This folder contains the 10 deep-dive files for the GBAS topic. GBAS is the local-area GNSS augmentation system that provides differential corrections and integrity data via a VHF data broadcast to support GLS (GBAS Landing System) precision approach and landing operations up to CAT III.

Files in this folder#

Reading order#

For a structured understanding: overview → components → blocks → threads → modules. Use enablers, performance_objectives, timeline, and references as reference material on demand.

Source basis#

Primary sources: Annex 10 Vol I (Aeronautical Telecommunications), Chapter 3 and Appendix B Section 3.6 — normative GBAS SARPs; Doc 8168 PANS-OPS Vol II, Part III Section 3 Chapter 6 — GLS procedure design; Doc 9849 GNSS Manual — implementation and evolution guidance.

Amendment 91 (7th Edition, applicable 8 November 2018) is the regulatory baseline for GAST C/D. Amendment 93 (8th Edition, applicable 2 November 2023) added ionospheric gradient mitigation provisions.

What is GBAS?#

Ground-Based Augmentation System (GBAS) is a local-area GNSS augmentation system that improves the accuracy, integrity, continuity, and availability of GNSS signals to support precision approach and landing at airports. GBAS is defined in Annex 10 Vol I as "an augmentation system in which the user receives augmentation information directly from a ground-based transmitter."

The service GBAS delivers to flight operations is called the GBAS Landing System (GLS). GLS is defined in PANS-OPS Doc 8168 as "a system for approach and landing operations utilizing GNSS, augmented by a ground-based augmentation system (GBAS), as the primary navigational reference." GLS charts are titled "GLS RWY XX" and carry a GBAS channel number and a Reference Path Identifier (RPI) in lieu of the ILS frequency.

Position in the ICAO/ATM framework#

GBAS is one of three GNSS augmentation architectures defined in Annex 10 Volume I, alongside ABAS (Aircraft-Based Augmentation System) and SBAS (Satellite-Based Augmentation System).

ABAS provides fault detection (RAIM) using the aircraft receiver alone and supports en-route through non-precision approach. SBAS broadcasts integrity and correction data from geostationary satellites, covering wide areas and supporting APV and CAT I. GBAS broadcasts from a single airport ground station and is the only GNSS augmentation architecture that supports CAT II and CAT III autoland operations under current SARPs.

GRAS (Ground-based Regional Augmentation System) extends GBAS coverage across a network of ground stations to support regional terminal and non-precision operations, but does not provide the precision approach service that GBAS does. No States currently plan GRAS implementations as a primary service.

Why GBAS matters in the CNS framework#

In the Communications, Navigation and Surveillance (CNS) triangle, GBAS sits firmly on the Navigation axis as the most advanced standardised ground-based precision approach navigation aid. Its significance is multi-dimensional:

Multi-approach coverage. A single GBAS station can support up to 49 approach procedures across all runway ends at an airport, covering all glide path angles (2.5 degrees to 3.5 degrees, or up to 3.2 degrees for CAT II/III), offset alignments, and displaced thresholds. This contrasts with ILS, which requires a separate facility per runway direction.

Airport access. GBAS can enable precision approach at runway ends where ILS installation is physically impractical due to terrain, airfield geometry, or cost. The VDB antenna siting is largely independent of runway centreline (unlike the ILS localiser), although it must be optimised for multipath and coverage.

Operational flexibility. GBAS supports multiple glide path angles on the same runway, enabling noise abatement approaches and simultaneous operations to parallel runways with independent path management.

Scalability. As fleet GBAS equipage grows, the ground station cost is amortised across more operations. Over 100 airlines currently operate GBAS-equipped aircraft totalling over 8,000 aircraft (Doc 9849, as of 2025).

GANP alignment. The GBAS CAT I capability is aligned with the ASBU NAVS thread (Navigation Systems) and APTA thread (Approach Procedures including Vertical Guidance), supporting precision approach modernisation at the Block 0/B1 level.

Relationship to ILS#

GBAS GLS is designed to be operationally compatible with ILS. The flight deck presentation is the same: the pilot sees a course deviation indicator and glide slope indicator driven by the airborne GBAS receiver rather than by ILS radio beams. Decision heights and RVR values are equivalent for the same operational category. Annex 10 §3.1.2.8 governs ILS/GBAS co-existence at aerodromes where both serve the same runway end, including the requirement for the ILS localiser not to radiate during GAST D operations when this could interfere with VDB reception.

This GBAS/ILS equivalency means states and airlines can phase in GBAS while maintaining ILS as a fallback without retraining pilots for a new flight-deck interface.

References#

• Annex 10 Vol I (Aeronautical Telecommunications), Chapter 3, §3.7.3.5 — normative GBAS performance and service volume definition.
• Annex 10 Vol I, Chapter 3, §3.1.2.8 — ILS/GBAS co-existence and localiser non-radiation requirement during GAST D.
• Doc 8168 (PANS-OPS) Vol II, Definitions — formal definitions of GBAS, GLS, and GBAS Approach Service Type (GAST).
• Doc 9849 (GNSS Manual), §1.4.2.3 — GBAS positioning service scope; GBAS designed primarily for APV and CAT I/II/III.
• Doc 9849 (GNSS Manual), §4.4.1 — system architecture overview.

The constituent elements of a GBAS installation#

A GBAS installation comprises a ground subsystem and an aircraft subsystem. Annex 10 Vol I Appendix B §3.6.1 states: "The GBAS shall consist of a ground subsystem and an aircraft subsystem. The GBAS ground subsystem shall provide data and corrections for the GNSS ranging signals over a digital VHF data broadcast to the aircraft subsystem."

