(
National
Differential
GPS) A ground-based augmentation system maintained by the U.S. Department of Transportation that provides GPS accuracy to within three feet. Initially developed for the Coast Guard, it was enhanced to cover the entire country, but is monitored from the Coast Guard's Navigation Center in Alexandria, VA. NDGPS is used by highway and road crews as well as by the police, but is available to anyone with an NDGPS receiver. See
GPS augmentation system and
GPS.