SBAS – A Satellite Based Augmentation System is a system that supports wide-area or regional augmentation through the use of additional satellite-broadcast messages.
Such systems are commonly composed of multiple ground stations, located at accurately-surveyed points. The ground stations take measurements of one or more of the GNSS satellites, the satellite signals, or other environmental factors which may impact the signal received by the users. Using these measurements, information messages are created and sent to one or more satellites for broadcast to the end users.
Source: http://en.wikipedia.org/wiki