconds. The twin solid rocket boosters have burned out of their propellant. Standing by for jettison.

T+plus 60 seconds. Now passing through the region of maximum aerodynamic pressure as the vehicle accelerates through the lower atmosphere.

T+plus 50 seconds. The launcher has broken through the sound barrier.

T+plus 30 seconds. The Delta 4 rocket is climbing away from Cape Canaveral with its main engine firing at full throttle and the two strap-on boosters giving a powerful extra kick.

T-minus 10, 9, 8, sequencer now controlling, T-minus 6, 5, main engine start, 3, 2, 1, SRB ignition and LIFTOFF! Liftoff of the Delta 4 rocket, building the future foundation of the Global Positioning System!

T-minus 12 seconds. Ignitors are armed and the residual hydrogen burnoff sparklers have fired beneath the main engine.

T-minus 22 seconds. The steering system for the twin solid rocket motor nozzles has been activated.

T-minus 30 seconds. Green board. All systems remain "go" for launch.

T-minus 35 seconds. Upper stage liquid hydrogen tank is confirmed at flight level.

T-minus 1 minute and counting. The ignition conditions for the RS-68 main engine are "go." The terminal countdown sequencer will take control at T-minus 8.5 seconds. Ignition of the RS-68 powerplant will follow at T-minus 5.5 seconds. The engine powers up to the 102 percent level of thrust for a computer-controlled checkout before liftoff.

T-minus 70 seconds. The Air Force-controlled Eastern Range has given its "go" for launch.

T-minus 75 seconds. Upper stage liquid oxygen tank has been secured at flight level.

T-minus 80 seconds. The liquid hydrogen fuel tank on the upper stage is being secured for launch.

T-minus 1 minute, 40 seconds. The first stage liquid hydrogen tank has reached flight level and pressure.

T-minus 2 minutes. The rocket's upper stage liquid oxygen tank is being secured.

T-minus 2 minutes, 35 seconds. The liquid oxygen tank in the first stage is confirmed at the proper level and pressure for flight.

T-minus 3 minutes, 15 seconds. Replenishment of liquid hydrogen and liquid oxygen to the Common Booster Core first stage is being secured in preparation to pressurize the tanks for launch.

T-minus 4 minutes and counting. Ordnance devices aboard the vehicle are being armed.

T-minus 4 minutes, 10 seconds. The systems of the first and upper stages of the Delta 4 rocket have switched from ground-fed power to internal batteries for launch.

T-minus 5 minutes and counting! The final phase of today's countdown has commenced for launch of the Delta 4 rocket carrying the GPS 2F-2 navigation satellite. Liftoff is set to occur at 2:41 a.m. EDT from pad 37B at Cape Canaveral Air Force Station.

The launch director has given permission to resume the count for liftoff at 2:41 a.m. EDT.

The GPS 2F-2 spacecraft atop the Delta 4 has switched from ground-fed power to its internal batteries for launch.

The launch pad swing arm retraction system pins are being pulled. The three arms will be rotated away from the Delta 4 rocket at liftoff.

The final readiness polls of the launch team and mission managers have been completed. No technical problems are being worked and all rocket, spacecraft and ground systems appear are "go" for liftoff at 2:41 a.m. EDT.

Standing by for the launch team readiness check for continuing the countdown.

A load relief file based on the upper level winds is being transmitted to the Delta 4 rocket's flight computer. This program informs the rocket on the wind conditions it can expect to see during ascent.

T-minus 5 minutes and holding. The countdown has entered a planned 15-minute hold. This pause is designed to give the launch team the opportunity to catch up on any work running behind schedule and verify all is in readiness for the final moments of the count. A series of management polls will be conducted during the hold to give approval to proceed with the launch.

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About three-and-a-half hours after launch, the Delta 4 rocket will deliver its satellite payload directly to the GPS constellation's altitude 11,000 nautical miles high. Ground controllers expect to have the new bird checked out and ready for use in about 30 days, the Air Force says. The new GPS 2F-2 spacecraft will take over the Plane D, Slot 2A location of the network, replacing a satellite that just celebrated its 20th birthday.

