GAST provides a standard way to describe what a GBAS facility delivers to airborne users, for example, precision level, update/latency behavior, protection levels, and whether autoland capability is supported. GAST is the basis for publishing GNSS Landing System (GLS) approaches and for certifying both ground systems and airborne equipment for particular minima.

• GAST‑C: A single‑frequency (GPS L1 C/A or GLONASS L1OF) service type currently standardized to support CAT I precision approaches. GAST‑C meets integrity and protection‑level targets appropriate for CAT I under the documented risk and continuity allocations.

• GAST‑D: A single‑frequency extension intended to enable lower minima up to CAT II/III. GAST‑D requires enhanced monitoring, improved Fault Detection and Exclusion (FDE), and stricter continuity and availability performance.

• GAST‑E / GAST‑F / GAST‑X: Service types defined for dual‑frequency, multi‑constellation GBAS implementations. These classes leverage multiple frequencies and constellations (for example, L1/L5, E1/E5 with GPS/Galileo/BeiDou) to improve resilience against ionospheric effects and to increase availability in challenging conditions, supporting robust CAT II/III capability.

Single‑frequency systems are more limited against ionospheric spatial gradients and certain multipath scenarios; they form the baseline for GAST‑C and GAST‑D definitions. Dual‑frequency, multi‑constellation systems reduce vulnerability to ionospheric errors, improve geometry and availability, and are central to GAST‑E/F/X concepts.

Higher GAST classes (aiming toward CAT II/III) impose tighter integrity risk allocations (lower allowable probability of hazardous misleading information), stricter continuity requirements, and lower protection levels. Achieving these targets typically requires enhanced monitoring, redundancy and refined FDE.

Authorized flight minima (decision height, runway visual range/RVR) for a specific GLS approach and aircraft depend on a combination of the factors below. In practice, a published GLS approach specifies the authorized minima based on the site’s declared GAST and the certified capabilities of participating aircraft and operators.

• The declared GAST and the certified performance of the site (protection levels and integrity risk).
• Airborne equipment capability and certification (compliance with relevant MOPS; autoland certification for CAT II/III).
• Operator and airframe approvals, including crew training and operational procedures.
• Procedure design elements (approach geometry, obstacle clearance, runway lighting and RVR reporting).

Local regulatory and operational constraints (national authority approvals, NOTAMs and airport‑specific limits).

GBAS ground equipment, airborne receivers and published procedures must conform to ICAO GBAS (GLS) SARPs and to MOPS produced by RTCA and EUROCAE. National authorities (for example, FAA, EASA) require demonstration of compliance with these standards and proof that continuity, availability and FDE performance meet the targets for the declared minima. Certification for CAT II/III minima is more demanding and requires rigorous demonstration of failure recovery and continuity within the required timeframes.

• Site design and redundancy: Additional reference receivers, strategic siting and infrastructure redundancy help meet strict continuity and integrity targets.
• Threat mitigation: Dual‑frequency and multi‑constellation processing, robust FDE, interference monitoring and anti‑spoofing measures reduce vulnerabilities such as ionospheric gradients and intentional interference.
• Ground‑to‑air latency and update rates: VDB message rates and latency must be managed so corrections and integrity alerts arrive in time for final approach guidance.
• Procedure validation: Flight checks and performance validation are required to confirm that protection levels and predicted performance hold in the operational environment.
• Airspace and airport integration: Publication of GLS procedures, pilot and ATC training, and contingency procedures for GBAS outages (for example, revert to ILS or visual minima) must be coordinated.

GAST‑C is the currently deployed, standardized service supporting CAT I operations at operational sites worldwide. GAST‑D and dual‑frequency GAST variants are under active development and validation to enable lower minima and autoland capability (CAT II/III). Progress depends on finalized MOPS/SARPs updates, successful field validations, and regulatory certification pathways for multi‑frequency, multi‑constellation GBAS.