The six key components are:

• Reference receivers
  • Installed on the airport, typically three or four units
  • Each tracks GNSS satellite signals and forms pseudo-range measurements
  • Diversity in placement reduces common-mode failures and improves fault detection
  • Siting is critical: must avoid multipath from buildings, aircraft on aprons, and terrain; must maintain clear sky view
  • For FAST D ground subsystems, the integrity bounding requirement (§3.6.7.1.2.2) mandates that the system can still bound errors with any one reference receiver excluded
• Ground processing unit
  • Aggregates pseudo-range measurements from all reference receivers
  • Computes differential corrections (pseudo-range corrections) for each tracked satellite, using carrier-smoothed code measurements
  • Applies approach-service-type-specific smoothing parameters: tau = 100 seconds for GAST A/B/C; for GAST D, tau = 100 seconds when sigma_iono is used for integrity bounding, or tau = 30 seconds when used for measurement weighting
  • Computes sigma_pr_gnd (ground subsystem error standard deviation) and broadcast ephemeris decorrelation parameters
  • Formats VDB messages: Type 1 (pseudo-range corrections), Type 2 (GBAS-related data, additional data blocks), Type 4 (FAS data blocks), Type 11 (additional data for GAST D)
  • Monitors integrity; sets alert flags if integrity cannot be assured
• VHF Data Broadcast (VDB) transmitter
  • Operates in 108.025-117.975 MHz band (shared with VOR/ILS)
  • Modulation: D8PSK (3-bit symbols), 10,500 symbols per second
  • TDMA frame structure: two time slots nominally assigned per facility (more if multiple antennas are used for coverage)
  • Polarization: GBAS/H (horizontal, mandatory Standard) or GBAS/E (elliptical, Recommended Practice); GBAS/E adds a vertical component enabling vertically-polarised avionics
  • EIRP provides minimum field strength of 215 microvolts per metre (-99 dBW/m2) throughout the service volume
  • Authentication: optional for GAST C; mandatory for GAST D (additional data block 4 carries VDB authentication parameters)
  • A single VDB frequency can carry up to 49 FAS data blocks, covering all runway ends at the airport
• FAS data block (Final Approach Segment data block)
  • Defines a unique approach geometry in space
  • Key parameters: Landing Threshold Point (LTP) or Fictitious Threshold Point (FTP), Flight Path Alignment Point (FPAP), Threshold Crossing Height (TCH), Glide Path Angle (GPA), Delta Length offset, approach performance designator (APD)
  • For GAST D: LTP must be at the runway threshold intersection; FPAP must be on the extended centreline; FTP is not permissible
  • CRC-protected to ensure data integrity through storage, loading, and transmission
  • Stored in the GBAS ground station and broadcast in Type 4 messages; also stored in on-board databases for approaches with channel numbers 40,000-99,999
  • Channel numbers 20,001-39,999 map to VDB frequency and time slot via a channel assignment formula
• Airborne GBAS receiver (Multi-Mode Receiver, MMR)
  • Tracks both the GNSS satellite signals and the VDB signal
  • On channel selection, automatically tunes the VDB frequency
  • Applies received pseudo-range corrections to its own measured pseudo-ranges
  • Computes vertical protection level (VPL) and lateral protection level (LPL) — upper confidence bounds on vertical and lateral position errors
  • Checks VPL against vertical alert limit (VAL) and LPL against lateral alert limit (LAL); provides alert if either is exceeded
  • Produces "ILS lookalike" deviation output — course deviation and glide slope deviation — displayed on the same flight-deck instruments as ILS
  • RTCA DO-253 (GBAS MOPS) and EUROCAE ED-114A are the airborne equipment minimum operational performance standards
• Integrity monitoring subsystem
  • Both ground and airborne elements participate in integrity
  • Ground monitors: detect satellite faults, reference receiver faults, ionospheric anomalies (gradient monitoring), and ephemeris errors
  • Signal-in-space integrity risk for GAST A/B/C: less than 1.5 x 10^-7 per approach (§3.6.7.1.2.1.1.1)
  • Signal-in-space integrity risk for GAST D (excluding ionospheric anomalies): less than 1.5 x 10^-7 per approach; probability of non-compliant information > 1.5 seconds less than 1 x 10^-9 per landing (§3.6.7.1.2.1.1.3)
  • Airborne monitors for GAST D: compare 30-second and 100-second smoothed position solutions (DV, DL parameters) to detect ionospheric anomalies not caught by the ground station

GBAS vs ILS: key architectural differences#

References#

• Annex 10 Vol I, Appendix B, §3.6.1 — GBAS ground and aircraft subsystem definition; GBAS message types.
• Annex 10 Vol I, Appendix B, §3.6.4.5 — FAS data block parameters (LTP/FTP, FPAP, TCH, GPA, APD, CRC).
• Annex 10 Vol I, Appendix B, §3.6.7.1.2.1 — ground subsystem integrity risk requirements for GAST A/B/C and GAST D.
• Annex 10 Vol I, Chapter 3, §3.7.3.5.4 — VDB signal characteristics (D8PSK, 10,500 symbols/s, TDMA, EIRP, polarization).
• Doc 9849 (GNSS Manual), §4.4.1 — GBAS architecture: reference receivers, VDB, FAS data block, multi-approach capability, TDMA, authentication.
• Doc 8168 (PANS-OPS) Vol II, Part III, Section 3, Chapter 6, §6.1.3.2 — FAS data block construction and CRC; §6.1.3.3 — GAST D FAS data constraints.

Mapping GAST to the GBAS block concept#

For GBAS, the "blocks" are the GBAS Approach Service Type (GAST) service levels. Each GAST level represents a defined tier of performance, a matched set of ground and airborne requirements, and a supported operational category. The progression from GAST C to GAST D to DFMC mirrors the block structure of ASBU: each tier builds on the prior one and requires its prerequisites.

The PANS-OPS Doc 8168 Vol II definition: "A GAST is defined as the matched set of airborne and ground performance and functional requirements that are intended to be used in concert in order to provide approach guidance with quantifiable performance."

GAST C — Category I (the current baseline)#

GAST C is the baseline service type supporting CAT I precision approach. It is the service type provided by the vast majority of the approximately 140 certified GBAS stations worldwide (as of 2025).

Operational performance: Decision Height (DH) typically 60 m (200 ft), RVR 550 m, equivalent to ILS CAT I.

Ground requirements: FAST C ground subsystem. Broadcasts Type 1, Type 2 (with additional data blocks 1-3), and Type 4 messages. Integrity risk: 1.5 x 10^-7 per approach. VDB authentication optional.

Airborne requirements: GAST C-capable MMR compliant with RTCA DO-253C (or EUROCAE ED-114A). Uses 100-second carrier-smoothing filter (tau = 100 s). Protection levels compared against GAST C alert limits.

