As defined by the International Civil Aviation Organization (ICAO), a Global Navigation Satellite System (GNSS) is a worldwide position and time determination system that includes one or more satellite constellations,
aircraft receivers, and system integrity monitoring, augmented as necessary to support the required navigation performance for the intended operation.

Current satellite navigation systems (GPS, GLONASS) were not designed to meet the real-time integrity monitoring capability required by the civil aviation navigation safety needs.
Sources of error such as satellite or ionospheric delays can introduce several meters of error in an aircraft's position.These errors must be corrected in real time for a precision approach where there is little or no visibility.
To overcome the above limitation, several Augmentation Systems have been put in place

Ground-Based Augmentation System (GBAS) is a system that provides differential corrections and integrity monitoring of Global Navigation Satellite Systems (GNSS).
GBAS provides navigation and precision approach service in the vicinity of the host airport (approximately 30 km), broadcasting its differential correction message via a very high frequency (VHF) radio data link from a ground-based transmitter.

GBAS yields the extremely high accuracy, availability, and integrity necessary for Category I, and eventually Category II, and III precision approaches.
GBAS demonstrated accuracy is less than one meter in both the horizontal and vertical axis. (see annex)