An Atlas 5 rocket successfully powered a replacement Global Positioning System satellite into space Wednesday to bolster the navigation utility for military and civil users around the world.

Read our launch story.

And here's our launch photo gallery.

"I'm extremely pleased with today's launch and delighted to be part of this mission that enhances our nation's critical GPS capability. Thanks to the superb efforts of the of the 45th and 50th Space Wings, United Launch Alliance, our industry partners, the Atlas 5 and GPS 2F launch teams, the GPS 2F-4 mission was successfully carried out," said Col.Bernie Gruber, director of the Space and Missile Systems Center's Global Positioning Systems Directorate.

"The GPS constellation remains healthy and continues to meet and exceed the performance standards to which the satellites were built. Our goal is to deliver sustained, reliable GPS capabilities to America's warfighters, our allies and civil users around the world, and this is done by maintaining GPS performance, fielding new capabilities and developing more robust modernized capabilities for the future."

MISSION SUCCESS! The Centaur upper stage completed its second burn and released the Global Positioning System 2F-4 satellite into the navigation network to complete tonight's launch of the Atlas 5 rocket.

After coasting for three hours to a point nearly 11,000 nautical miles above the planet south of Australia, the RL10 upper stage engine was re-ignited for a 90-second burn to circularize the orbit. The Boeing-built payload was deployed minutes later.

It is another successful flight for the United Launch Alliance Atlas 5 vehicle, which has completed 38 missions in the past decade, 9 in just the past 12 months and four this year.

"The ULA team is honored to place another next-generation GPS satellite on orbit for our US Air Force customer," said Jim Sponnick, ULA vice president, Mission Operations.

"Today's successful delivery of the GPS 2F-4 mission represents the 70th launch success in the 77 months since ULA was formed – an accomplishment made possible by seamless integration of the customer and industry team; reliable production and launch operation processes; and a one-launch-at-a-time focus on mission success for these critical space assets."

The next Atlas 5 will use the vehicle's most powerful version flying today, the mighty 551 with five strap-on solid rocket boosters, to launch the Navy's second MUOS mobile communications satellite on July 19 during a window of 8:51 to 9:35 a.m. EDT.

The satellite arrived at the Cape on Monday from Lockheed Martin's factory in Sunnyvale, California.

But more immediately for ULA, a Delta 4 rocket with the Air Force's WGS 5 communications satellite is set for launch next Wednesday night at 8:26 p.m. from Complex 37 at Cape Canaveral.

T+plus 3 hours, 24 minutes. SPACECRAFT SEPARATION! The Global Positioning System 2F-4 satellite has been released from the Centaur upper stage to complete today's flight from the Cape Canaveral.

T+plus 3 hours, 20 minutes. MECO 2. Main engine cutoff confirmed. Centaur has completed its second burn of the day to rendezvous with the GPS constellation for deployment of a replacement spacecraft.

This was a planned 90-second firing by the Centaur's single Pratt & Whitney Rocketdyne RL10 engine to circularize the orbit. The first burn had put the rocket into a transfer orbit.

We will pause our live updates at this time. Check back just after 9 p.m. EDT for confirmation of the second Centaur burn and deploy of the GPS 2F-4 satellite to complete today's mission.

T+plus 20 minutes. That first burn by Centaur inserted the rocket into a transfer orbit with a high point of 10,983 nautical miles, a low point of 90 miles and inclination of 55 degrees. The numbers are right on target.

T+plus 17 minutes, 15 seconds. MECO 1. Centaur's main engine has shut down following its first burn today, achieving a transfer orbit to reach the GPS network around Earth. The rocket will coast in this orbit for about 3 hours before the RL10 engine re-ignites to circularize the orbit and then deploys the satellite.

T+plus 16 minutes, 45 seconds. Centaur systems continue to be in good shape as the rocket nears the planned orbit.

T+plus 16 minutes, 25 seconds. The rocket traveling at 20,567 mph.

