BY JUSTIN RAY

Follow the countdown and launch of the Boeing Delta 2 rocket with the U.S. Air Force's latest GPS navigation spacecraft. Reload this page for the very latest on the mission.

1550 GMT (11:50 a.m. EDT)

T-minus 150 minutes and counting. The Terminal Countdown is now underway for this afternoon's launch of the Delta 2 rocket and the Global Positioning System 2R-15 satellite. The next three hours will be spent fueling the rocket, activating systems and performing final testing before liftoff at 2:50 p.m. EDT (1850 GMT) from pad 17A at Cape Canaveral Air Force Station in Florida.

The Complex 17 area is verified cleared of workers. A warning horn will be 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 hazardous operations such as the pressurization of helium and nitrogen storage tanks inside the rocket's first and second stages, along with the second stage fuel and oxidizer tanks.

The countdown currently stands at T-minus 150 minutes. However, there are a pair of holds -- totaling 30 minutes in duration -- planned at T-minus 20 minutes and T-minus 4 minutes.

1541 GMT (11:41 a.m. EDT)

The launch team members have been polled for a "ready" status to resume the countdown as planned at the end of this hold. No constraints were voiced. Clocks will start ticking again at 11:50 a.m. EDT as the Terminal Count begins.

1450 GMT (10:50 a.m. EDT)

T-minus 150 minutes and holding. Clocks are entering a planned 60-minute built-in hold in the countdown. Holds are scheduled into the count to give workers a chance to catch up on any activities that may be running behind.

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.

Liftoff of the Delta 2 rocket remains scheduled for 2:50 p.m. EDT from Cape Canaveral, Florida.

1350 GMT (9:50 a.m. EDT)

Clocks have entered the final five hours in the countdown to today's liftoff of the Boeing Delta 2 rocket and the GPS 2R-15 spacecraft. The launch team is gathering inside the control center at Cape Canaveral. A readiness poll of the members will be conducted around 11:40 a.m. in advance of starting the Terminal Countdown.

1200 GMT (8:00 a.m. EDT)

This morning's edition of the launch weather forecast is available here.

1115 GMT (7:15 a.m. EDT)

The most recent launch of a new Global Positioning System satellite was exactly a year ago today. At Cape Canaveral's pad 17A this morning, preparations are underway for liftoff of another replacement spacecraft to bolster the orbiting navigation network.

Crews have just rolled the mobile service gantry away from the Boeing Delta 2 rocket as countdown activities proceed toward a 2:50 p.m. EDT liftoff.

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 at Cape Canaveral Air Force Station.

The next few hours will be spent configuring launch pad equipment and securing the site.

The Terminal Countdown begins at 11:50 a.m. EDT, followed about 20 minutes later by loading the first stage with kerosene propellant. Liquid oxygen starts flowing into the first stage just before 1 p.m.

Today's launch will last 68 minutes from liftoff through deployment of the GPS 2R-15 satellite. The 4,500-pound satellite, built by Lockheed Martin, is nestled inside the nose cone of the three-stage rocket.

The spacecraft is headed for an orbit 11,000 miles above Earth where it will fill a slot being vacated by a 14-year-old satellite showing signs of its age.

The GPS constellation uses two dozen primary satellites and a handful of backups to provide precision location and time information to military and civilian users around the world.

SUNDAY, SEPTEMBER 24, 2006

A fresh navigation satellite to keep the Global Positioning System healthy awaits its ride into space Monday afternoon aboard a Boeing Delta 2 rocket.

Activities are proceeding for liftoff from Cape Canaveral's pad 17A at 2:50 p.m. EDT (1850 GMT).

The mission dress rehearsal was held Friday. Technicians filled the second stage with its load of storable propellants on Saturday. Engineering inspections, the final buttoning up equipment and ordnance connections were underway Sunday.

Countdown operations start early Monday with retraction of the mobile service gantry from around the 125-foot tall rocket. The Terminal Count picks up three hours before launch time -- 11:50 a.m. EDT -- in advance of fueling the vehicle's first stage with a highly fined kerosene propellant and supercold liquid oxygen.

"We're really looking forward to putting up another successful GPS 2R-M (satellite)," said Lt. Col. Ron Fortson, the Air Force launch director and commander of the 1st Space Launch Squadron at Cape Canaveral.

"We hope for mission success, we anticipate mission success," added Col. James Planeaux, the mission director for Monday's liftoff and director of the Air Force's Delta Launch Systems Group.

Weather forecasters predict an 80 percent chance that conditions will be acceptable for launch. See the latest forecast here.

The $75 million GPS satellite is the second with modernized features that will transmit additional signals for military and civilian users, promising greater accuracy, added resistance to interference and enhanced performance.

"By launching just one or two, that does not give an immediate full operational benefit. But it starts us down the path," said Col. Allan Ballenger, commander of the Global Positioning System Wing. "The more satellites we populate into the constellation, the greater the benefit."

The Air Force has a half-dozen more of these replacement satellites to launch in the next couple of years, plus a new generation packed with more upgrades being built.

THURSDAY, SEPTEMBER 21, 2006

With a boost from Boeing's workhorse Delta 2 rocket, the next step in modernizing the Global Positioning System for military and civilian users will be taken Monday afternoon.

The three-stage launcher is scheduled for blastoff at 2:50 p.m. EDT (1850 GMT) from Cape Canaveral to deliver the GPS 2R-15 navigation spacecraft into Earth orbit. The day's launch window extends through 3:03 p.m.

Weather forecasters predict a 70 percent chance of favorable conditions. Clouds associated with rain showers or thunderstorms are the prime worry. See the early forecast here.

"We are extremely pleased to be able to continue our support for the nationally critical GPS constellation," said Col. James Planeaux, the Air Force mission director for the GPS 2R-15 launch.

"Lots of hard work and a tremendous team effort between our GPS customer, Boeing, the 45th Space Wing, the Aerospace Corporation, and the Launch and Range Systems Wing have gotten us ready to fly. The Delta 2 launch vehicle is a proven performer, and we are looking forward to continuing that tradition with a successful launch."

GPS 2R-15 is the second in a new breed of updated craft built to replace aging satellites in space. The modernized craft transmit additional signals and feature improvements aimed at greater accuracy, tougher resistance to interference and enhanced performance for users around the world.

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

Millions of people across the globe use GPS every day, and that commercial marketplace for GPS technology has exploded in recent years, growing from $16 billion in 2003 to what analysts predict will be $68 billion in 2010, according to the U.S. Department of Transportation.

Lockheed Martin has built 21 of the GPS Block 2R satellites for the Air Force, with 14 already launched. The first with added modernization updates was successfully deployed a year ago.

The spaceborne GPS network features 24 primary and several backup satellites flying in six orbital groupings. The Air Force began launching the fleet in 1989 and continues to send up new satellites to keep the navigation system in good health. There are 29 operational GPS satellites today.

GPS 2R-15 will assume the Plane A, Slot 2 position, taking over for the GPS 2-12 craft launched in February 1992. The aging satellite, which has long surpassed its expected life span, is showing symptoms of failure in its internal clock system, the Navstar Global Positioning Systems Wing at the Space and Missile Systems Center told Spaceflight Now.

To get the last bit of usefulness out of the old satellite, controllers plan to relocate it to the A4 slot within the GPS constellation to act as a backup for another craft, the Wing said.

Watch this page for comprehensive live coverage during Monday's countdown and launch.

And check out our Cue Card for a quick-look at the mission.