File image of Titan 4B on the launch pad. Photo: Air Force TV/Spaceflight Now

A puzzling problem with a guidance computer is keeping a U.S. Air Force Titan 4B rocket and its sophisticated communications satellite cargo grounded at Cape Canaveral, Florida.

Originally scheduled for blastoff a few days ago, the massive Titan 4B was put on an indefinite hold in late January after concerns were raised with an Inertial Navigation Unit like the one aboard the rocket.

An INU still in the factory failed routine acceptance testing last month, prompting the Air Force to postpone the launch to ensure the unit aboard the Titan 4 was healthy.

Built by Honeywell, the INU serves as the vehicle's navigation brain during launch. The faulty unit was manufactured for use by an Atlas rocket but has many of the same components and software as the Titan 4 INUs.

The factory unit has undergone several retests since the failure and has performed properly, the Air Force said in a statement. What caused the initial problem is still unclear, however.

During a senior-level Mission Risk Decision Board meeting last Friday, officials determined more testing would be needed before allowing the Lockheed Martin-built Titan 4B to lift off with the fourth Milstar military tactical communications relay satellite.

The INU on the Titan 4B has not been retested during this delay, the Air Force says, because it has already passed acceptance testing and is believed to be fine. Engineers are performing additional tests and analysis on similar hardware to unravel the mystery of the factory test failure.

Officials don't want any problems to occur during the upcoming launch. The rocket and satellite are valued at over $1 billion, and the last Milstar was lost in a launch failure nearly two years ago because of a computer software error.

The mission patch for this launch. Photo: Lockheed Martin

The earliest launch could occur is next Tuesday at 2:52 p.m. EST (1952 GMT), the opening of a four-hour window.

However, that launch opportunity depends on both the INU concern being resolved and space shuttle Atlantis blasting off as scheduled on Wednesday from neighboring Kennedy Space Center.

The Air Force says the computer issue must be cleared before launch pad crews can load nitrogen tetroxide oxidizer into the rocket's core stages. That work is scheduled for Thursday, followed by three days of final battery installation and activation chores and Range checks. The countdown would begin on Monday.

But for that plan to be implemented, Atlantis needs to fly on Wednesday evening so the Titan's Complex 40 launch pad can be opened to workers on Thursday for fueling. The shuttle's launch site is pad 39A located just a few miles away.

If the shuttle is delayed and uses backup launch attempts available on Thursday or Friday, that would keep fueling of the Titan on hold and postpone its blastoff in a domino effect, the Air Force statement said.

This is the first of three Titan 4B launches planned from Cape Canaveral this year. Two more are scheduled at Vandenberg Air Force Base in California.