Maturity: SARPs codified through successive amendments culminating in Amdt 91. First operational CAT I GBAS deployments started in the late 2000s and early 2010s (Sydney 2012, Newark circa 2009, Frankfurt 2012 — see timeline.md). Widely deployed; commercially mature.

GAST C with extended CAT II. Doc 9849 §7.9.4.3-4 notes that in some regions, GAST C can support CAT II minima with additional ionospheric monitoring (e.g. using SBAS ionospheric information), adapted FAS data (APD = 1, FASLAL reduced to 10 m), and specific regulatory approval. This is not a standard GAST C approval.

GAST D — Category II/III Autoland#

GAST D was standardised in Annex 10 Amendment 91 (7th Edition, applicable 8 November 2018). It extends the service to support Category II/III operations including autoland and guided take-off.

Operational performance: Category II (DH 30-60 m, RVR 300-550 m) and Category III (DH below 30 m or no DH, RVR below 300 m to RVR 0).

Ground requirements: FAST D ground subsystem (must also meet all FAST C requirements). Additional Type 11 message (GAST D parameters: sigma_pr_gnd_D, ephemeris decorrelation parameters for GAST D). Broadcasts additional data block 3 (GAST D parameters) in Type 2. VDB authentication mandatory. Integrity risk (excluding ionospheric anomalies): 1.5 x 10^-7 per approach; probability of non-compliant information > 1.5 s: less than 1 x 10^-9 per landing.

Airborne requirements: GAST D-capable avionics. Dual time-constant processing: 30-second and 100-second carrier-smoothing filters. DV and DL parameters computed and checked. Protection levels computed using GAST D sigma values. Alert limits tighter than GAST C (Table B-136/B-137 in Annex 10 Appendix B).

FAS data constraints for GAST D: LTP must be at the runway threshold; FTP is not permissible; FPAP must be on the extended runway centreline; authentication protocol must be applied.

Status: Amendment 91 SARPs are in force. Prototype GAST D stations validated the SARPs pre-adoption. Commercial GAST D deployments are limited; most airports have retained ILS CAT II/III alongside GBAS GAST C. The business case for GAST D is strongest at new airports without existing ILS CAT II/III investment (Doc 9849 §7.4.5.3).

DFMC GBAS — Dual-Frequency Multi-Constellation (future)#

Doc 9849 §6.9.3-4 and Annex 10 Amdt 93 Note 2 (§3.6 Note 2) confirm that the current GBAS SARPs are applicable only to GPS L1 C/A and GLONASS L1OF (single-frequency). DFMC GBAS is the next evolutionary step.

Drivers:

• Single-frequency GBAS is vulnerable to ionospheric anomalies (large gradients). The GAST D mitigation (dual time-constants, ground monitoring) addresses this partially but siting constraints remain in equatorial regions.
• DFMC provides direct ionospheric delay measurement from two frequencies, removing the need for a threat model.
• Multi-constellation improves satellite geometry availability for CAT II/III operations globally.

Expected capabilities: Support for all GAST service types (CAT I through CAT III) with higher availability. Surface movement guidance. Low-visibility take-off. Guided departures. Future services for advanced air mobility autoland.

SARPs status: Annex 10 Amendment 93 (applicable 2 November 2023) introduced ionospheric gradient mitigation provisions as a step toward DFMC. Full DFMC GBAS SARPs are under development by the Navigation Systems Panel (NSP). The current §3.6 note states: "GBAS SARPs have not yet been updated to support dual-frequency multi-constellation (DFMC) use."

Augmentation family context#

References#

• Annex 10 Vol I, Appendix B, §3.6.1.1 — GBAS service types: positioning service and approach service.
• Annex 10 Vol I, Appendix B, §3.6.7.1.2.1 — integrity risk requirements for GAST A/B/C and GAST D.
• Annex 10 Vol I, Amendment 91 (applicable 8 November 2018) — GAST C/D codification, GBAS Facility Classification, FAST D requirements.
• Annex 10 Vol I, Amendment 93 (applicable 2 November 2023) — ionospheric gradient mitigation provisions for GBAS.
• Doc 8168 (PANS-OPS) Vol II, Definitions, §GBAS Approach Service Type (GAST) — formal four-type definition; GAST A/B for APV; GAST C for CAT I; GAST D for CAT II/III.
• Doc 9849 (GNSS Manual), §6.9.2 — Amendment 91 CAT II/III support; applicable 8 November 2018.
• Doc 9849 (GNSS Manual), §6.9.3-4 — DFMC GBAS evolution: multi-frequency, multi-constellation, future capabilities.
• Doc 9849 (GNSS Manual), §7.9.4.3-4 — GAST C/D operational mapping; GAST C with extended CAT II.

Overview#

GBAS can be understood through six functional threads. Each thread is a distinct engineering or operational axis that must be addressed to implement and sustain GBAS service. The threads are interdependent: deficiency in one (e.g. siting) can limit performance on another (e.g. integrity).

The six threads:

