Ground‑Based Augmentation System (GBAS)

The Ground‑Based Augmentation System (GBAS) is a ground‑based system that augments the Global Positioning System (GPS) to provide enhanced navigation performance for aircraft. A GBAS installation typically consists of:

• Reference Receivers: A network of GPS reference receivers located at surveyed positions around the airport. These receivers measure GPS signal errors (ionospheric, tropospheric, satellite clock, and ephemeris errors).
• Central Processing Station: A ground computer that processes the reference receiver data to generate differential corrections and integrity parameters.
• VHF Data Broadcast (VDB): A VHF transmitter that broadcasts the correction and integrity data to aircraft within the coverage area (typically about 20 nautical miles).

Aircraft equipped with a GBAS receiver use the broadcast data to compute a highly accurate, integrity‑monitored position, enabling precision approaches (CAT I, II, III) with lateral and vertical guidance. GBAS is increasingly positioned as a longer-term successor to the Instrument Landing System (ILS), with a growing number of installations at major airports worldwide, offering multiple approach paths and greater flexibility than a fixed ILS beam can provide.

GBAS exists to deliver the same category of precision approach guidance as ILS, lateral and vertical, down to CAT III minima, from a single ground installation that can support every runway and multiple approach paths at an airport rather than requiring a dedicated localizer and glide slope pair for each runway end. Its purpose is achieved by applying the same correction-and-integrity principle as wide-area SBAS, but concentrated locally around one airport using its own reference receivers and VHF broadcast, which gives it tighter local accuracy and lower latency than a wide-area system could provide for approach-critical guidance. That local scope is also what lets GBAS support curved or offset approach paths that a fixed ILS beam geometry cannot offer, giving airports more flexibility in how aircraft are guided to each runway.

GBAS is used for a wide range of applications across all phases of flight:

• Precision Approaches: GBAS supports CAT I, II, and III precision approaches, providing lateral and vertical guidance to the runway. It allows for multiple approach paths (e.g., curved approaches) and can serve all runway ends from a single installation.
• Terminal Area Navigation: GBAS provides accurate positioning for departure and arrival procedures, including RNAV/RNP operations.
• En‑route Navigation: GBAS corrections can improve en‑route GPS accuracy, though coverage is typically limited to the airport vicinity.
• Surface Navigation: GBAS can provide accurate positioning for airport surface movement guidance, including A‑SMGCS applications.
• Remote and Offshore Operations: GBAS is particularly valuable in areas where ILS installations are impractical or expensive, such as offshore heliports or remote airstrips.

Reference receivers are distributed around the airfield on surveyed monuments, spaced apart and sited with a clear view of the sky to minimise multipath and ensure the array as a whole sees consistent satellite geometry; their antennas are mounted on short masts or building rooftops away from large reflective surfaces. The central processing station and VHF data broadcast (VDB) transmitter are housed together in an equipment building, with the VDB antenna mounted on a mast sited to give unobstructed line of sight over the runway and approach/departure paths it serves. Cabling from each reference receiver back to the processing station is run in duct, and the facility is connected to airport standby power in the same way as other critical navigation aids, given its role in supporting precision approaches.

The GBAS system operates through a multi‑step process:

• Reference Measurements: The reference receivers track all visible GPS satellites and measure pseudorange and carrier‑phase observables. These measurements are compared with the known positions of the receivers to calculate ranging errors.
• Differential Corrections: The central processing station computes differential corrections for each satellite in view. These corrections include ionospheric delay, tropospheric delay, satellite clock errors, and ephemeris errors.
• Integrity Monitoring: The system continuously monitors the health of the GPS constellation and the ground equipment. It generates integrity parameters — a ground accuracy designator (σ_pr_gnd) and per-receiver B-values derived by cross-checking the redundant reference receivers against each other — that allow the aircraft receiver to assess the reliability of the corrections and detect a faulty measurement before it propagates into the position solution.
• Data Broadcast: The VDB transmitter broadcasts the corrections, integrity data, and approach path information (waypoints, glide path angle, etc.) on a VHF frequency (108.025–117.950 MHz, in 25 kHz channel steps, paired with the approach procedure).
• Aircraft Processing: The aircraft's GBAS receiver uses the broadcast data to correct its GPS measurements, compute a precise position, and generate lateral and vertical deviation signals for the cockpit display (similar to ILS).

A GBAS ground facility's control logic centres on redundant reference receivers feeding a central integrity processor. Multiple GNSS reference receivers, sited around the airfield, each independently compute pseudorange corrections; the processor cross-checks these against one another to detect a faulty receiver or a satellite anomaly before it can propagate to an aircraft. Corrections, approach path definitions and integrity parameters are packaged and broadcast continuously over the VHF Data Broadcast (VDB) link at the required message rate, so aircraft are never operating on stale data. If the processor cannot certify the correction data within the required integrity bounds, it withholds or flags the broadcast rather than transmit an unverified correction, with status reported to the ATC and maintenance monitoring system.

The GBAS ground facility comprises a small number of precisely surveyed GPS reference receiver antennas mounted on short masts around the airfield, each connected via shielded, lightning-protected cabling back to a central processing cabinet housing the redundant reference receivers, integrity-monitoring processors, and VHF Data Broadcast (VDB) transmitter. The VDB transmit antenna is mounted at an elevated, unobstructed location to achieve its rated coverage radius, with the equipment cabinet built as a weatherproof, temperature-controlled enclosure similar to other NAVAID ground stations. Internally, the reference receivers and VDB transmitter chains are duplicated for hot-standby redundancy, with automatic switchover logic and continuous integrity monitoring built into the processing electronics to meet the stringent CAT I/II/III integrity and continuity requirements. Surge protection and isolation are fitted at every antenna feed and external interface given the facility's exposure to lightning and RF interference, and the cabinet's power section connects to mains, UPS, and generator backup for uninterrupted operation.