Definition
A precision approach and landing system that uses GPS signals corrected and enhanced by a Ground-Based Augmentation System (GBAS) located at or near the airport. A GBAS ground station receives GPS signals, calculates correction data, and broadcasts it via VHF data link to aircraft on approach, allowing them to fly precision approaches with vertical and lateral guidance comparable to an ILS.
Plain English
A landing system that uses GPS, but with a local airport ground station that fixes small GPS errors and sends the corrections to the aircraft so it can fly an accurate approach down to the runway.
Context Anchor
Seen on instrument approach charts and in visibility or approach minimums discussions for runways served by GBAS equipment.
Derivation
GBAS stands for Ground-Based Augmentation System. 'Augmentation' comes from Latin 'augmentare', meaning 'to increase' or 'add to'. The ground station 'adds to' the basic GPS signal by supplying local corrections, making it accurate enough for landing.
Why Pilots Care
Enables precision approaches with lower decision altitudes and visibility minima than basic GPS approaches, improving access to runways in marginal weather.
Intuition Check
Do not assume GLS means the airplane lands itself. GLS provides precise guidance to the runway; the pilot or approved aircraft system still must fly the approach correctly.
Example Sentence 1
The crew briefed the GLS approach to Runway 28R, noting that the GBAS ground station was reporting normal status.
Example Sentence 2
Pilots briefing for an arrival check the chart to confirm the airport offers a GLS procedure before selecting it in the flight management system.