• Thread 1 — Signal-in-space integrity
  • The core promise of GBAS is bounded integrity: the system must provide an alert before a position error exceeds the alert limit for the intended operation.
  • Ground integrity: the ground subsystem must detect and alert on satellite faults, reference receiver faults, ionospheric anomalies, and ephemeris errors before they cause out-of-tolerance guidance.
  • For GAST A/B/C: integrity risk less than 1.5 x 10^-7 per approach (Annex 10 App B §3.6.7.1.2.1.1.1).
  • For GAST D (excluding ionospheric): same 1.5 x 10^-7 bound plus probability of non-compliant information transmitted for more than 1.5 seconds less than 1 x 10^-9 per landing.
  • Ionospheric threat: single-frequency GBAS relies on a threat model (sigma_vert_iono_gradient) to bound ionospheric gradient errors. Amdt 93 added provisions for ionospheric gradient mitigation. DFMC GBAS will directly measure the ionospheric delay.
  • Airborne integrity (GAST D): airborne equipment monitors the difference between 30-second and 100-second smoothed position solutions (DV vertical, DL lateral) to detect ionospheric anomalies not caught by the ground station.
  • Protection levels VPL and LPL are computed from broadcast sigma values and checked against the service-type alert limits (Tables B-136 and B-137 for GAST C and D).
• Thread 2 — VHF Data Broadcast (VDB) datalink
  • The VDB is the physical communication link from the ground station to the aircraft.
  • 108.025-117.975 MHz band; D8PSK modulation; 10,500 symbols/second; TDMA frame structure.
  • Two time slots nominally assigned; multiple antennas can be used with more time slots to solve coverage problems from building blockage.
  • Polarization: GBAS/H (horizontal, Standard) and GBAS/E (elliptical, Recommended Practice). Most civil aircraft use horizontally polarised antennas.
  • ILS compatibility: Amdt 92 addressed VDB compatibility with ILS and VOR signals in the same band. Annex 10 §3.1.2.8 governs ILS/GBAS co-existence where both serve the same runway end.
  • Authentication (Amdt 91): optional for GAST C, mandatory for GAST D (additional data block 4). Protects against spoofing of the VDB signal.
  • Service volume: minimum 45 km (28 NM) radius from the VDB antenna in azimuth, and coverage down to 12 ft AGL in the additional GAST D service volume extension (§3.7.3.5.3.2).
  • The broadcast FAS data block in Type 4 messages defines each approach uniquely by channel number and Reference Path Identifier.
• Thread 3 — GLS approach procedure design
  • GLS procedure construction follows PANS-OPS Doc 8168 Vol II, Part III, Section 3, Chapter 6.
  • The FAS data block defines the approach geometry: LTP/FPAP baseline, TCH, GPA, and delta-length offset. The procedure designer computes the path geodetically and formats it into the FAS data block with CRC protection.
  • Glide path angle range: 2.5 degrees (minimum) to 3.5 degrees (maximum), or 3.2 degrees maximum for CAT II/III operations.
  • Course width: 210 m (plus/minus 105 m) at the threshold.
  • Reference datum height: 15 m (50 ft).
  • GAST D specific: LTP at the runway threshold; FPAP on the extended centreline; FTP not permitted; authentication applied; APD coded per desired performance.
  • Offset azimuth approaches (GLS CAT I with offset track) are permitted under PANS-OPS Vol II §6.6.
  • Unlike ILS, the GLS guidance geometry does not depend on the physical location of any ground transmitter antenna. The path is entirely defined by the FAS data block, giving designers flexibility to optimise for obstacles, noise, and terrain.
• Thread 4 — Airworthiness and avionics certification
  • Airborne GBAS equipment must comply with: RTCA DO-253 (current revision) — GBAS Airborne Minimum Operational Performance Standards (MOPS); EUROCAE ED-114A — European equivalent.
  • Aircraft certification for CAT I: typically STC or TC amendment to aircraft flight manual authorising GLS CAT I.
  • Aircraft certification for CAT II/III autoland: requires full all-weather operations approval under EASA CS-AWO or FAA Order 6750.24. This includes autopilot/autothrottle integration, flare law, rollout guidance, and go-around capability.
  • MMR (Multi-Mode Receiver): the standard avionics unit integrating ILS, MLS, and GBAS landing system functions in a single unit. The pilot selects a GLS channel in exactly the same manner as dialling an ILS frequency.
  • NOTAM and AIP management: states must publish GBAS facility information (VDB polarization type, service volumes, channel assignments) in the AIP. GBAS outage NOTAMs are required when the station is out of service (Doc 9849 §7.9.4.8).
• Thread 5 — Siting, multipath, and interference
  • Reference receiver siting is the primary ground engineering challenge. Sources of error include multipath from buildings, taxiing aircraft, and terrain; signal blockage; RF interference from co-channel or adjacent-channel sources.
  • Doc 9849 §4.4.1.3: "Unlike ILS and MLS, antenna location is relatively independent of the runway configuration, but requires the careful evaluation of local sources of interference, signal blockage, airport protection area and multipath."
  • VDB antenna siting: must provide coverage throughout the service volume, down to the minimum height above the runway surface.
  • Equatorial regions: single-frequency GBAS faces more stringent siting constraints or lower availability in regions with active ionospheric scintillation. DFMC GBAS will reduce this constraint by directly measuring ionospheric delay.
  • Protection areas: states define airport GBAS protection zones to restrict obstructions that could cause multipath or blockage for reference receivers. These are published in the AIP.
  • Note: Amdt 92 specifically addressed VDB compatibility with ILS and VOR signals operating in the same frequency band.
• Thread 6 — Multi-constellation and DFMC evolution
  • Current SARPs (Annex 10 Amdt 93): GPS L1 C/A and GLONASS L1OF only. Note §3.6 Note 2 explicitly states "GBAS SARPs have not yet been updated to support dual-frequency multi-constellation (DFMC) use."
  • Russia deployed GPS+GLONASS dual-constellation GBAS stations. This confirms that multi-constellation is already operationally proven under the current L1-only SARPs.
  • Amdt 93 ionospheric gradient mitigation: preparatory step for DFMC, enabling tighter bounding with enhanced monitoring.
  • DFMC GBAS: NSP working group developing new SARPs. Direct ionospheric delay measurement from L1+L5 (GPS), E1+E5 (Galileo), and other dual-frequency pairs. Enables CAT II/III at airports in equatorial and high-ionospheric-activity regions.
  • Expected DFMC GBAS capabilities beyond CAT I/II/III: surface movement guidance, low-visibility take-off, guided departure procedures, AAM precision landing (Doc 9849 §6.9.4).

References#

• Annex 10 Vol I, Appendix B, §3.6.5.5.1 — protection level computation for all GBAS approach service types (VPL, LPL).
• Annex 10 Vol I, Appendix B, §3.6.7.1.2.1 — ground subsystem signal-in-space integrity risk; §3.6.7.1.2.1.1.3 — GAST D non-compliant information probability.
• Annex 10 Vol I, Appendix B, §3.6.4.3.2.2 — GAST D additional data block 3 parameters.
• Annex 10 Vol I, §3.7.3.5.4 — VDB signal specification (D8PSK, TDMA, polarization, EIRP).
• Annex 10 Vol I, §3.1.2.8 — ILS/GBAS co-existence; localiser non-radiation during GAST D.
• Doc 8168 (PANS-OPS) Vol II, Part III Section 3 Chapter 6, §6.1.3 — GLS standard conditions and FAS data constraints.
• Doc 9849 (GNSS Manual), §4.4.1.3 — siting considerations (multipath, interference, signal blockage).
• Doc 9849 (GNSS Manual), §6.9.3-4 — DFMC evolution roadmap.

