The Wide Area Augmentation System (WAAS) is an extremely accurate navigation system developed for civil aviation by the Federal Aviation Administration (FAA) in conjunction with the United States Department of Transportation (DOT). The stated accuracy is within 3 meters of the true position 95% of the time. This is achieved via a network of ground stations located throughout the north-western hemisphere which monitor the GPS signal and measure the difference between that signal and their surveyed location. These ground stations send the measured differences to a master relay station which sends corrections to two geostationary satellites over the East and West coasts. Those satellites transmit the correction values back to Earth, where WAAS-enabled GPS receivers apply the correction to their computed GPS position.

Before WAAS, the U.S. National Airspace System (NAS) did not have the ability to provide horizontal and vertical navigation for precision approaches for all users at all locations, as ground-based systems are quite expensive. WAAS provides service for all classes of aircraft in all flight operations, including en route navigation, airport departures, and airport arrivals, including precision approaches throughout the NAS.

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