T+plus 16 minutes, 15 seconds. Everything looking normal with one minute to go in this burn.

T+plus 15 minutes, 25 seconds. RL10 engine parameters still look good.

T+plus 14 minutes, 45 seconds. The rocket traveling over 17,000 mph.

T+plus 13 minutes. Centaur remains on course and looking good.

T+plus 12 minutes, 15 seconds. About five minutes are left in this burn of Centaur.

T+plus 10 minutes, 30 seconds. Centaur is 240 miles in altitude, 1,457 miles downrange from the launch pad, traveling at 13,826 mph.

T+plus 10 minutes. All systems reported stable as the Centaur fires to reach an initial Earth orbit.

T+plus 8 minutes, 15 seconds. The rocket is 218 miles in altitude and traveling over 12,377 mph already.

T+plus 8 minutes. The RL10 continues to perform well, burning liquid hydrogen and liquid oxygen propellants.

T+plus 7 minutes, 20 seconds. The rocket is 194 miles in altitude and traveling at 11,00o mph.

T+plus 6 minutes, 35 seconds. The rocket has conducted a planned roll to improve the link with NASA's orbiting Tracking and Data Relay Satellite System.

T+plus 6 minutes. Centaur performance is reported right on target.

T+plus 5 minutes, 45 seconds. The rocket is tracking right down the planned flight path.

T+plus 5 minutes. Centaur engine readings look good as this burn gets underway.

T+plus 4 minutes, 45 seconds. Payload fairing jettison confirmed.

T+plus 4 minutes, 27 seconds. Centaur has ignited! The RL10 engine is up and running at full thrust for its first of two planned firings today.

T+plus 4 minutes, 17 seconds. The Atlas 5's Common Core Booster first stage has been jettisoned, and the Centaur upper stage's liquid hydrogen and liquid oxygen systems are being readied for engine start.

T+plus 4 minutes, 7 seconds. BECO. Booster Engine Cutoff is confirmed as the RD-180 powerplant on the first stage completes its burn. Standing by to fire the retro thrusters and separate the spent stage.

T+plus 3 minutes, 58 seconds. The vehicle is accelerating at 4.6 g's.

T+plus 3 minutes, 25 seconds. The rocket now weighs only a quarter of what it did at liftoff.

T+plus 3 minutes, 10 seconds. RD-180 is performing well as the rocket climbs away from the planet.

T+plus 2 minutes, 55 seconds. Reaction control system has been activated.

T+plus 2 minutes. The rocket is tracking on its northeasterly course.

T+plus 1 minutes, 45 seconds. The RD-180 main engine continues to fire normally, burning a mixture of highly refined kerosene and liquid oxygen.

T+plus 95 seconds. Maximum dynamic pressure.

T+plus 85 seconds. Mach 1.

T+plus 60 seconds. One minute into the ascent. It will take 3 hours and 24 minutes for the two-stage rocket to deploy the GPS 2F-4 satellite into the navigation network tonight.

T+plus 40 seconds. Good engine performance is reported from the rocket as the booster plots a northeasterly course from the Cape to reach the GPS constellation.

T+plus 15 seconds. The vehicle has cleared the towers at Complex 41 on 860,000 pounds of thrust from the RD-180 main engine.

LIFTOFF! Liftoff of the Atlas 5 rocket, giving lift to the GPS 2F-4 satellite to bolster the navigation utility for military and civil users around the world.

T-minus 20 seconds. "Go Atlas" and "Go Centaur" was just called by launch team during a final status check.

T-minus 40 seconds. Centaur's liquid oxygen and liquid hydrogen tanks are stable at flight pressures.

T-minus 55 seconds. Range is green.

T-minus 1 minute. Now 60 seconds away from launching an upgraded satellite to sustain and enhance the navigation and timing capabilities from the Global Positioning System.

T-minus 90 seconds. The rocket's safety system has been armed.