Anatomy of a GAST-D CAT III GLS approach#

This file walks through a complete GAST-D CAT III GLS approach — from pre-flight database validation through touchdown and roll-out. This worked example illustrates how all the GBAS components and threads converge in a single operational event.

Scenario#

Aircraft: B777-300ER with GAST-D capable MMR and autoland system. Airport: A major international airport with a certified FAST-D GBAS ground station on a runway already clear of ILS CAT III obstacle surfaces. Meteorological conditions: RVR 100 m (CAT III). The ILS for the same runway end is decommissioned or switched off per Annex 10 §3.1.2.8 (the localiser must not radiate during GAST D operations unless interference compliance is demonstrated).

Step 1: Pre-flight — FAS data block validation#

The airline's Flight Management System (FMS) carries an onboard GBAS database containing FAS data blocks for GLS approaches. Each FAS data block contains a CRC-protected set of parameters:

• LTP: the Landing Threshold Point at the runway threshold elevation.
• FPAP: Flight Path Alignment Point on the extended centreline.
• TCH: Threshold Crossing Height (standard 15 m / 50 ft).
• GPA: Glide Path Angle (3.0 degrees nominal; max 3.2 degrees for CAT II/III operations).
• Approach Performance Designator (APD): coded 1 or 2 to indicate GAST D performance.
• Delta Length offset: 0 for GAST D (FPAP correctly positioned at stop end).

The avionics validates the CRC against the stored data before the approach is accepted. If the CRC fails, the approach is rejected.

Step 2: Approach briefing and crew actions#

The crew selects the GLS approach from the FMS approach page. They enter the GBAS channel number (e.g. 20143) assigned to the CAT III runway. The MMR automatically tunes the VDB frequency and begins receiving the broadcast. The flight deck indication shows "GLS" on the nav source annunciator.

The approach performance designator is checked against the published minima. For CAT III, the DH is typically 15 m (50 ft) or less (or no DH in some CAT IIIc concepts), and the required RVR is 175-200 m (CAT IIIb) or lower.

Step 3: Final approach capture and precision segment#

At approximately 18-20 NM from the threshold (within the GBAS service volume of 45 km), the MMR locks onto the VDB and begins receiving pseudo-range corrections. The airborne equipment processes:

Type 1 message: pseudo-range corrections for each satellite in view. Type 2 message: GBAS reference data, additional data block 3 (GAST D parameters: sigma_pr_gnd_D, Kmd_e_D parameters, sigma_vert_iono_ gradient_D, YEIG, MEIG). Type 4 message: the FAS data block for the selected channel. Type 11 message: GAST D ephemeris decorrelation parameters.

The MMR applies corrections to its own GNSS measurements, computes the approach path deviation, and derives:

Vertical Protection Level (VPL) using GAST D sigma_pr_gnd_D values. Lateral Protection Level (LPL) using GAST D lateral bounding. DV and DL: difference between 30-second and 100-second smoothed position solutions. These detect ionospheric anomalies.

If VPL exceeds the GAST D VAL (from Table B-137) or LPL exceeds the GAST D LAL (from Table B-136), the avionics alerts the crew before the decision altitude. The GAST D VAL tightens progressively below 60 m height above threshold, matching the increasing criticality.

Step 4: Autopilot-coupled approach and flare#

Below 300 ft (nominally), the autopilot is engaged in autoland mode coupled to the GLS deviation signals from the MMR. The flight deck CDI and glide slope display are indistinguishable from an ILS presentation. Annex 10 confirmation of this design intent: the FAS data block "enables the computation of ILS lookalike deviation guidance" (Doc 9849 §4.4.1.6).

The deviation guidance drives the autoland system through the flare law. The GBAS continues to update corrections at the VDB frame rate. Because the VDB carries angle-coded deviations referenced to the WGS-84 path, the geometry remains precisely defined all the way to the runway surface. Unlike ILS, there is no "false glide slope" risk from multipath at low heights (because the guidance is GNSS-derived, not beam-based).

Step 5: Touchdown, roll-out, and deceleration#

At touchdown, the GBAS GAST D additional service volume (Annex 10 §3.7.3.5.3.2) provides VDB coverage to 12 ft AGL and lateral roll-out guidance. Guided take-off (an extension of the GAST D capability) uses the same GBAS signal to maintain centreline guidance during the ground roll. The FPAP is correctly positioned at the stop end of the runway (delta-length = 0, per §3.6.7.2.4.5).

Step 6: Post-landing monitoring#

The GBAS system continues to monitor satellite geometry. If the operational category degrades (e.g. from CAT III to CAT I due to a reference receiver failure), the GBAS ground station broadcasts a downgrade notification via the Type 2 message and the avionics annotate the current guidance level. The crew and ATC are notified. This fall- back capability is a planned feature of the GBAS architecture.

Module summary: key numbers#

References#

• Annex 10 Vol I, Appendix B, §3.6.4.3.2.2 — GAST D additional data block 3 parameters.
• Annex 10 Vol I, Appendix B, §3.6.5.5.1 — protection level computation for all GAST; GAST D DV/DL parameters.
• Annex 10 Vol I, Appendix B, §3.6.7.1.2.1.1.3 — non-compliant information probability for GAST D.
• Annex 10 Vol I, §3.7.3.5.3.2 — additional GAST D service volume for autoland and guided take-off.
• Annex 10 Vol I, Appendix B, §3.6.7.2.4.4-4.5 — LTP/FTP and FPAP location constraints for GAST D FAS.
• Doc 8168 (PANS-OPS) Vol II, Part III Section 3 Chapter 6, §6.1.3.3 — GAST D FAS data constraints (LTP at threshold; no FTP; FPAP on extended centreline; authentication).
• Doc 9849 (GNSS Manual), §4.4.1.6 — FAS data block provides ILS-lookalike deviation guidance.
• Doc 9849 (GNSS Manual), §7.9.4.3 — GAST C supports CAT I; GAST D supports CAT II/III.

