Tracking white and yellow blips on a computer screen, Tech. Sgt. Nora Hemphill uses the world's most advanced radar to make sure none of the planes flying toward North America are Soviet bombers.

From a darkened command center on the outskirts of Bangor, Air Force technicians like Hemphill have monitored the radar round-the-clock since April 1990 to guard against the start of World War III.But beginning Saturday, the technicians will be keeping a lookout for Soviet bombers only 40 hours a week because the Pentagon is trying to save money.

The Over-The-Horizon Backscatter radar, created at a cost of $1.05 billion as a linchpin in North America's air defense, has become the latest victim of military budget cuts brought on by the end of the Cold War.

The Pentagon originally wanted to shut down the radar in Maine and a companion system at Mountain Home Air Force Base in Idaho that watches over the West Coast. But protests from Maine Sens. George Mitchell and William Cohen persuaded the Air Force to keep the Maine system operating part-time.

By shutting down the West Coast radar July 1 and reducing operations in Maine, the Pentagon will save about $38 million a year, officials estimated.

The radar's supporters in the Air Force concede that staffing the system 40 hours a week won't provide a credible early warning against a bomber attack.

But they say the radar is just one part of a classified system of sensors and intelligence gathering that guards North America's perimeter.

"If this radar was the only thing that indicated the bombers were on their way, I would be worried about operating 40 hours a week," said Col. Harris N. Madson, commander of the Air Force's 776th Radar Squadron, the unit staffing the system at the Bangor Air National Guard base in central Maine.