The Federal Aviation Administration (FAA) in the US have released the first edition of their Wide Area Augmentation System (WAAS) Performance Standard to the general public.
The document gives a good overview of how WAAS works as well as illustrating what level of performance will be available where.
WAAS
The US Global Positioning System (GPS) Standard Positioning Service (SPS) consists of spacebased positioning, navigation, and timing (PNT) signals generated from space vehicles orbiting the earth and delivered free of direct user fees for civil, commercial, and scientific uses worldwide. The Wide Area Augmentation System (WAAS) provides an augmentation signal to GPS, delivered free of direct user fees, that provides correction and integrity information intended to improve PNT service over the US and portions of Canada and Mexico.
WAAS is the first operational implementation of an International Civil Aviation Organization (ICAO) compliant Space Based augmentation System (SBAS). This WAAS Performance Standard (WAAS PS) specifies the levels of navigation performance that will be available to suitably equipped users who use both the GPS SPS broadcast signals and the WAAS augmentation signal.
For more information, please visit http://pnt.gov/public/docs/2008-WAASPS.pdf