CNS enablers#

Navigation (primary). GBAS is itself a navigation enabler, but it depends on the health of the GNSS core constellations it augments. GPS L1 C/A and GLONASS L1OF are the currently standardised ranging sources under Annex 10 App B §3.6. Galileo and BDS are being incorporated through DFMC provisions. Minimum satellite geometry (PDOP, sufficient number of tracked satellites) is required for protection levels to remain below alert limits. GNSS signal health monitoring via NOTAMs is required when satellites are flagged as unhealthy.

Communications. ATC must be notified of GBAS outages and service downgrades. The GBAS ground station communicates status to the ANSP operations centre via maintenance interfaces. ATC provides GBAS channel number to pilots on initial approach frequency assignment (or it appears on the approach chart). No specific ATC datalink is required for routine GLS operations, but CPDLC can be used for pre-departure clearances that include approach type selection.

Surveillance. Not a direct enabler for GLS guidance, but surface movement surveillance (A-SMGCS) is complementary for low-visibility operations on the ground. CAT II/III operations require a clear Obstacle Free Zone (OFZ) verified through surveillance; this is an Annex 14 requirement, not a GBAS requirement per se.

Procedures and operations#

Approach procedure design. Procedure designers must be trained in PANS-OPS Vol II Part III Section 3 Chapter 6 GLS criteria. This is a specialist skill distinct from standard ILS/VOR procedure design because of the FAS data block creation, geodetic computations, and GBAS-specific parameters (APD, FPAP positioning for GAST D). Doc 9368 (IFP Construction Manual) Attachment C.5 provides the FAS data block encoding guidance.

AIP promulgation. GBAS channel numbers, Reference Path Identifiers (RPI), VDB frequency, VDB polarization type, and service volumes must be published in the State AIP. Chart identification follows PANS-OPS Vol II Section 5 (and Part I Section 4 for PBN GLS combination charts): "GLS RWY XX" with optional route indicator for duplicate procedures.

NOTAM. GBAS outage NOTAMs are required when the station is unserviceable. If a GBAS element failure results in service downgrade (e.g. CAT III to CAT I) rather than full outage, a downgrade NOTAM is also required (Doc 9849 §2.6 and §7.9.4.8).

Operational approval. Operators must obtain regulatory approval for GBAS CAT I operations (via Flight Operations Inspector approval or equivalent). CAT II/III GBAS operations require a full All-Weather Operations (AWO) approval including demonstrated aircraft capability, crew training, simulator validation, and regulatory acceptance of the approach service type.

Training#

Flight crew. Initial familiarisation: GBAS GLS operation is intentionally similar to ILS. Pilots trained on ILS can transition to GLS with relatively brief type-rating differences training. Key training elements:

• Channel number selection vs. frequency dial.
• Understanding the GBAS annunciation on the EHSI/PFD.
• GBAS-specific failure modes (VDB signal loss; protection level exceedance; service type downgrade).
• CAT III autoland decision process for GAST D.

Maintenance. Ground station maintenance technicians require training on:

• Reference receiver calibration and periodic verification.
• VDB transmitter maintenance and power measurement.
• GBAS processor health monitoring and parameter verification.
• Quarterly/annual site survey (multipath monitoring).
• Authentication key management for GAST D stations.

ATC. Controllers at airports with GBAS need familiarisation with GLS approach type, GBAS channel numbers, NOTAM interpretation for service degradation, and phraseology (no change from ILS for routine operations; specific procedures for service downgrade advisories).

Regulation and certification (ground station)#

ICAO SARPs compliance. Ground stations must comply with Annex 10 Vol I (current edition, Amdt 94 from 27 November 2025 the latest amendment). Flight inspection of GBAS includes verification of VDB coverage, EIRP, signal quality, and protection level bounding.

FAA. FAA authorises GBAS stations under Technical Standard Order (TSO) TSO-C161a (ground equipment) and TSO-C162a. Site Acceptance (SCA) is required before operational service. FAA Order 8400.13D governs GBAS operational approval.

EASA. EASA certifies GBAS installations under ETSO-C161a/C162a equivalents. CS-AWO governs airworthiness for GBAS CAT II/III operations on the aircraft side. Airspace authority (national CAA) approves the ground station and the published procedures.

ICAO Annex 10 / Annex 14 intersection. Annex 14 inner approach, inner transitional, and balked landing surfaces must not be penetrated for CAT II/III operations (Annex 14 Section 4). This is an aerodrome design enabler independent of GBAS, but it is a prerequisite for any CAT II/III precision approach — ILS, MLS, or GLS alike.

Institutional and coordination#

Frequency coordination. GBAS VDB frequencies in the 108-117.975 MHz band are coordinated through the national frequency assignment process in accordance with Annex 10 and Doc 9718 (Handbook on Radio Frequency Spectrum Requirements). The GBAS station must be protected from co-channel and adjacent-channel interference from ILS localisers and VOR stations.

GNSS Signal-in-Space monitoring. States with GBAS must monitor GNSS signal quality. Annex 10 Section 3.9 addresses status monitoring and NOTAM requirements when GNSS elements are unhealthy. The GBAS ground station performs its own real-time satellite monitoring and can exclude unhealthy satellites from the corrections it broadcasts.

Multilateration / interferer hunting. Radio Frequency Interference (RFI) to GNSS is a growing concern. States operating GBAS should have RFI detection and mitigation procedures. Doc 9849 Chapter 5 provides guidance on GNSS interference threats.

Fleet equipage. GBAS capacity benefit depends on equipped aircraft share. Airline operators must equip aircraft with GAST C (or GAST D for CAT II/III) MMRs. The cost of MMR installation is borne by the airline. Business case analyses (Doc 9849 Chapter 7) must factor in the equipage transition period.

References#

• Annex 10 Vol I, §3.7.3.5.4.6 — VDB unwanted emissions; band protection from ILS/VOR.
• Annex 10 Vol I, Appendix B, §3.6.7.2 — ground subsystem functional requirements; message formats.
• Doc 8168 (PANS-OPS) Vol II, Part III Section 3 Chapter 6, §6.7 — GLS promulgation requirements (channel number, RPI, polarization).
• Doc 9849 (GNSS Manual), §7.4.5.3 — business case for CAT II/III GBAS; airfield infrastructure costs.
• Doc 9849 (GNSS Manual), §7.9.4.8 — GBAS NOTAM and AIP requirements; outage and service degradation.
• Doc 9849 (GNSS Manual), Chapter 5 — GNSS interference threats and mitigation.
• Doc 9718 (Handbook on Radio Frequency Spectrum Requirements) Vol II — GBAS/ILS/VOR frequency assignment criteria (authoritative source — not in local library).