T-minus 1 minute, 45 seconds. Liquid oxygen and liquid hydrogen propellant topping to the Centaur upper stage is being secured.

T-minus 1 minute, 55 seconds. The launch sequencer has been commanded to start.

T-minus 2 minutes. The Atlas first stage and Centaur upper stage are now switching from ground power to internal batteries.

T-minus 2 minutes, 30 seconds. The first stage RP-1 kerosene fuel tank and the liquid oxygen have stepped up to proper flight pressure levels.

T-minus 3 minutes. The Atlas first stage liquid oxygen replenishment is being secured so the tank can be pressurized for launch.

T-minus 3 minutes, 50 seconds. The ground pyrotechnics are enabled.

T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket with a replacement satellite for the GPS navigation system.

Standing by to pick up the count.

The GPS spacecraft is running on internal power and configured for launch.

ULA and the Air Force have given their respective final approvals to resume the countdown as scheduled.

Polling of the team by Atlas launch conductor just occurred. All systems are "go" for a liftoff today at 5:38 p.m. EDT.

Standing by for the final readiness check to be conducted. The launch team will be polled for a "go" or "no go" to proceed with the count.

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The countdown remains on target for liftoff at 5:38 p.m. EDT (2128 GMT) when today's 18-minute launch opportunity opens.

The kerosene-fueled first stage will be burn for the first four minutes of flight before separating to allow the cryogenic Centaur upper stage to ignite for a 13-minute initial burn to reach a preliminary parking orbit. The nose cone is jettisoned four-and-a-half minutes after launch.

The rocket settles into a 3-hour coast to reach the GPS constellation altitude 11,000 nautical miles up before restarting the RL10 upper stage engine for a 90-second circularization burn.

The satellite is deployed 3 hours and 24 minutes after liftoff into a circular orbit 11,047 nautical miles with a inclination of 55 degrees.

Here's a look at some stats about today's mission. This will be:

• The 620th launch for Atlas program since 1957
• The 330th Atlas to occur from Cape Canaveral
• The 209th mission for the Centaur upper stage
• The 186th use of Centaur by an Atlas rocket
• The 435th production RL10 engine to be launched
• The 44th flight of the RD-180 main engine
• The 38th launch of an Atlas 5 since 2002
• The 59th Evolved Expendable Launch Vehicle flight
• The 13th Atlas 5 dedicated to the Defense Department
• The 32nd Atlas 5 to occur from the Cape
• The 26th daytime Atlas 5 from the Cape
• The 70th United Launch Alliance flight overall
• The 30th Atlas 5 under United Launch Alliance
• The 26th 400-series flight of the Atlas 5
• The 18th Atlas 5 to fly in the 401 configuration
• The 64th GPS satellite to launch
• The 43rd Boeing-built GPS satellite
• The fourth GPS Block 2F satellite
• The 12th GPS launch on an Atlas rocket
• The first GPS 2F on an Atlas 5
• The fourth Atlas launch of 2013

Twenty minutes from liftoff. Weather is GO for launch.

T-minus 4 minutes and holding. The countdown has entered the planned 25-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff.

The fuel-fill sequence for the first stage main engine is starting.

T-minus 5 minutes. Standing by to go into the final built-in hold.

The GPS 2F-4 satellite is switching to internal power for launch.

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Today marks the 38th flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 37 missions since debuting in August 2002, the tally shows 12 flights dedicated to the Defense Department, 9 commercial missions with communications spacecraft, 9 for NASA and 7 with spy satellites for the National Reconnaissance Office.

Just 45 minutes until liftoff time. Today's launch is the fourth for the Block 2F series of GPS satellites built by Boeing. In fact, it is the company's 43rd GPS launch overall, following 11 Block 1 spacecraft, 9 Block 2 and 19 Block 2A. All of the satellites have combined to provide over 500 years of cumulative on-orbit service, said Jan Heide, Boeing's GPS 2F program director.

The latest generation, of which a total of 12 are being produced, began flying in 2010.