The Air Force launch weather officer has given his final planned briefing to mission managers in the countdown. The chance of isolated showers has been removed from the forecast and the odds of acceptable launch conditions have improved to 90 percent.

T-minus 30 minutes and counting. The countdown is headed to the T-minus 5 minute point where a 15-minute hold is planned. Liftoff is targeted for 2:41 a.m., which is the opening of today's 19-minute launch window to reach the desired location within the GPS constellation for replacing an aging satellite.

The latest tweaks to the launch weather forecast now calls for a few clouds at 2,500 feet, scattered at 15,000 feet and a broken deck at 22,000 feet, isolated showers in the vicinity, north-northwesterly winds of 5 to 10 knots and a temperature of 77 degrees F.

The odds of acceptable launch weather remain at 80 percent.

The ULA launch team and management are guiding the countdown from the Delta Operations Center, located about 9,200 feet from the pad. The engineers overseeing the rocket and ground systems are located on the third floor and the Mission Directors Center room is on the fourth floor. Both rooms have a view of pad 37B and the Delta 4 rocket out their windows. Other rooms are also set up for engineering support.

The DOC was formerly built to support the Titan-Centaur program but was refurbished to support Delta 4.

The RS-68 first stage main engine spin start pressurization operation is starting.

No issues are being worked in the countdown and the weather continues to look good for launch at 2:41 a.m.

The safety system checks are underway at this point in the countdown.

The countdown is entering the final 90 minutes until the Delta 4 rocket launch from Cape Canaveral at 2:41 a.m. EDT. Here's a look at some stats about today's mission. This will be:

• The 355th Delta rocket launch since 1960
• The 17th Delta 4 rocket mission since 2002
• The 14th Delta 4 rocket launch from Cape Canaveral
• The 8th Medium+ 4,2 configuration to fly
• The 10th Delta 4 under the ULA banner
• The 3rd Delta 4 launch this year
• The 62nd GPS satellite to launch
• The 51st GPS launch on a Delta rocket
• The 2nd GPS Block 2F satellite

The RF checks are complete.

Radio frequency link checks between tracking facilities at the Cape and the launch vehicle are underway.

The flight slews and commanding tests for the vehicle steering systems are complete and satisfactory.

Now confirmation that propellant conditioning has been completed on the first stage as well. The Delta 4 rocket is fueled up and ready for today's launch.

Both cryo tanks on the upper stage have completed their pre-flight conditioning and remain in topping.

The launch team is kicking off the steering checks for the RS-68 main engine and RL10 upper stage engine, plus commanding tests with the solid rocket motors.

Check out this photo gallery showing the GPS 2F-2 satellite being encapsulated in the rocket's nose cone last month.

The thermal insulation inspections have been completed following fueling and there were no problems reported tonight.

The first stage liquid hydrogen tank has entered the topping phase after the post-fueling tests were accomplished. That puts all four cryogenic fuel tanks aboard the Delta 4 rocket in topping mode.

The newly revised weather forecast for the 2:41 a.m. EDT launch time now calls for a few low clouds, scattered mid-level clouds and a broken deck of high clouds, isolated showers in the vicinity, west-southwesterly winds of 5 to 10 knots and a temperature of 77 degrees F.

The odds of favorable weather have jumped to 80 percent.

Weather is "go" for launch! The evening storms have cleared away, current conditions are within limits for liftoff and the forecast is looking very positive now.

Engineers are conducting the standard evaluation of the Delta 4 rocket's thermal insulation following the loading of supercold propellants into the vehicle.

Upper stage liquid oxygen topping is beginning.

All is quiet at this point in the countdown with fueling operations nearly complete. The countdown remains on target for a liftoff at 2:41 a.m. EDT.

And the upper stage liquid oxygen filling has been completed, now heading into post-loading tests.

The first stage liquid oxygen system is conditioned for flight.

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Weather forecasters believe the constraint for anvil clouds will be cleared perhaps in the next half-hour and that the conditions should be favorable for the remainder of the countdown to launch.

The topping mode is starting for the upper stage's liquid hydrogen tank.