The GBAS performance lens#

GBAS performance is characterised through the four GNSS performance parameters defined in Annex 10 and the broader ATM Key Performance Areas (KPAs). The four signal-in-space parameters are:

• Accuracy — the difference between estimated and true position. Measured as the 95th percentile error.
• Integrity — the ability to provide timely warnings when the system cannot be trusted. Measured as protection levels vs. alert limits.
• Continuity — the probability of unscheduled loss of service during a planned operation.
• Availability — the fraction of time the system is simultaneously accurate, has integrity, and is continuous.

Numeric signal-in-space requirements (from Annex 10 Table 3.7.2.4-1)#

Note: For CAT I precision approach, a VAL greater than 10 m requires a system-specific safety analysis (Annex 10 Attachment D §3.3.6-3.3.10; Doc 9849 §7.9.3.2).

KPA contribution matrix#

The matrix below scores GBAS contribution to each Key Performance Area by service level (1 = some benefit, 2 = clear benefit, 3 = primary driver). Scoring is editorial — consistent with the ICAO performance framework from Doc 9854 and Doc 9883.

KPA narrative#

Safety. GBAS delivers the same or better safety performance as ILS for equivalent operational categories. The protection level / alert limit architecture provides a mathematical guarantee that an alert is issued before position errors reach the hazard threshold. GAST D adds tighter integrity requirements (1 x 10^-9 per landing for non-compliant information transmission) and dual time-constant monitoring for ionospheric anomalies.

Capacity. A single GBAS serves all runways at an airport, enabling precision approach to runway ends that do not have ILS, thereby unlocking additional capacity at constrained airports. CAT II/III operations reduce low-visibility ground holds and diversions, sustaining throughput in adverse conditions. Multi-approach support also enables independent parallel approaches without per-runway ILS investments.

Flight efficiency. GLS supports flexible glide path angles (higher angles reduce noise exposure and fuel burn on approach). Continuous Descent Approach (CDA) profiles can be more consistently maintained with GBAS angular guidance than with ILS (which has critical areas near the localiser that restrict CDA in some configurations). DFMC GBAS is expected to further enable noise-optimised curved approaches.

Predictability. CAT II/III capability reduces weather-related diversions and delays. The integrity system provides timely alerts, enabling crews to execute go-arounds before losing positional accuracy, rather than discovering the problem at DH.

Environment. Higher GLS glide path angles (up to 3.5 degrees for CAT I, 3.2 degrees for CAT II/III vs. standard 3.0 degrees) can reduce overland noise exposure. Optimised approach angles reduce fuel burn on final approach. Guided take-off support (GAST D) can enable noise-reduced departure profiles.

Cost-effectiveness. GBAS ground station cost is comparable to ILS, but it serves all runways at an airport. For airports with more than 2-3 runway ends requiring precision approach, the multi-runway coverage of GBAS offers better per-runway cost than multiple ILS installations. Doc 9849 §7.4 provides the formal business case framework.

Access and equity. GBAS enables precision approach at runway ends where ILS is impractical: mountainous terrain, runway geometry preventing ILS siting, or developing-state airports without ILS maintenance expertise. The multi-approach capability allows Annex 10-compliant precision approach to be extended to secondary runways at regional airports.

Key Performance Indicators#

Safety KPIs:

• Approach incidents per 10,000 GLS approaches (target: consistent with ILS equivalent category).
• Protection level exceedance rate (should be zero in nominal operation).
• False alarm rate (continuity events): less than 1-8 x 10^-6 per 15-second approach period.

Capacity KPIs:

• Additional runways equipped with precision approach capability (vs. baseline ILS-only).
• Low-visibility throughput: maintained aircraft movements per hour in RVR below 400 m.

Flight efficiency KPIs:

• Average GLS glide path angle vs. standard ILS (3.0 degrees baseline); tracks noise-optimised approach usage.
• Track-mile deviation on final: GLS vs. ILS (should be comparable or better).

Cost-effectiveness KPIs:

• Ground station cost per runway end served (GBAS vs. ILS).
• Fleet GBAS equipage share at hub airports.

References#

• Annex 10 Vol I, Chapter 3, Table 3.7.2.4-1 — GNSS performance requirements (accuracy, integrity, continuity, availability) for all operations including CAT I.
• Annex 10 Vol I, Appendix B, Table B-136 and Table B-137 — GAST C and D lateral and vertical alert limits by height above threshold.
• Doc 9849 (GNSS Manual), §2.2 — accuracy, integrity, continuity, availability definitions and the protection level / alert limit architecture.
• Doc 9849 (GNSS Manual), §7.4 — GBAS business case analysis framework; ground station and airfield infrastructure costs.
• Doc 9849 (GNSS Manual), §7.9.4 — GBAS procedure design and minima; operational KPI comparisons with ILS.
• Doc 9854 (Global ATM Operational Concept) — eleven KPA framework source document (authoritative source — not in local library).
• Doc 9883 (Manual on Global Performance of the Air Navigation System) — KPI definitions (authoritative source — not in local library).

Historical evolution and key dates#

Amendment cadence summary#

Deployment milestones (selected)#

Note: Specific deployment dates for Frankfurt, Sydney, and GAST D trial sites are derived from Doc 9849 context and publicly available ANSP announcements; exact certification dates should be verified from national CAA records (authoritative source — not in local library).

References#

• Annex 10 Vol I (Aeronautical Telecommunications), Amendment history table — Amendments 77 through 94 with adoption, effective, and applicable dates.
• Doc 8168 (PANS-OPS) Vol II, Amendment history — GLS CAT I criteria introduced 2004 (OCP/13); GLS definition 2010.
• Doc 9849 (GNSS Manual), Foreword §1 — status as of 2025; approximately 140 certified stations; over 8,000 equipped aircraft.
• Doc 9849 (GNSS Manual), §6.9.2 — Amendment 91 applicable 8 November 2018; CAT II/III SARPs.