"They are providing greater navigational accuracy through improvements in atomic clock technology, a more secure and jam-resistant military signal, an improved L5 (signal) that will eventually assist in commercial aviation and search-and-rescue operations, and an onboard reprogrammable processor. And this is with a 12-year design life for longterm service and reduced operating costs," Heide said.

GPS 2F-4 should complete its initial testing program and be turned over from Boeing to the Air Force in June for final checkout and activation in the navigation constellation.

The liquid hydrogen tank in the Centaur upper stage just reached the 97 percent level. Topping is now beginning.

Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.

Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is nearing completion as the countdown continues for a liftoff at 5:38 p.m. EDT. Weather remains GO.

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Centaur's liquid hydrogen tank is 30 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney Rocketdyne-made RL10 engine.

Chilldown of the liquid hydrogen system has been accomplished. The launch team has received the "go" to begin filling the Centaur upper stage with the supercold fuel.

First stage liquid oxygen tank is 50 percent full thus far. Chilled to Minus-298 degrees F, the liquid oxygen will be used with RP-1 kerosene by the RD-180 main engine on the first stage during the initial four minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket after rollout yesterday.

The first stage liquid oxygen tank has reached the 30 percent mark.

The Centaur engine chilldown sequence is being initiated.

The first stage liquid oxygen loading is transitioning from slow-fill to fast-fill mode.

The Centaur liquid oxygen tank reached the 95 percent level. The topping off process is starting now.

The chilldown conditioning of liquid hydrogen propellant lines at Complex 41 is starting to prepare the plumbing for transferring the Minus-423 degree F fuel into the rocket. The Centaur holds about 12,300 gallons of the cryogenic propellant.

Centaur liquid oxygen is 75 percent loaded.

The conditioning of the systems for the first stage liquid oxygen tank have been completed. And a "go" has been given to begin pumping supercold liquid oxygen into the Atlas 5's first stage.

The Common Core Booster stage's liquid oxygen tank is the largest tank to be filled today. It holds 48,750 gallons of cryogenic oxidizer for the RD-180 main engine.

Now two hours to launch. Passing the 20 percent level on the Centaur upper stage's liquid oxygen tank.

Filling of the Centaur upper stage with about 4,100 gallons of liquid oxygen has begun at Cape Canaveral's Complex 41 following the thermal conditioning of the transfer pipes.

The liquid oxygen -- chilled to Minus-298 degrees F -- will be consumed during the launch by the Centaur's single RL10 engine along with liquid hydrogen to be pumped into the stage a little later in the countdown. The Centaur will perform two firings to propel the GPS satellite into its intended orbit today.

The Centaur liquid oxygen pad storage area has been prepped. The next step is conditioning the transfer lines, which is now beginning to prepare the plumbing for flowing the cryogenic oxidizer.

T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket on a mission to deploy the Air Force's GPS 2F-4 navigation satellite.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 20 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 5:38 p.m. EDT from Cape Canaveral's Complex 41.

In the next couple of minutes, chilldown thermal conditioning of the mobile launch platform upon which the rocket stands will begin. This is meant to ease the shock on equipment when supercold cryogenic propellants start flowing into the rocket.

All console operators have reported GO status during the pre-fueling readiness poll. The ULA launch director also voiced his approval for moving forward with the countdown as scheduled today.

Loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket will be getting underway a short time from now.

The ULA launch conductor at the Atlas Spaceflight Operations Center is briefing his team on procedures before entering into the final two hours of the countdown.

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T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the day that will lead to the 5:38 p.m. EDT launch of the Atlas 5 rocket.

This initial pause was designed to give the team some margin in the countdown timeline to deal with technical issues or any work that could fall behind schedule before fueling starts. But all is going smoothly today, with officials not reporting any troubles in the count.

The final hold will occur at T-minus 4 minutes.

