Satellite-Based Augmentation Systems (SBAS) are used to enhance the accuracy and precision of an existing GPS system. GPS by itself isn’t sufficiently accurate to do things like help land airplanes or other applications where there is a need for high accuracy combined with high consequences for failure. The other acronym of note is WAAS, or Wide Area Augmentation System. This is the SBAS system for the United States and most of North America, developed not by the military, but by the Federal Aviation Administration and the Department of Transportation. Almost all GPS receivers sold today incorporate SBAS features for at least WAAS, and often other SBAS systems like EGNOS (European Union) and MSAS (Japan). Supporting multiple platforms has become increasingly common, even in consumer-grade products. The WASS supplements the space-borne GPS signal with a series of 25 ground stations that compare the GPS signal to geodetically known points. The stations then send corrections over a landline to a master station, which retransmits a unique corrected GPS signal to each of four orbiting WAAS satellites. In turn, the satellites finally broadcast a corrected signal to WAAS-enabled GPS receivers. The WAAS signal serves three purposes. It provides additional ranging for GPS receivers, it provides GPS satellite integrity data, and most importantly, it provides wide-area differential corrections to the civilian GPS signal. Currently the WAAS system consists of 3 satellites in geosynchronous orbit.
Uncorrected, the civilian GPS signal has an accuracy of around 10 meters. WAAS-enabled receivers differentially correct for much of this error in realtime by comparing the corrected WAAS signal to the incoming GPS signal and eliminating much of the atmospheric, ephemeris and clock error, thus increasing the accuracy of position solutions to around three meters. In practice, WAAS accuracy is often under 1.5 meters. This is often more than sufficient accuracy for most casual GPS applications, but not quite good enough for mapping and survey applications. Applications that require both accuracy and precision tend to demand much higher-end hardware to achieve the necessary results.