Ground-Based and Satellite-Based Augmentation Systems
Core concepts
Augmentation improves GNSS accuracy, integrity and availability. Ground-based augmentation (GBAS) uses a ground station to compute corrections and integrity parameters for a local area, broadcasting them to aircraft on VHF; it supports precision approach (GLS). Satellite-based augmentation (SBAS) uses geostationary satellites to broadcast wide-area corrections and integrity information, enabling LPV approaches. Key concepts are the correction message, integrity bounds, the protection level, and the difference between accuracy and integrity. Also relevant are the coverage areas of SBAS and the fact that GBAS is local while SBAS is regional.
Traps in questions
Common traps: confusing GBAS with SBAS coverage; assuming SBAS gives Cat III capability (it typically supports LPV, not autoland); and forgetting that augmentation does not remove the need for RAIM prediction in some cases. Questions may ask about the effect of ionospheric activity on SBAS integrity. Also remember that GBAS supports multiple approaches at one airport with a single ground station.
Worked example
An aircraft on an SBAS LPV approach receives an integrity alert; the protection level exceeds the alert limit. The crew must abandon the approach and follow the missed approach procedure, because integrity, not just accuracy, has failed.
Rule of thumb
Accuracy is how close the position is; integrity is the confidence that the error is within bounds. Augmentation mainly improves integrity and availability, not just accuracy.