The final hands-on work has wrapped up at the launch pad and technicians have departed the complex. Safety officials just confirmed that the surrounding danger area has been cleared of all workers for the remainder of the countdown.

Hold-fire checks were performed with the Eastern Range to ensure safety personnel can hold the countdown if necessary.

Weather conditions remain ideal at Cape Canaveral today. The latest forecast given to managers before fueling still calls for a 90 percent chance of favorable conditions during the 5:38 to 5:56 p.m. EDT opportunity, with the only minor concern being cumulus clouds.

The outlook for the launch window now predicts scattered clouds at 3,000 feet, good visibility, southeasterly winds of 14 gusting to 18 knots, a temperature of 75 degrees F and humidity level of 60 percent.

Guidance system testing has been accomplished as the countdown goes smoothly today at Cape Canaveral.

Atlas first stage propulsion, pneumatic and hydraulic preps have been accomplished, along with Centaur hydrogen. And launch pad cameras have been verified configured for monitoring fueling operations.

The hazard area roadblocks around the launch site's safety perimeter have been established. Also, the launch team has configured the pad's water deluge system.

Now entering the final five hours of the countdown to the United Launch Alliance Atlas 5 rocket flight carrying the Air Force's Global Positioning System 2F-4 satellite.

Weighing about 3,400 pounds at launch, the satellite will be injected by the Atlas 5 directly into the navigation network 11,000 nautical miles above Earth tonight. Separation of the spacecraft from the rocket occurs about 3 hours and 24 minutes after liftoff.

"We are looking forward to yet another successful launch; tremendous progress is being made with the GPS 2F space vehicles. The first three satellites are on-orbit and meeting all mission requirements and the atomic clocks on-board the payloads are providing the best accuracy ever," said Col. Bernie Gruber, director of the Space and Missile Systems Center's Global Positioning Systems Directorate.

"This is just the beginning of the modernization plan to improve operations, sustainment and overall GPS service for military and civil users around the globe. I would like to commend the 45th and the 50th Space Wings, United Launch Alliance, our industry partners, the Atlas 5 and GPS 2F launch teams. Thanks to the hard work and focus on mission success, we are ready to launch the fourth GPS 2F satellite."

In today's first weather briefing to mission managers, all current conditions are observed GO for launch of the Atlas 5 rocket and odds for the 5:38 p.m. EDT liftoff time have improved to 90 percent favorable. There is only a very small concern for a cumulus cloud rule violation during the launch window.

The outlook for the launch window now predicts scattered clouds at 3,000 feet, good visibility, southeasterly winds of 14 gusting to 18 knots, a temperature of 76 degrees F and humidity level of 60 percent.

If the launch should slip to Thursday for some reason, there is a 70 percent chance of acceptable wether due to cumulus and thick clouds.

The Atlas-Centaur rocket has been powered up at Complex 41 and guidance system testing is getting started for today's launch, as the countdown progresses as planned.

The countdown begins now for today's launch of the Atlas 5 rocket to deploy the Air Force's Global Positioning System 2F-4 navigation satellite for military and civil users across the world.

Clocks are picking up the seven-hour, 10-minute sequence of work that will prepare the booser, payload and ground systems for blastoff at 5:38 p.m. EDT (2138 GMT).

Soon the launch team will begin powering up the rocket to commence standard pre-flight tests. Over the subsequent few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed, along with a test of the rocket's guidance system and the first stage propulsion and hydraulic preps, internal battery checks and testing of the C-band system used to track the rocket as it flies downrange, plus a test of the S-band telemetry relay system. The Complex 41 site will be cleared of all personnel at 2:33 p.m.

A planned half-hour hold begins at 2:48 p.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 3:16 p.m. to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage around 3:35 p.m., followed by the first stage filling around 3:48 p.m. Liquid hydrogen fuel loading for Centaur will be completed a short time later.

A final hold is scheduled at the T-minus 4 minute mark starting at 5:14 p.m. That 20-minute pause will give everyone a chance to finish any late work and assess the status of the rocket, payload, Range and weather before proceeding into the last moments of the countdown.

