MT1 – Pseudorange Corrections: 100s smoothed

Purpose
Message Type 1 (MT1) provides the core differential corrections that allow an aircraft to remove most of the common-mode GNSS errors seen at the airport.

Key Characteristics

• Contains pseudorange corrections for the satellites in view and supported by the GBAS installation.

• Corrections are derived from the ground reference receiver network and represent localised adjustments to satellite ranging signals.

• Typically includes:

  • Satellite identifiers (e.g. PRN/SV ID)

  • Correction values and associated quality indicators

  • Time tags linking corrections to GNSS time

Airborne Use

• The GBAS-capable receiver applies MT1 corrections to smoothed GNSS measurements.

• This reduces errors due to satellite clock and ephemeris, as well as most local atmospheric effects within the coverage area.

• Corrected measurements form the basis for the precision position solution used in GLS approaches.

MT2 – Integrity, Protection and System Parameters

Purpose
Message Type 2 (MT2) provides integrity and protection-related information and key system parameters needed for safe use of GBAS corrections.

Key Characteristics

• Contains parameters that define:

  • Integrity bounds and protection-level components

  • Allocation of error budgets, including residual ionospheric and tropospheric effects

  • Validity intervals and conditions for using the broadcast corrections

• May include:

  • Ground system configuration information

  • Indicators of which satellites are approved or excluded for use

  • Parameters supporting airborne computation of Vertical and Lateral Protection Levels (VPL/LPL)

Airborne Use

• GLS receivers use MT2 data to:

  • Determine which satellites are eligible for use in the position solution.

  • Compute protection levels consistent with the declared GAST and minima.

  • Detect conditions in which the GBAS service is not valid (e.g. flagged anomalies).

In combination with MT1, MT2 enables the receiver to form a corrected and integrity-bounded solution suitable for precision approach.

MT4 – Final Approach Segment and Approach Path Data

Purpose
Message Type 4 (MT4) defines the approach path geometry and associated data for a GLS procedure, effectively describing the virtual localiser and glideslope that the aircraft will follow.

Key Characteristics

• Contains Final Approach Segment (FAS) data for one or more GLS approaches served by the GBAS installation.

• Typically encodes:

  • Runway and approach identifiers / reference path identifiers

  • Touchdown and threshold coordinates and elevations

  • Final approach course (localiser-equivalent) and glide path angle

  • Lateral and vertical path definitions, including any offsets

  • Additional parameters such as reference datum, path width and limits

Airborne Use

• The receiver selects the appropriate GLS approach using identifiers that match the published procedure.

• MT4 data allow the airborne system to:

  • Generate deviation signals equivalent to ILS localiser and glideslope.

  • Align the guidance with the physical runway and obstacle clearance surfaces.

• Flight guidance systems and, where available, autoland systems use this data to fly the precision approach down to the authorized minima.

MT11 – Pseudorange Corrections: 30s smoothed

Purpose
Message Type 11 (MT11) contains differential corrections derived from carrier smoothed pseudoranges using a time constant of 30 seconds. MT11 also includes alternative parameters for integrity bounding and for optimal weighting of measurements.

Key Characteristics

• The use of a shorter time constant reduces the differential error resulting from ionospheric activity.

• Supports enhanced integrity monitoring for higher GAST levels (e.g. CAT II/III services) where ionospheric and other spatial effects become more critical.

Airborne Use

• Receivers use MT11 data in combination with MT1/MT2 to:

  • Adjust protection levels and integrity calculations for spatially dependent effects.

  • Determine whether specific approach segments or geometries remain within acceptable risk bounds.

• This allows GBAS to support lower minima and more demanding operations without excessive conservatism in protection levels.