DENVER — A problem that rendered as many as 10,000 U.S. military GPS receivers useless for days is a warning to safeguard a system that enemies would love to disrupt, a defense expert says.

The Air Force has not said how many weapons, planes or other systems were affected or whether any were in use in Iraq or Afghanistan. But the problem, blamed on incompatible software, highlights the military's reliance on the Global Positioning System and the need to protect technology that has become essential for protecting troops, tracking vehicles and targeting weapons.

"Everything that moves uses it," said John Pike, director of Globalsecurity.org, which tracks military and homeland security news. "It is so central to the American style of war that you just couldn't leave home without it."

The problem occurred when new software was installed Jan. 11 in ground control systems for GPS satellites, the Air Force said.

Officials said that between 8,000 and 10,000 receivers could have been affected, out of more than 800,000 in use across the military.

In a series of e-mails to the Associated Press, the Air Force initially blamed a contractor for defective software in the affected receivers but later said it was a compatibility issue rather than a defect.

The Air Force said it hadn't tested the affected receivers before installing the new software in the ground control system.

One program still in development was interrupted but no weapon systems already in use were disabled as a result of the problem, the Air Force said.

An Air Force document said the Navy's X-47B, a jet-powered, carrier-based drone under development, was interrupted by the glitch. Air Force officials would not comment beyond that on what systems were affected.

At least 100 U.S. defense systems rely on GPS, including aircraft, ships, armored vehicles, bombs and artillery shells.

Because GPS makes weapons more accurate, the military needs fewer warheads and fewer personnel to strike targets. But a leaner, GPS-dependent military becomes vulnerable if the GPS is knocked out.

James Lewis, a senior fellow at the Center for Strategic and International Studies, said the glitch was a warning "in the context where people are every day trying to figure out how to disrupt GPS."

The Air Force said it took less than two weeks for the military to identify the cause and begin devising and installing a temporary fix. It did not say how long it took to install the temporary fix everywhere it was needed but said a permanent fix is being distributed.

The military said it ran tests on some types of receivers before it upgraded with the new software in January, but that the tests didn't include the receivers that had problems.

The Air Force said the software upgrade was to accommodate a new generation of GPS satellites, called Block IIF.

Space and Missile Systems Center spokesman Joe Davidson said in an e-mail to the Associated Press that the system is safe from hackers or attack, noting that control rooms are on secure bases and communications are encrypted.

The military also has tried to limit the potential for human error by making the GPS control system highly automated, Davidson said.

Iraq tried jamming GPS signals during the 2003 U.S. invasion, but the U.S. took out the jammer with a GPS-guided bomb, Hasik said.

The organizational skills required to jam GPS over a broad area are beyond the capability of groups like the Taliban and most Third World nations, Hasik said.

"The harder you try to mess with it, the more energy you need. And the more energy you use, the easier it is for me to find your jammer," Hasik said.

More worrisome, Hasik said, is the potential for an accident within U.S. ranks that can produce anything from an errant bomb to sending troops or weaponry on the wrong course.

In 2001, a GPS-guided bomb dropped by a Navy F-18 missed its target by a mile and landed in a residential Kabul neighborhood, possibly killing four people. The military said wrong coordinates had been entered into the targeting system.