Introduction
While SBAS covers entire continents, Ground-Based Augmentation Systems (GBAS) take the opposite approach: extremely high-precision corrections broadcast from a single airport ground station over a short range. GBAS was developed specifically to replace the Instrument Landing System (ILS) for precision approaches, offering flexibility that fixed-frequency ILS installations cannot match.
System Architecture
A GBAS installation consists of four elements:
- Reference Receivers: Typically four dual-frequency GNSS receivers at precisely surveyed locations around the airport. Using four provides redundancy and allows detection of a failed reference receiver through consistency checking.
- GBAS Ground Facility (GGF): Processes reference receiver data, computes differential corrections and integrity bounds, and formats the VDB message.
- VHF Data Broadcast (VDB): A VHF transmitter operating in the 108 to 118 MHz aviation band broadcasts corrections on a time-division multiple-access basis at 31.5 kbits/s. This is the same frequency range as VOR navigation aids, so no new antenna infrastructure is required on most aircraft.
- Airborne Equipment: GBAS-capable aircraft carry a VDB receiver and software that applies the corrections to compute a precise approach path defined by the Final Approach Segment (FAS) data block broadcast by the ground station.
Range and Coverage
The VDB signal provides usable coverage over a radius of approximately 20 to 50 km from the ground station, depending on terrain, transmitter power, and aircraft altitude. A single GBAS installation can support multiple runway ends and multiple approach procedures simultaneously - simply by broadcasting different FAS data blocks. This is a significant advantage over ILS, where each runway end requires a separate, carefully calibrated ILS installation.
GBAS vs ILS: A Direct Comparison
| Characteristic | ILS | GBAS |
|---|---|---|
| Frequency | 108 to 112 MHz (LOC) + 329 to 335 MHz (GP) | 108 to 118 MHz (VDB) |
| Coverage | Single runway end only | Multiple runway ends from one station |
| Approach paths | Fixed glide slope and localiser | Curved, segmented, and multiple glidepath angles possible |
| CAT III capability | Yes (ILS CAT III) | Yes (GBAS CAT III - emerging) |
| Infrastructure cost | High per runway end | One installation serves entire airport |
| Sensitivity to interference | Moderate (RF interference) | Must handle multipath at reference antennas |
Approach Categories
Like ILS, GBAS approaches are categorised by their minimum Decision Height (DH) and visibility requirements:
- CAT I (GBAS Approach Service Type C - GAST C): Decision Height 200 ft, Runway Visual Range (RVR) 550 m. The most widely deployed GBAS capability today. Equivalent to ILS CAT I but using GNSS.
- CAT II/III (GAST D): Decision Height below 100 ft, potentially autoland. Requires additional integrity monitoring and dual-frequency capability. Several airports including Frankfurt and Zurich have operational GBAS CAT III capability.
Operational Deployments
GBAS CAT I systems are operational at airports including Bremen, Malaga, Frankfurt, Sydney, and several US airports under FAA certification. The technology is gaining acceptance as an alternative to new ILS installations, particularly at airports planning runway extensions or new approaches where installing multiple ILS systems would be prohibitively expensive.
Summary
GBAS represents the highest-performance end of the GNSS augmentation spectrum for aviation - providing integrity and accuracy sufficient for autoland in near-zero visibility. Its ability to serve an entire airport from a single ground installation, supporting curved approaches and multiple runway ends simultaneously, gives it significant economic and operational advantages over ILS. As GAST D certification matures, GBAS is poised to become the primary precision approach technology at major airports worldwide.