Ground Based Augmentation System, or GBAS, improves ordinary GPS so aircraft can fly very precise landing approaches. A GBAS Landing System approach, called GLS, gives pilots vertical and horizontal guidance to a runway in a way that feels similar to an Instrument Landing System. This matters because accurate guidance helps aircraft land safely in low visibility and supports efficient airport operations.
GLS can serve multiple runway ends from one ground station, which can reduce the need for separate radio navigation equipment near each runway.
Key Facts
- GBAS = Ground Based Augmentation System, the ground equipment that improves GNSS accuracy and integrity near an airport.
- GLS = GBAS Landing System, the aircraft approach procedure that uses GBAS corrections for precision landing guidance.
- Position error correction = measured GPS error at the GBAS station sent to the aircraft as a correction message.
- Glide path angle is often about 3 degrees, so descent rate depends on groundspeed: descent rate ≈ groundspeed x tan(3 degrees).
- For small angles, tan(3 degrees) ≈ 0.052, so altitude loss ≈ horizontal distance x 0.052.
- A 3 degree glide path descends about 318 ft per nautical mile because 6076 ft x tan(3 degrees) ≈ 318 ft.
Vocabulary
- GBAS
- A ground based system that monitors GNSS signals, computes corrections, checks integrity, and broadcasts guidance data to aircraft near an airport.
- GLS
- A precision approach type in which an aircraft uses GBAS corrected satellite navigation to follow lateral and vertical guidance to a runway.
- GNSS
- A general term for satellite navigation systems such as GPS, Galileo, and other global constellations.
- Integrity
- The ability of a navigation system to warn users quickly when its information should not be used for safety critical guidance.
- Glide path
- The planned sloping path that guides an aircraft from final approach altitude down toward the runway touchdown zone.
Common Mistakes to Avoid
- Confusing GBAS with GLS is wrong because GBAS is the airport ground system, while GLS is the aircraft approach operation that uses it.
- Thinking GLS uses only raw GPS is wrong because GLS depends on correction and integrity messages broadcast from a local GBAS station.
- Assuming one GBAS antenna serves only one runway is wrong because one certified GBAS installation can support multiple approach paths at the same airport.
- Treating GLS as identical to ILS is wrong because ILS uses local radio beams from runway equipment, while GLS uses satellite positioning corrected by ground based augmentation.
Practice Questions
- 1 An aircraft is 5 nautical miles from the runway on a 3 degree glide path. Using 318 ft per nautical mile, about how many feet above the touchdown zone should it be?
- 2 A jet flies a GLS approach at a groundspeed of 140 knots. Using a 3 degree glide path and 318 ft per nautical mile, estimate the descent rate in feet per minute.
- 3 Explain why a GBAS station located at the airport can improve GPS landing guidance more effectively than using uncorrected GPS alone.