The chilldown of the upper stage liquid oxygen system is reported complete, clearing the way for loading the rocket's tank with 4,500 gallons. This is the last of the rocket's four cryogenic supplies to be filled in today's countdown to launch.

Post-filling tests on the first stage liquid oxygen system are complete. The tank is now entering topping mode.

Fast-fill loading of the upper stage liquid hydrogen tank has been accomplished as fueling proceeds very smoothly this evening at Complex 37.

The "go" has been given for the upper stage liquid oxygen chilldown in advance of filling that tank.

Fast-filling of the first stage liquid hydrogen reservoir, the latest of the tanks to be loaded today, is wrapping up. After post-filling checks and valve tests, the tank will be placed in topping mode.

The first stage liquid oxygen loading just finished. The tank has been loaded with its supercold oxidizer that is chilled to Minus-298 degrees F. Topping will be completed as the count rolls on.

One of the two weather rules that had been "red" just went "green" as conditions get better. The field mills that measure the lightning potential were out of limits throughout the evening, but those readings are "go" for launch now.

The launch team is actively loading the upper stage's liquid hydrogen tank with 10,000 gallons of propellant for the RL10 engine.

Now four-and-a-half hours till launch. The Delta 4 rocket's Common Booster Core first stage and upper stage are being loaded with supercold liquid oxygen and liquid hydrogen.

Complex 37 has two giant sphere-shaped fuel tanks to store the cryogenic liquid oxygen and liquid hydrogen. The LOX tank holds 250,000 gallons and LH2 sphere about 850,000 gallons.

The cryogenics are fed from the storage tanks through pipelines to the pad. For the Common Booster Core, the propellants are routed up to the launch table upon which the rocket sits. Tail service masts, the large box-like structures at the base of the vehicle, feed the oxygen and hydrogen to the booster via separate umbilicals.

The upper stage receives its cryos from the middle swing arm that extends from the Fixed Umbilical Tower to the front-side of the rocket.

The chilldown of the upper stage liquid hydrogen system is reported complete, clearing the way for loading the rocket's tank.

A current check of the weather shows two rules -- anvil clouds and the electrical potential in the air -- remain "red" at this time. But the lightning advisory that's been in effect all evening will be allowed to expire at 10 p.m. as conditions gradually improve.

First stage liquid oxygen tanking operation is switching from "slow-fill" to "fast-fill" mode. The rocket will be loaded with 40,000 gallons of supercold LOX.

As tonight's countdown continuing to go very smoothly, the launch team is preparing to start fueling the Delta 4 rocket's upper stage. The "go" has been given to start the chilldown conditioning of the upper stage liquid hydrogen system.

The first stage liquid oxygen chilldown is complete. "Slow-fill" mode is beginning to load a small percentage of the tank. The process then speeds up to the "fast-fill" mode until the tank is nearly full.

First stage liquid hydrogen tanking operation is switching from "slow-fill" to "fast-fill" mode.

And now the chilldown of Delta's first stage liquid oxygen system is starting. This preps the tank and pumping to guard against shock when the supercold oxidizer begins flowing into the rocket a short time from now.

Liquid hydrogen flow to the first stage is confirmed. Some 110,000 gallons of LH2 will fill the rocket's tank.

The cold gas chilldown conditioning of the liquid hydrogen system has been accomplished. Liquid hydrogen propellant will begin to flow into the first stage in "slow-fill" mode. That is sped up to "fast-fill" after a small portion of the tank is loaded.

Chilled to Minus-423 degrees Fahrenheit, the liquid hydrogen will be consumed by the RS-68 main engine along with liquid oxygen during the first four minutes of the launch.

And fueling officially has begun. The hydrogen system's cold gas chilldown conditioning is underway.

A "go" has been given to start the cold gas chilldown conditioning of the first stage liquid hydrogen system. This is the precursor to actually filling the vehicle with propellant.

T-minus 6 hours, 15 minutes and counting! The Terminal Countdown begins now for tonight's flight of the United Launch Alliance Delta 4 rocket with the Global Positioning System 2F-2 satellite.

The count has a planned built-in hold at T-minus 5 minutes lasting some 15 minutes, leading to liftoff at 2:41 a.m. EDT (0641 GMT).

