The Global Positioning System; February 1996; Scientific American Magazine; by Herring; 7 Page(s)

Turning onto the final approach, the Boeing 737 airliner responded smoothly to the command of its computerized autopilot, setting up for what looked to be another perfect landing. Although automatic approaches are routinely performed in bad weather, this particular aircraft was not using the normal navigational signals beamed up from the airport to complete its so-called Category IIIA landing--the kind used when the pilot cannot see the runway until after the airplane touches down. The jet's occupants were relying instead on satellites of the U.S. Department of Defense's Global Positioning System (GPS) high in orbit overhead. These modern navigational benchmarks, floating in space at an altitude of more than 20,000 kilometers, were supposed to guide the swiftly moving aircraft safely to the ground.

As the 737 neared the runway, the GPS signals indicated that the ground loomed only 300 feet below the landing gear, and the airliner slowed its descent. Having completed numerous landings that day, the engineers on board had grown confident in the craft's satelliteguided abilities. But on this attempt the autopilot suddenly sounded an alarm: the GPS equipment had lost contact with a critical satellite. The airplane's human pilot quickly took over control from the computer and throttled up the engines to abort a potentially disastrous landing.