How do GBAS and SBAS ground stations correct satellite navigation for a pilot?
Ground receivers at known positions find the errors in the satellite signals, a broadcast carries the corrections and a trust check to the aircraft, and the antennas on the airfield share its clear sites and cable routes with the lighting.
Satellite navigation on its own has small errors. A GBAS (ground-based augmentation system) ground station fixes this at one airport. Several reference receivers at surveyed positions track the satellites, the station compares what they report with where the receivers really are, and it works out corrections and an integrity check. These are sent to aircraft on approach by VHF data broadcast, and the aircraft receiver uses them to fly a GLS precision approach. One GBAS station can serve several runways.
An SBAS (satellite-based augmentation system) does the same job over a whole region. Many ground reference stations feed a central station, and the corrections are sent up to a geostationary satellite and broadcast to aircraft from there, with no local broadcast needed. India's system is GAGAN. Both systems also tell the aircraft when the signal cannot be trusted, which is as important as the correction itself.
Neither is part of the AGL system, but a GBAS station stands on the airfield and meets the lighting on the ground. The reference receivers need an open view of the sky, free of metal that could reflect the signal, so masts, fences, vehicles, new cable routes and works near them can spoil the corrections. Any such change is agreed with the navaid owner first, so that the navigation system and the lighting both stay right. Approach and runway lighting are still needed to land.
Frequently Asked Questions
Q1What is the difference between GBAS and SBAS?
Both correct satellite navigation signals and tell the aircraft whether it can trust them. A GBAS ground station sits at one airport and sends its corrections by a local VHF data broadcast to aircraft nearby. An SBAS covers a whole region: ground stations spread across the region feed a central station, and the corrections go up to a geostationary satellite and are broadcast from there.
Q2What does a GBAS ground station do?
Several reference receivers at precisely surveyed positions on the airfield track the navigation satellites. The ground station compares what the satellites say with the known positions, works out the corrections, adds integrity information, and sends them to aircraft on approach by VHF data broadcast. The aircraft receiver applies them to guide a precision approach.
Q3Is GBAS or SBAS part of airfield ground lighting?
No. They are navigation systems owned and maintained by the air navigation service provider, or in India by the national authorities and agencies that run them. They meet the AGL system on the ground, because GBAS antennas and cabinets stand on the airfield and share its clear sites, cable routes, power and works.
Q4Why do the antennas need a clear view of the sky?
The reference receivers must see the satellites without reflections. Metal masts, fences, vehicles and buildings near an antenna can reflect the signal, which is called multipath, and a small error there is passed on as a correction error to every aircraft. Sites are chosen for open sky and kept free of new structures.
Q5Do GBAS or SBAS approaches replace approach lighting?
No. They replace or supplement the radio guidance, for example an ILS, but the pilot still needs the visual aids to land. Approach lighting, runway lighting and markings are still required, and the lighting category needed for the minima does not change because the radio guidance is satellite based.
Q6What should be done before working near a GBAS site?
Tell the navaid owner before the work and agree the dates and the method. Keep tall equipment, large metal objects and new cable routes away from the reference receivers and the VHF antenna, and do not move or knock the antennas, because their surveyed positions matter. Ask whether the station needs a check afterwards.
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