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...