ICAO Normative References#

• Annex 10 Vol I (Aeronautical Telecommunications), Chapter 3, §3.7.3.5 — GBAS and GRAS performance requirements, service volume, VDB signal characteristics, navigation data requirements.
• Annex 10 Vol I, Chapter 3, §3.1.2.8 — ILS/GBAS co-existence; ILS localiser non-radiation requirement during GAST D low-visibility operations.
• Annex 10 Vol I, Appendix B, §3.6 — detailed GBAS and GRAS SARPs: ground subsystem, aircraft subsystem, message types (Type 1/2/4/11), pseudo-range corrections, sigma parameters, FAS data block, protection levels, alert limits, integrity monitoring.
• Annex 10 Vol I, Appendix B, §3.6.4.5 — FAS data block definition; LTP/FTP, FPAP, TCH, GPA, APD, CRC; GAST A/B/C/D approach performance designator encoding.
• Annex 10 Vol I, Appendix B, §3.6.5.5.1 — protection level computation (VPL, LPL) for all GBAS approach service types; GAST D DV/DL dual time-constant parameters.
• Annex 10 Vol I, Appendix B, §3.6.7.1.2.1 — ground subsystem integrity risk: GAST A/B/C less than 1.5 x 10^-7 per approach; GAST D less than 1.5 x 10^-7 and less than 1 x 10^-9 per landing for non-compliant transmission.
• Annex 10 Vol I, Appendix B, §3.6.7.2.4 — LTP/FTP and FPAP placement requirements for GAST D FAS data blocks.
• Annex 10 Vol I, Chapter 3, Table 3.7.2.4-1 — GNSS signal-in-space performance requirements (accuracy, integrity, continuity, availability) for all phases of flight including CAT I/II/III.
• Annex 10 Vol I, Appendix B, Tables B-136 and B-137 — GAST C and D lateral and vertical alert limits by height above threshold.
• Annex 10 Vol I, Amendment 91 (7th Edition, applicable 8 November 2018) — introduction of GAST A/B/C/D classification; GBAS Facility Classification; FAST D requirements; VDB authentication.
• Annex 10 Vol I, Amendment 92 (applicable November 2020) — GBAS VDB compatibility with ILS and VOR; clarifications and corrections to GBAS provisions.
• Annex 10 Vol I, Amendment 93 (8th Edition, applicable 2 November 2023) — ionospheric gradient mitigation provisions for GBAS; BDS and Galileo SARPs (DFMC groundwork).
• Annex 10 Vol I, Amendment 94 (applicable 27 November 2025) — GBAS frequency assignment planning; GPS/Galileo/SBAS/GBAS updates.

PANS and Procedures References#

• Doc 8168 (PANS-OPS) Vol I, §5.4.3.3 — altimeter cross-check note for ILS/MLS/GLS vs. APV/Baro-VNAV.
• Doc 8168 (PANS-OPS) Vol II, Definitions — formal definitions of GBAS, GLS, and GBAS Approach Service Type (GAST); four-type GAST framework.
• Doc 8168 (PANS-OPS) Vol II, Part III Section 3 Chapter 6 — GLS precision approach design criteria: FAS data block, standard conditions, glide path angle range (2.5 to 3.5 degrees; 3.2 degrees max for CAT II/III), course width 210 m, reference datum height 15 m; GAST D FAS constraints; offset azimuth GLS.
• Doc 8168 (PANS-OPS) Vol II, Part I Section 4 Chapter 9 — procedure chart identification including GLS RWY XX and route indicator.
• Doc 8168 (PANS-OPS) Vol II, Part III Section 2 Chapter 6 — FAS data block application for SBAS and GBAS; Appendix B: GBAS FAS data block parameters for procedure designers.

Implementation and Guidance Documents#

• Doc 9849 (GNSS Manual), §4.4 — GBAS system architecture: reference receivers, VDB, FAS data block, multi-approach capability (up to 49 approaches per station), TDMA, polarization (GBAS/H and GBAS/E), authentication.
• Doc 9849 (GNSS Manual), §2.2 — accuracy, integrity, continuity, availability definitions; protection level / alert limit architecture (VPL/LPL vs. VAL/LAL).
• Doc 9849 (GNSS Manual), §6.9 — GBAS evolution: Amendment 91 CAT II/III support; DFMC roadmap (multi-frequency, multi-constellation, enhanced capabilities including surface movement and AAM).
• Doc 9849 (GNSS Manual), §7.4 — GBAS business case analysis; cost comparison with ILS; airfield infrastructure considerations for CAT II/III.
• Doc 9849 (GNSS Manual), §7.9.4 — GBAS operational implementation: procedure design, GAST C/D mapping to minima, FAS data constraints, NOTAM and AIP requirements.
• Doc 9849 (GNSS Manual), Foreword / §1 — status as of 2025: approximately 140 certified GBAS stations; over 100 airlines with GBAS equipage; over 8,000 GBAS-equipped aircraft.

Authoritative External References#

• RTCA DO-253 (current revision), Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment — GBAS airborne MOPS (authoritative source — not in local library).
• EUROCAE ED-114A, Minimum Operational Performance Specification for the Airborne Ground Based Augmentation System — European equivalent to DO-253 (authoritative source — not in local library).
• EASA CS-AWO (Certification Specifications — All Weather Operations) — airworthiness basis for GBAS CAT II/III aircraft certification in Europe (authoritative source — not in local library).
• FAA Order 8400.13D — operational approval for GBAS CAT I and II/III approaches; airline operational approval criteria (authoritative source — not in local library).
• FAA TSO-C161a and TSO-C162a — Technical Standard Orders for GBAS ground equipment and airborne equipment (authoritative source — not in local library).
• Doc 9368 (Instrument Flight Procedures Construction Manual) — Attachment C.5: FAS data block encoding and example; CRC computation (authoritative source — not in local library).
• Doc 9718 (Handbook on Radio Frequency Spectrum Requirements for Civil Aviation) Vol II — GBAS, ILS, and VOR frequency assignment and geographical separation criteria (authoritative source — not in local library).