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associated broadcast frequency. The FAS block for a particular approach procedure is described in Appendix B, 3.6.4.5.1 and
Table B-66.
23/11/06 ATT D-26
Attachment D Annex 10 — Aeronautical Communications
7.11.2 FAS path definition
7.11.2.1 Lateral orientation. The LTP/FTP is typically at or near the runway threshold. However, to satisfy operational
needs or physical constraints, the LTP/FTP may not be at the threshold. The FPAP is used in conjunction with the LTP/FTP
to define the lateral reference plane for the approach. For a straight-in approach aligned with the runway, the FPAP will be at
or beyond the stop end of the runway. The FPAP is not placed before the stop end of the runway.
7.11.2.2 ΔLength offset. The Δlength offset defines the distance from the end of the runway to the FPAP. This
parameter is provided to enable the aircraft equipment to compute the distance to the end of the runway. If the Δlength offset
is not set to appropriately indicate the end of the runway relative to the FPAP, the service provider should ensure the
parameter is coded as “not provided”.
7.11.2.3 Vertical orientation. Local vertical for the approach is defined as normal to the WGS-84 ellipsoid at the
LTP/FTP and may differ significantly from the local gravity vector. The local level plane for the approach is defined as a
plane perpendicular to the local vertical passing through the LTP/FTP (i.e. tangent to the ellipsoid at the LTP/FTP). The
datum crossing point (DCP) is a point at a height defined by TCH above the LTP/FTP. The FAS path is defined as a line with
an angle (defined by the GPA) relative to the local level plane passing through the DCP. The GPIP is the point where the
final approach path intercepts the local level plane. The GPIP may actually be above or below the runway surface depending
on the curvature of the runway.
7.11.3 “ILS look-alike” deviation computations. For compatibility with existing aircraft designs, it is desirable for
aircraft equipment to output guidance information in the form of deviations relative to a desired flight path defined by the
FAS path. The Type 4 message includes parameters that support the computation of deviations that are consistent with ILS
requirements.
7.11.3.1 Lateral deviation definition. Figure D-6 illustrates the relationship between the FPAP and the origin of the
lateral angular deviations. The course width parameter and FPAP are used to define the origin and sensitivity of the lateral
deviations. By adjusting the location of the FPAP and the value of the course width, the course width and sensitivity of a
GBAS can be set to the desired values. They may be set to match the course width and sensitivity of an existing ILS. This
may be necessary, for example, for compatibility with existing visual landing aids.
7.11.3.1.1 Lateral deviation reference. The lateral deviation reference plane is the plane that includes the LTP/FTP,
FPAP and a vector normal to the WGS-84 ellipsoid at the LTP/FTP. The rectilinear lateral deviation is the distance of the
computed aircraft position from the lateral deviation reference plane. The angular lateral deviation is a corresponding angular
displacement referenced to the GBAS azimuth reference point (GARP). The GARP is defined to be beyond the FPAP along
the procedure centre line by a fixed offset value of 305 m (1 000 ft).
7.11.3.1.2 Lateral displacement sensitivity. The lateral displacement sensitivity is determined by the aircraft equipment
from the course width provided in the FAS data block. The service provider is responsible for setting the course width
parameter to a value that results in the appropriate angle for full scale deflection (i.e. 0.155 DDM or 150 μA) taking into
account any operational constraints.
7.11.3.2 Vertical deviations. Vertical deviations are computed by the aircraft equipment with respect to a GBAS
elevation reference point (GERP). The GERP may be at the GPIP or laterally offset from the GPIP by a fixed GERP offset
value of 150 m. Use of the offset GERP allows the glide path deviations to produce the same hyperbolic effects that are
normal characteristics of ILS and MLS (below 200 ft). The decision to offset the GERP or not is made by the aircraft
equipment in accordance with requirements driven by compatibility with existing aircraft systems. Service providers should
be aware that users may compute vertical deviations using a GERP which is placed at either location. Sensitivity of vertical
deviations is set automatically in the aircraft equipment as a function of the GPA. The relationship between GPA and the
vertical deviation sensitivity is equivalent to the glide path displacement sensitivity provided by ILS.
7.11.4 Approaches not aligned with the runway. Some operations may require the definition of a FAS path that is not
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