The launch window extends from 5:38 to 5:56 p.m. EDT (2138-2156 GMT).

Here's our photo gallery showing the Atlas 5's on the pad following today's rollout.

An Atlas 5 rocket stands atop its launch pad at Cape Canaveral for blastoff Wednesday to deploy a replacement satellite to strengthen the Global Positioning System for U.S. military forces and the worldwide economy.

The United Launch Alliance booster was rolled out to the Complex 41 pad shortly after 10 a.m. EDT today aboard a mobile platform, emerging from the assembly building where the rocket's two stages and the payload were integrated over the past month.

Countdown clocks begin ticking Wednesday at 10:28 a.m. EDT, leading to activation of the rocket, final testing and system preps. Fueling operations start at 3:30 p.m.

The day's available launch opportunity begins at 5:38 p.m. and closes at 5:56 p.m. EDT to send the rocket on a northeasterly heading into the GPS constellation.

The launch is targeting an aging satellite in the orbiting network that has twice outlived its design life, having been deployed in 1996. The Atlas 5 mission will exchange that bird with a new one possessing significant upgrades.

Known as GPS 2F-4, the $121.3 million spacecraft should become operational later this summer to replace the GPS 2A-25 satellite in the Plane C, Slot 2 position of the constellation that circles the planet at 11,000 nautical miles.

It joins three other Block 2F craft, the new Boeing-built series currently being fielded by the Air Force to update the system and add new features.

"GPS 2F satellites represent part of our overall modernization in GPS, inducing improved signals, enhancing our position, navigation and timing services that support not only our warfighters but growing civil and commercial needs of the U.S. and global economy," said Col. Steve Steiner, chief of the GPS Space Systems Division at the Los Angeles Air Force Base.

"2F satellites are critical to sustaining and modernizing GPS for the years to come."

It will be the first time an Atlas rocket has launched a GPS satellite since 1985. Delta rockets have performed all of the deployments since then.

But with the Air Force's Evolved Expendable Launch Vehicle program of Atlas 5 and Delta 4, both rocket fleets are capable of supplying the Pentagon with the payload delivery capabilities for all military satellites.

"We're very confident what we're going to do here with the Atlas 5 is going to be fine and deliver equally good performance that we've seen so far with GPS launches on Delta 4," said Walt Lauderdale of the Launch and Range Systems Directorate at the Air Force's Space and Missile Systems Center and the alternate mission director for this launch.

This particular mission, to carry a GPS 2F satellite, has been on the books awaiting a suitable manifest slot on Atlas, Lauderdale said, adding that another Atlas 5 flight for the navigation network could come next summer.

"The ability for ULA to successfully and efficiently integrate and launch a satellite on two launch systems provides significant operational flexibility for our customers," said Tony Taliancich, director of United Launch Alliance's customer program office.

The GPS phenomenon began as a military tool but blossomed into a worldwide economic driver.

"From a military standpoint, clearly it is hugely important that GPS is able to provide precision, day or night, all-weather guidance and timing to numerous platforms and weapons that we have. GPS is critical to how we fight wars on land, sea and air. Space systems truly are an embedded part of how the U.S. military operates these days," said Steiner.

Countless commercial uses have been found for the revolutionary GPS system, including air, sea and ground transportation, surveying, precision agriculture, infrastructure monitoring, geodesy, volcano and earthquake research and even helping hikers navigate their way.

"The timing capabilities of the vehicle are also integral to the U.S. economy," said Jan Heide, Boeing's GPS 2F program director. "Every ATM transaction that happens essentially is facilitated by GPS timing capabilities; cellphone is another fantastic example; the ability to walk up and pay at the pump to get gas, fantastic capability. It's almost integrated into everything that current societies are accustomed to having. It's like telephone utility service that people just have an expectation that the satellite and the mission capability are going to be there for them."

