In July of 2003, the Federal Aviation Administration (FAA) commissioned the Wide Area Augmentation System (WAAS) for instrument flight use in the U.S. The WAAS system provides increased accuracy and reliability on a real-time basis to users of the Global Positioning System (GPS), and transmits horizontal and vertical guidance allowing pilots to fly under satellite-based instrument guidance to as low as 350 feet above runways (later milestones include reducing this to 250 feet). WAAS will also decrease pilot reliance on ground-based navigation aids and allow instrument approaches at airports with no ground-based landing capability. Certified avionics are required to use this system for flight navigation; several companies have released systems and many others are in development.
WAAS itself is based on a series of ground control stations and geostationary satellites; the ground control stations provide by-the-second corrections which are uplinked to the satellites, and these satellites then broadcast the information for use by GPS receivers. The correction signal also includes information on current GPS errors, providing warning of problems within six seconds of occurrence. Reliability is assured through redundancy, as each WAAS ground site includes 3 base stations.
Ground-based surveyors have been able to access this system for some time, with WAAS-enabled receivers providing accuracies of approximately 3 meters in near real time. Many new receivers are WAAS-enabled, and some older receivers may be upgraded to use the WAAS signals; check with the manufacturer for details. WAAS sites are also being incorporated into the National Continuously Operating Reference Station (CORS) network, with WAAS data eventually accessible through the National Geodetic Survey (due to some equipment changes, the WAAS data is not currently available through this mechanism).