Technicians have finished all hands-on work at the launch pad and Complex 37 has been cleared of personnel in advance of fueling the rocket's two stages this evening. Pad systems and equipment stand ready for fueling activities, which should kick off shortly.

No technical issues are being worked in the countdown and the current weather conditions are acceptable for fueling to start on schedule around 8:30 p.m.

The launch team has been polled to verify all consoles are manned and ready to begin the Terminal Countdown when this hold ends at 8:11 p.m. Officials are pressing onward with optimism the weather will improve over the next few hours.

Stormy weather is blanketing Central Florida tonight, bringing rain, thunder and lightning to the Space Coast. At the present time, a few of the launch commit criteria weather rules are "red" due to the cloud cover and electrical potential. But for the overnight launch time at 2:41 a.m. EDT, forecasters still predict a 60 percent chance of acceptable conditions then.

T-minus 6 hours, 15 minutes and holding. The countdown has entered a planned 60-minute built-in hold, a pre-planned pause designed to give the team time to catch up on any work that could be running behind schedule. Once the clocks resume ticking at 8:11 p.m. EDT, the Terminal Countdown phase of tonight's launch operation will begin.

Check out today's gantry rollback photo gallery taken by ULA's Pat Corkery.

The 330-foot tall mobile service tower has been retracted from the Delta 4 rocket at Cape Canaveral's pad 37B for tonight's launch that will place the GPS 2F-2 satellite into Earth orbit.

The wheeled structure just moved along rail tracks to its launch position about the length of a football field away from the rocket. The 9-million pound tower shielded the Delta from the elements during the its stay on the pad, provided workers 360-degree access to the various areas on the vehicle and was used to attach the strap-on solid motors and the payload during the launch campaign. The tower is 90-feet wide and 40-feet deep.

Crews will spend the next couple of hours securing the complex for launch before leaving the danger area around the pad. All workers must be clear of the area for the start of hazardous operations in the countdown, which include fueling the vehicle later tonight.

The Terminal Countdown will begin ticking at 8:11 p.m. EDT, leading to the multi-hour process of loading the Delta 4's Common Booster Core first stage and the second stage with supercold liquid oxygen and liquid hydrogen propellants.

Testing of communications links between the rocket and Air Force Eastern Range will occur after fueling is accomplished. Steering checks of the first stage RS-68 engine and second stage RL10 powerplant are on tap in the last hour of the count.

A 15-minute build-in hold is slated for T-minus 5 minutes, during which time teams will go through final polling to grant clearance to launch. The Delta 4 will transition to internal power as the count resumes, ordnance will be armed and the propellant tanks pressurized as clocks target the main engine ignition time at T-minus 5.5 seconds.

Liftoff remains scheduled for 2:41 a.m. EDT, the opening of a 19-minute window that extends to 3:00 a.m. EDT (0641-0700 GMT).

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Racing to beat any afternoon storms, tower rollback has begun for tonight's launch of the Delta 4 rocket.

The United Launch Alliance team will give it another shot today, hoping the weather allows rollback of the mobile gantry to uncover the Delta 4 rocket for Saturday's early morning blastoff carrying the Global Positioning System 2F-2 satellite.

"Combination of a weak low pressure trough moving down from the north and daily development of east coast sea breeze will again lead to scattered afternoon and evening thunderstorms across the Space Coast on Friday," the Air Force weather team reported this morning.

"Sea breeze will move farther inland over southern Brevard County and remain closer to the coast over northern Brevard County. Thunderstorms along the boundaries will move slowly from west-northwest to east-southeast during the afternoon and evening hours.

"The low pressure trough will also lead to a higher coverage of thunderstorms over the coast waters after sunset. This low pressure trough will remain across central or southern Florida and influence the weather throughout the weekend and into early next week."

Once the 2:41 a.m. EDT launch time arrives tomorrow morning, meteorologists predict a 60 percent chance that weather will be acceptable for liftoff.

"Main concern during the launch window will be for any remnant cloud cover associated with the earlier thunderstorm activity over the land and water," forecasters say.

