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Abstract
The Global Positioning System has proved itself very successful. The satellite system, pioneered by the Defense Department, had proved its military worth in the Gulf War. There is one obstacle to civilian use of GPS, but even that is surmountable: a GPS security feature known as "selective availability." Selective availability is a purposeful degradation in GPS navigation and timing accuracy that DoD creates by intentionally varying the precise time of the clocks on bard satellites that broadcast signals to GPS receivers. With selective availability, the civilian signal is accurate only to within 100 meters. When the Federal Aviation Administration recommended that GPS be augmented for its use in air traffic control, both the Secretary of Defense and the Secretary of Transportation endorsed the idea. The FAA unveiled the system it would use to get around DoD's selective availability capability. However, the Defense Department suddenly changed course, citing potential interference and national security risk.