Definition
A precision approach and landing system that uses a ground station at or near an airport to broadcast correction and integrity data to GPS-equipped aircraft, allowing them to fly precise approaches with vertical and lateral guidance comparable to an Instrument Landing System (ILS). The ground station monitors GPS signals, calculates corrections, and transmits them via VHF data broadcast to aircraft within range, enabling precision approaches to suitably equipped runways.
Plain English
It's a way of using GPS for accurate landings. A ground station at the airport checks the GPS signals, fixes any errors, and sends that information to aircraft so they can fly a very accurate approach down to the runway -- similar to what an ILS does, but using satellites instead of runway-mounted radio beams.
Context Anchor
Pilots encounter GLS when briefing or flying an instrument approach chart labeled as a GLS approach, and when confirming the aircraft has the equipment needed to use it.
Derivation
Augmentation' comes from Latin augere, meaning 'to increase' or 'add to.' The ground station adds correction data to the basic GPS signal, increasing its accuracy enough for precision landings. 'Ground based' simply means the augmentation comes from a station on the ground, as opposed to a satellite-based system like WAAS.
Why Pilots Care
Enables precision approaches at airports lacking traditional ground-based landing systems, supporting lower landing minima and greater operational flexibility.
Analogy
Think of satellite navigation as a location estimate, and GBAS as a local airport station that knows exactly where it is and sends corrections. GLS uses those corrections to guide the airplane down the approach path more accurately.
Intuition Check
GLS does not mean the ground equipment lands the airplane by itself. It means the airplane uses corrected satellite navigation to give the pilot or autopilot accurate approach guidance.
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
Airports with GBAS installations now publish GLS procedures that allow Category I landings with satellite-based guidance.