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Definition (ICAO Annex 10,
§3.1.1, §3.1.5.1.3 ):
ILS glide path. That
locus of points in the vertical plane containing the runway centre line
at which the DDM is zero, which, of all such loci, is the closest to the
horizontal plane.
ILS reference datum (Point "T").
A point of a specified height located above the intersection of the
runway centre line and the threshold through which the downward extended
straight portion of the ILS glide path passes.
The height of the point T above the
threshold is the threshold crossing height (TCH). The TCH and and the
glidepath angle are the most safety critical, basic operational system
parameters of the ILS glideslope subsystem.
ICAO Annex 10 (Att. C §2.4.9
Fig. C5 ; see Fig. 1) does not specifiy a method to determine the
TCH but gives a crude first guideline to determine the position of the
glidepath antenna.
The underlying theory and the technical
statements in ICAO DOC8071 are identical.
Problem:
Clear and unique definition of
the TCH exists in ICAO Annex 10.
TCH is determined by the glidepath
antenna position and antenna geometry.
Antenna position and antenna geometry
depend on the geometry of the antenna reflection plane in front of the
glidepath antenna.
Installation of the antenna requires
the determination of these parameters in advance.
Theoretically/numerically determined
TCH and practically measured TCH by flightcheck should be identical.
Requires that the basic procedures
are identical and that both are consistent with the definition of ICAO
Annex 10.
In the general case, the reflection
plane is tridimensional and the methods must be adapted to this (see Fig. 2 and Fig. 3).
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