Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix I
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DO-253D Change 1
GBAS Approach Service Types (GAST)
A GBAS Approach Service Type 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.
Four GBAS Approach Service Types are defined. Service Types A-C are the services that
are currently described in the SARPs [ICAO Annex 10 Volume I through amendment 91].
GBAS Service Types A & B (identified with the terms APV I and APV II) use vertical
guidance relative to a glide path, but the facility or navigation system may not satisfy all
of the requirements associated with precision approach. This MOPS only supports GAST
C, and does not include requirements for GAST A or B. Although the SARPs do not
characterize the collection of airborne and ground functional and performance
requirements as service types, the definition can be applied. It should be recognized that
the resultant levels of position domain navigation performance defined in table A-126.96.36.199-
1 of the ICAO Annex 10 SARPs are the result of a specific combination of ground and
airborne functional and performance requirements. In other words, the standard signal-in-
space performance levels defined in table A-188.8.131.52-1 are a characteristic of the Service
Type. However, the position domain based signal-in-space performance requirements are
not a complete definition of the service.
The fourth service type, GAST D is defined by this MOPS, the LAAS ICD RTCA/DO-
246E, and ICAO Annex 10 SARPs revisions for GBAS GAST D. Just as with the other
service types, GAST D defines a matched set of standard airborne and ground functional
requirements that when combined results in position and guidance information with
quantifiable performance. However, GAST D is different than the other defined services
in that some aspects of the airborne functional requirements are not standardized. These
non-standard characteristics allow the position domain NSE performance of the system to
be tailored to the needs of a specific aircraft implementation.
Unless otherwise noted, performance requirements for GAST C are met by GAST D.
GAST D includes some additional requirements not included in GAST C that are intended
to support CAT II/III operations. The ground subsystem functional and performance
requirements for GAST D are a superset of the requirements for GAST C, (i.e., all GAST
D capable ground subsystems will support GAST C). From the airborne equipment
perspective, the functional requirements depend on the active service type. Although the
airborne functional requirements are somewhat different between GAST C and GAST D,
the Signal in Space performance achieved is the same with one small exception. For GAST
D, the mitigation of errors induced by anomalous ionospheric conditions has been partially
allocated to the airborne system. Consequently, the SIS integrity bounding process (i.e.,
VPL and HPL) does not necessarily bound the errors due to the anomalous ionospheric
conditions. For all other error contributors, the SIS bounding provided by the protection
level computations is equivalent between GAST C and GAST D.
For GAST D, the additional accuracy performance requirements (beyond the baseline SIS
requirements defined for GAST C) are specified in the pseudorange domain. When
combined by the airborne equipment, the resultant position domain accuracy depends on
the specific geometry screening done by the airborne equipment. There is a functional
requirement included in GAST D for the airborne equipment to perform geometry
screening to limit the normal and faulted NSE performance at the output of the system.
However, the specific metrics to be used by the airborne in the screening will depend on
the NSE performance required by the specific airplane type in order to meet the relevant
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