Watch this page for live updates throughout the countdown and flight, plus live streaming video.

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The United Launch Alliance Atlas 5 rocket's rollout to the pad is underway!

This slow half-hour drive from the 30-story Vertical Integration Facility to Cape Canaveral's Complex 41 pad uses a pair of specially-made "trackmobiles" to carry the rocket's 1.4-million pound mobile launching platform along rail tracks for the 1,800-foot trip.

The 189-foot-tall satellite booster is moving to Cape Canaveral's Complex 41 for launch of GPS 2F-4, the latest satellite to replenish and upgrade the navigation network.

The two-stage rocket and the GPS 2F-4 spacecraft were put together inside the assembly building over the past month in preparation for this rollout event. The Atlas 5 is designed to spend minimal time at the launch pad, which does not include a service gantry like other sites.

The Launch Readiness Review was conducted this morning and gave approval for Tuesday's rollout of the Atlas 5 rocket to the Complex 41 pad at Cape Canaveral for Wednesday's blastoff carrying a GPS satellite for the Air Force.

"There's been a lot of work accomplished to date on this mission. There's hundreds of people on the satellite and the launch team that have worked very hard to make sure we are prepared for a successful launch and mission," said Walt Lauderdale of the Launch and Range Systems Directorate at the Air Force's Space and Missile Systems Center and the alternate mission director for this launch.

Wednesday's launch opportunity extends from 5:38 p.m. to 5:56 p.m. EDT.

Weather forecasters are predicting an 80 percent chance of favorable conditions for the 18-minute window. The outlook predicts just some scattered clouds, good visibility, easterly winds of 14 gusting to 18 knots, a temperature around 77 degrees and only a slight chance for violating the cumulus cloud rule.

This will mark the 38th launch of an Atlas 5 booster in the past decade, the 18th in the 401 configuration with a four-meter nose cone and no strap-on solid motors and the fifth Atlas flight since December. It also represents United Launch Alliance's 70th mission in six years and the 30th using an Atlas 5.

It is the first time an Atlas has launched a GPS satellite in over a quarter-century, but it is Atlas 5's 20th mission in service to U.S. national security payloads.

The weather outlook for Wednesday's liftoff of the Atlas 5 rocket calls for an 80 percent chance of acceptable conditions during the 5:38 to 5:56 p.m. EDT launch window.

"On launch day, high pressure continues to build down the peninsula with fair weather. On-shore east winds will result in a small coastal shower threat. No thunderstorms are expected. The primary concern for launch day is cumulus clouds," Air Force meteorologists report.

The launch time outlook calls for scattered low- and high-level clouds, good visibility, easterly winds at 14 gusting to 18 knots and a temperature of 77 degrees F.

The launch time and window for May 15 have been adjusted slightly. Liftoff is now targeted for 5:38 p.m. EDT, at the opening of an 18-minute launch opportunity that extends to 5:56 p.m. EDT (2138-2156 GMT).

If the flight is delayed for some reason, the window opens four minutes earlier per day.

An Atlas rocket and a Global Positioning System satellite were joined together for the first time in 28 years today, as the pairing of booster and payload were united for their May 15 launch from Cape Canaveral.

Already shrouded in the launcher's 39-foot-tall, 14-foot-diameter aluminum nose cone, the spacecraft was hauled to the Atlas assembly building and hoisted atop the Centaur upper stage by United Launch Alliance workers.

Read our full story.

Going for its fifth flight since December in an unprecedented tempo, United Launch Alliance workers have begun stacking an Atlas 5 rocket for blastoff May 15 carrying a vital satellite to replenish the Global Positioning System.

The GPS 2F-4 spacecraft will ascend into a constellation of satellites circling 11,000 nautical miles above Earth that provides precision navigation and timing to U.S. military forces and civilian users worldwide.

The May 15 liftoff is scheduled during a window stretching from 5:39 to 5:58 p.m. EDT (2139-2158 GMT).

Read our full story.