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DO-253D Change 1
© 2019 RTCA, Inc.
GBAS ID Selection
For determining Precision Approach Guidance Outputs, when a FAS Data Block has been
selected, the PAN function shall [LAAS-113] only use messages for which the GBAS
Identification (ID) matches the GBAS ID in the Type 4 message containing the selected
FAS Data Block.
Approach Service Type (AST) Selection
18.104.22.168.3.1 AST Selection for Single Approach Service Type Capable Equipment
LAAS equipment that supports a single Approach Service Type applies functional
requirements for GAST C. In this case no selection is required; the selected Approach
Service Type and the active Approach Service Type are both considered to be GAST C.
22.214.171.124.3.2 AST Selection for Multiple Approach Service Type Capable Airborne Equipment
Note: The conditions for a change in the active Approach Service Type are the collection
of many functional requirements in this MOPS, including but not limited to satellite
geometry, integrity bounds, correction data, and ranging source and position
domain monitoring. Automatic downgrade from GAST D to GAST C is required
by this MOPS. Upgrade from GAST C to GAST D is not precluded by this MOPS.
All GAST D functional, performance, and monitoring requirements must be
satisfied prior to an upgrade from GAST C. Design flexibility is allowed regarding
the choice to upgrade or not, as well as the timing for an upgrade. This design
flexibility enables compatibility with a range of aircraft integrations.
The selected Approach Service Type for LAAS Equipment that supports multiple types of
approach service shall [LAAS-364] not be reflected until the RPDS derived from the
channel number (Section 2.3.5) or input to the PAN is found in a FAS Data Block in any
received Type 4 message.
LAAS Equipment that supports multiple types of approach services shall [LAAS-365] set
the selected Approach Service Type to the highest GAST supported by both the equipment
and the type of service indicated in the Approach Performance Designator (APD) specified
in the selected Final Approach Segment Data Block from the Type 4 message.
126.96.36.199.3.3 Approach Service Type Output
LAAS Equipment that supports multiple types of approach services shall [LAAS-366]
output the selected Approach Service Type as well as the active Approach Service Type.
When the conditions are satisfied for a change in either the selected or active Approach
Service Type, the output(s) shall [LAAS-367] reflect the change within 2 seconds.
Precision Approach Region
The Precision Approach Region (PAR) associated with an approach procedure is the region
defined by the following conditions (see Figure 2-5):
a) Distance to the Landing Threshold Point (LTP)/Fictitious Threshold Point (FTP)
in the horizontal plane < 10 nmi;
b) Bearing to the indicated end of the runway is within ± 90 degrees of the FAS
c) Proportional lateral deviation is within twice full scale; and
d) Aircraft position is below twice the full-scale fly down on the proportional vertical
deviations or below a height of 200 feet above the LTP/FTP.
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