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DO-253D Change 1
LAAS Ground Subsystem
It is assumed that the LAAS ground subsystem is operating in conformance with the
requirements stated in the ICAO Annex 10 Standards and Recommended Practices
(SARPs) through Amendment 91. These standards were developed to support two types
of services: precision approach services and a differentially corrected positioning service.
The services provided by the ground subsystem are indicated by the Type 2 Message of
the VHF data broadcast.
Note: Appendix I discusses how the organization of requirements for GASTs map into
the classification scheme for GBAS services, facilities and equipment.
The primary distinguishing feature between the two ground system services is the integrity
level supported by the signal-in-space. The precision approach service provides approach
and landing guidance with vertical and lateral integrity. The differentially corrected
positioning service provides PVT with horizontal integrity. The integrity risk exposure
times for these two services are specified over different periods of time. The integrity risk
exposure time for the approach service is characterized on a per approach basis and where
applicable on a per landing basis, while the integrity for the positioning service is
characterized on a per hour basis.
It is assumed that the two levels of approach services referred to as GAST C or GAST D
could be supported by the ground system. The GAST C level is intended to support CAT
I operations; whereas GAST D is intended to support CAT II/III operations. A primary
distinguishing factor between GAST C and GAST D is that the responsibility for ensuring
integrity and continuity is largely shifted from the ground facility in GAST C to the aircraft
in GAST D. This is accomplished by further standardizing ground system performance in
the pseudorange domain for GAST D and allowing the aircraft to restrict satellite geometry
to achieve desired position domain performance.
This document specifies a standard that will provide interoperability between LAAS
signal-in-space and receiving LAAS equipment developed by different manufacturers. It
is assumed that the signals-in-space for LAAS and WAAS are compliant with ICAO Annex
10 for GBAS and SBAS, respectively.
It is assumed that the interference environment specified in Appendix D for which
GPS/LAAS receivers must operate bounds the actual operational environment. If the
interference environment is higher than that specified in Appendix D, the availability and
continuity of LAAS will be reduced; however, there is no increase in the risk of misleading
information (see Section 184.108.40.206).
The test procedures specified in Sections 2 through 4 are intended to be used as the
recommended means of demonstrating compliance with the minimum acceptable
performance parameters specified herein. Although specific test procedures are cited, it is
recognized that other methods may be suitable. These alternate procedures may be used if
they provide equivalent information. In such cases, the procedures cited herein should be
used as one criterion in evaluating the acceptability of the alternate procedures.
Environmental tests are specified in Section 2.4 . These tests, like bench test, are performed
at the equipment level. The procedures and their associated limit requirements provide a
laboratory means of determining the electrical and mechanical performance of the
equipment under conditions expected to be encountered in actual aeronautical operations.
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