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necessary to exclude some or all moving traffic depending on interference potential and category of operation. It would
be advisable to have the aerodrome boundaries include all the sensitive areas so that adequate control can be exercised
over all moving traffic to prevent unacceptable interference to the ILS signals. If these areas fall outside the aerodrome
boundaries, it is essential that the cooperation of appropriate authorities be obtained to ensure adequate control.
Operational procedures need to be developed for the protection of sensitive areas.
2.1.10.3 The size of the sensitive area depends on a number of factors including the type of ILS antenna, the
topography, and the size and orientation of man-made objects, including large aircraft and vehicles. Modern designs of
localizer and glide path antennas can be very effective in reducing the disturbance possibilities and hence the extent of the
sensitive areas. Because of the greater potential of the larger types of aircraft for disturbing ILS signals, the sensitive areas for
these aircraft extend a considerable distance beyond the critical areas. The problem is aggravated by increased traffic density
on the ground.
2.1.10.3.1 In the case of the localizer, any large objects illuminated by the main directional radiation of the antenna
must be considered as possible sources of unacceptable signal interference. This will include aircraft on the runway and on
some taxiways. The dimensions of the sensitive areas required to protect Category I, II and III operations will vary, the
largest being required for Category III. Only the least disturbance can be tolerated for Category III, but an out-of-tolerance
course along the runway surface would have no effect on Category I or II operations. If the course structure is already
marginal due to static multipath effects, less additional interference will cause an unacceptable signal. In such cases a largersize
sensitive area may have to be recognized.
2.1.10.3.2 In the case of the glide path, experience has shown that any object penetrating a surface above the reflection
plane of the glide path antenna and within azimuth coverage of the antenna must be considered as a source of signal
interference. The angle of the surface above the horizontal plane of the antenna is dependent on the type of glide path antenna
array in use at the time. Very large aircraft, when parked or taxiing within several thousand feet of the glide path antenna and
directly between it and the approach path, will usually cause serious disturbance to the glide path signal. On the other hand,
the effect of small aircraft beyond a few hundred feet of the glide path antenna has been shown to be negligible.
2.1.10.3.3 Experience has shown that the major features affecting the reflection and diffraction of the ILS signal to
produce multipath interference are the height and orientation of the vertical surfaces of aircraft and vehicles. The maximum
height of vertical surface likely to be encountered must be established, together with the “worst case” orientation. This is
because certain orientations can cause out-of-tolerance localizer or glide path deviations at greater distances than parallel or
perpendicular orientations.
2.1.10.4 Computer or model techniques can be employed to calculate the probable location, magnitude and duration of
ILS disturbances caused by objects, whether by structures or by aircraft of various sizes and orientation at different locations.
Issues involved with these techniques include the following:
a) computerized mathematical models are in general use and are applied by personnel with a wide variety of
experience levels. However, engineering knowledge of and judgement about the appropriate assumptions and
limitations are required when applying such models to specific multipath environments. ILS performance
information relative to this subject should normally be made available by the ILS equipment manufacturer;
23/11/06 ATT C-8
Attachment C Annex 10 — Aeronautical Communications
75 m
(250 ft)
300 m (1 000 ft) or the near end of the runway
whichever is the greater
Centre of localizer array
Toward approach end of runway 120 m
Critical area (400 ft)
X Sensitive area
Y
Y
B-747 B-747 B-727
Typically 27 m (90 ft)
Example 1 Example 2 Example 3
Aircraft type
Localizer antenna Typically 16 m (50 ft) Typically 16 m (50 ft)
(Directional dual (Semi-directional, (Semi-directional,
frequency, 14 elements) 8 elements) 8 elements)
X 600 m (2 000 ft) 600 m (2 000 ft) 300 m (1 000 ft)
Y 60 m ( 200 ft) 110 m ( 350 ft) 60 m ( 200 ft)
X 1 220 m (4 000 ft) 2 750 m (9 000 ft) 300 m (1 000 ft)
Y 90 m ( 300 ft) 210 m ( 700 ft) 60 m ( 200 ft)
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