The outlook includes a few low clouds, scattered mid-level clouds and a broken deck of high clouds, isolated showers in the vicinity, easterly winds of 5 to 10 knots and a temperature of 77 degrees F.

The odds of favorable weather for the backup launch opportunities on Sunday and Monday mornings also stand at 60 percent.

Although there's no Delta launch tonight after all, check out these photos taken at the pad earlier today.

After having to scrub tonight's launch attempt because stormy weather prevented the team from retracting the protective mobile service gantry away from the Delta 4 rocket, officials plan to try the launch again in 24 hours. The new target liftoff time is 2:41 a.m. EDT on Saturday, with a window extending 19 minutes to 3:00 a.m.

DELAY! Tonight's launch has been scrubbed due to bad weather. Rollback of the launch pad tower, which was due to take place this afternoon, had been delayed for a couple hours due to weather concerns. No word at this point when the launch might be rescheduled.

The second new-generation satellite for the Global Positioning System is awaiting a predawn blastoff into the navigation network Friday from Cape Canaveral.

The GPS 2F-2 spacecraft is targeting a 2:45 a.m. EDT launch atop a Delta 4 rocket. A 19-minute window extending to 3:04 a.m. EDT will be available if conditions prevent an on-time liftoff.

We will have complete live coverage of the countdown and launch on this page.

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Forecasters are predicting a 70 percent chance of favorable weather, with the only concern being cloud cover.

"On launch day conditions aloft will consist of an upper level high pressure center over the southeast U.S. This will result in east-southeast flow in the mid and upper levels across Florida. At the surface, a high pressure ridge axis will be across southern Florida resulting in southwest flow in the lower levels. Thunderstorms will develop along the sea breeze initially, then along thunderstorm outflow boundaries across central Florida. These thunderstorms will move slowly from west-southwest to east-northeast during the late afternoon and evening hours," Air Force meteorologists say.

"At the beginning of the launch window any remaining thunderstorm activity is expected to be either dissipated or offshore. Main concern during the launch window will be for any remnant cloud cover associated with the earlier thunderstorm activity.

The launch time outlook includes a few clouds, good visibility, south-southwesterly winds of 5 to 10 knots and a temperature around 77 degrees F.

Boeing developed a dozen Block 2F satellites to sustain and further upgrade the Global Positioning System for the U.S. Air Force. The first was launched 14 months ago.

About three-and-a-half hours after launch, the Delta 4 rocket will deliver the satellite directly to the GPS constellation's altitude 11,000 miles high. Ground controllers expect to have the new bird checked out and ready for use in about a month.

The new GPS 2F-2 spacecraft take over the Plane D, Slot 2A location of the network, which is divided into six orbital groupings with multiple satellites flying in each.

The craft replaces the long-lived GPS 2A-11 satellite launched by a Delta 2 rocket from Cape Canaveral on July 3, 1991.

The Delta 4 rocket launch carrying the next Global Positioning System satellite into orbit from Cape Canaveral has been delayed by 24 hours.

Originally planned for liftoff Thursday, the mission has been postponed to Friday morning at 2:45 a.m. EDT (0645 GMT). A reason for the rescheduling was not immediately announced.

Weather forecasters are predicting a 70 percent chance of acceptable conditions for the predawn blastoff. The chance of lingering cloud cover from evening thunderstorms will pose the main concern during the launch window.

The outlook calls for scattered low, mid and high clouds, isolated showers or thunderstorms in the area, southwesterly winds of 8 to 12 knots and a temperature of 78 degrees F.

The 20-story rocket will haul the GPS 2F-2 spacecraft directly into the navigation network to replace an aging bird for providing highly precise location and timing information for military and civilian users around the globe.

Watch this page for complete live coverage of the launch.

A Global Positioning System satellite has been loaded aboard its ride to space, arriving Monday at Cape Canaveral's Complex 37 for mounting atop the United Launch Alliance Delta 4 rocket.

Liftoff of the GPS 2F-2 spacecraft is scheduled for July 14 during a 19-minute window extending from 2:49 to 3:08 a.m. EDT (0649-0708 GMT).

The launch is timed to deliver the satellite into the GPS constellation to replace a 20-year craft that has long exceeded its expected lifespan.

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