WAAS, or Wide Area Augmentation System, was developed by the US Department of Transportation and the Federal Aviation Administration to improve the accuracy of GPS receivers all across North America. A standard GPS receiver calculates its position by measuring timed radio signals from multiple satellites in orbit around the earth. These signals are subject to many sources of error like atmospheric disturbances, satellite orbit errors, and clock drift. GPS receivers without any correction have an accuracy of about 15 metres or less.
This, in part, led to the development of Differential GPS Signals (DGPS). Fixed stations on the ground receive the satellite GPS positions, which then broadcast the difference between the GPS position and the station's actual position to DGPS capable receivers. DGPS greatly improved the positioning accuracy, but these ground stations are costly to maintain, have limited range, and require special receivers. The WAAS system is based on the DGPS idea of using fixed ground reference stations, except that the correction signals are broadcast over a network of satellites. This makes WAAS available over a much wider area (hence the name) compared to DGPS . Today, WAAS signals are available throughout the contiguous United States and Canada and parts of Mexico and Alaska.