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affected by:
a) the runway length available;
b) the operating limits envisaged.
Where the application of the foregoing criteria permits, the preferred angle of the ILS glide path would be 3 degrees.
2.4.5 An ILS reference datum and glide path should then be selected, having regard to the foregoing criteria, and the
ability of the site to provide the clearance required by the Procedures for Air Navigation Services — Aircraft Operations
(PANS-OPS, Doc 8168) should be determined by calculation and confirmed, where possible, by flight test.
2.4.6 Where the selected ILS reference datum, the ILS glide path angle and the other relevant equipment characteristics
do not provide the required clearances, the following alternative course of action should be investigated:
a) removal of the offending obstacle;
b) selection of an alternative height for the ILS reference datum, taking into account the criteria indicated in 2.4.3 and
2.4.5;
c) selection of an alternative acceptable ILS glide path angle;
d) variation of the obstacle clearance limit to cater for the offending obstacle.
2.4.7 To enable more effective use of land adjacent to Category III — ILS glide path sites and to reduce siting
requirements and sensitive areas at these sites, it is desirable that the signals forming the horizontal radiation pattern from the
Category III — ILS glide path antenna system be reduced to as low a value as practicable outside the azimuth coverage limits
specified in Chapter 3, 3.1.5.3. Another acceptable method is to rotate in azimuth the glide path antennas away from
multipath sources thus reducing the amount of radiated signals at specific angles while still maintaining the azimuth coverage
limits.
ATT C-21 23/11/06
Annex 10 — Aeronautical Communications Volume I
2.4.8 ILS glide path curvature. In many cases the ILS glide path is formed as a conic surface originating at the glide
path aerial system. Owing to the lateral placement of the origin of this conic surface from the runway centre line, the locus of
the glide path in the vertical plane along the runway centre line is a hyperbola. Curvature of the glide path occurs in the
threshold region and progressively increases until touchdown.
2.4.9 Relationship between siting of glide path antenna and glide path threshold crossing height. The longitudinal
position of the glide path antenna should be selected so as to meet the recommendation made in Chapter 3, 3.1.5.1.4, in
respect to the height of the ILS reference datum above the runway threshold. The height of the ILS reference datum above the
runway threshold is then a function of the longitudinal position of the glide path antenna, of the longitudinal slope of the
glide path reflection plane and of the position of the runway threshold in respect to the glide path reflection plane. This
situation is described pictorially in Figure C-5. The longitudinal position of the glide path antenna is then calculated as
follows:
H + Y
D =
tan (θ + α)
where
D = the horizontal distance between O and P;
H = the nominal threshold crossing height;
Y = the vertical height of the runway threshold above P';
θ = the nominal ILS glide path angle;
α = the longitudinal downslope of the glide path reflection plane.
Note.— In the above formula α is to be taken as positive in the case of a downslope from the antenna towards the
threshold. Y is taken as positive if the threshold is above the reflection plane intersection line.
2.4.10 The foregoing guidance material in respect of the longitudinal placement to the glide path antenna in relation to
the runway threshold, which takes into account the fact that the runway may not be in the glide path reflection plane, and that
the glide path reflection plane may be sloped, is based on geometrical abstractions. The material implicitly assumes that the
glide path locus in the vertical plane, containing the runway centre line, is a perfect hyperbola; consequently, the glide path
extension is implicitly assumed as the asymptote to this hyperbola.
2.4.11 In fact, however, the glide path is often quite irregular. The mean ILS glide path angle can be ascertained only
by flight tests; the mean observed position of that part of the glide path between ILS Points A and B being represented as a
straight line, and the ILS glide path angle being the angle measured between that straight line and its vertical projection on
the horizontal plane.
2.4.12 It is important to recognize that the effect of glide path irregularities if averaged within the region between the
middle marker and the threshold will likely tend to project a reference datum which is actually different from the ILS
reference datum. This reference datum, defined here as the achieved ILS reference datum, is considered to be of important
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