Update for Sept. 8: The Air Force has announced that liftoff is scheduled for 11:12 p.m. EDT (0312 GMT).

The United States is poised to place a giant reconnaissance satellite into orbit from Cape Canaveral for the first time in over five years Monday night using Lockheed Martin's heavy-lifting Titan 4B rocket.

Exactly when the $500 million rocket will thunder off remains classified because of the mission's hush-hush nature. But the Air Force has announced a broad eight-hour period from 7:45 p.m. to 3:45 a.m. EDT (2345-0745 GMT) in which liftoff is expected to occur.

The Titan 4 rocket's launch pad bathed in spotlights. Photo: Lockheed Martin

The Titan 4 will follow a 93.9-degree flight azimuth, heading eastward from the Complex 40 launch pad as it travels downrange over the Atlantic Ocean. The launch will last nearly six hours and include three firings of the Centaur upper stage to hurl the clandestine cargo into the desired orbit.

Details about the spacecraft are top-secret. The National Reconnaissance Office, the government agency based in Chantilly, Virginia that is responsible for U.S. spy satellites, will operate the craft.

"All of our launches are important because they are part of the larger national security capability. We look forward to getting down there and having a successful launch," said Art Haubold, a spokesman for the NRO.

Outside observers suspect the cargo is a sophisticated eavesdropping satellite, commonly called an Advanced ORION, designed to intercept communications, electronic signals and telemetry from Earth. Such craft are positioned in geostationary orbit 22,300 miles above the planet.

Space watchers point out that two similar launches occurred in May 1995 and May 1998. Those flights of the Titan 4 also featured east-bound trajectories, liquid-fueled Centaur upper stages and the largest payload fairing made for the vehicle -- a nose cone 86-feet in length.

This will be the first attempt to launch a National Reconnaissance Office cargo of this size from the Cape since August 1998. A Titan 4A vehicle, with a 76-foot long fairing, malfunctioned about 40 seconds after liftoff, destroying what is believed to have been a MERCURY-class intelligence-gathering craft.

Monday night's launch -- dubbed B-36 -- was postponed from last year citing problems with the payload. The Air Force allowed a launch of the sixth and final Milstar communications satellite to fly first, successfully lofting the mission in April.

Plans to carry out the NRO launch last month were thwarted by a propellant leak at the launch pad, followed by a glitch with a sensor on the vehicle and an undisclosed issue with the cargo.

Efforts to understand an errant reading from a fuel tank sensor led engineers to work around the device to permit the launch to be rescheduled.

"One of several fuel probes gave an intermittent reading while we were filling the tank. The sensor should indicate covered once fuel is over the top of the probe. But for several milliseconds it indicated uncovered. The problem has not repeated since we began troubleshooting," the Air Force officials at the Space and Missile Systems Center told Spaceflight Now.

"Lockheed Martin came up with a creative way to support a similar sensor that is normally used for telemetry. By installing a short wire harness to this fuel probe, they were able to substitute a new probe and abandon the questionable probe in place.

"It allowed us to use the guidance computer logic without changing the majority vote software. This retains all the protection we need to ensure a successful mission."

Since hooking up the telemetry sensor was performed outside the tank, the rocket remained fueled with its storable hypergolic propellants throughout the delay.

The payload's problem was not revealed to the public due to its classified nature.

All is now set for Monday's countdown and launch. The final pre-launch readiness review was held Sunday afternoon, clearing the way for the flight.

Weather forecasters are calling for an 80 percent chance of favorable conditions for the launch attempt.

We will provide live updates in our Mission Status Center during the final count and the launch from the Cape Canaveral press site.