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boundaries of the airport may also cause difficulty to the ILS course quality, even though they meet restrictions with regard
to obstruction heights.
2.1.10.5.1 Should the environment of an airport in terms of large fixed objects such as tall buildings cause the structure
of the localizer and/or glide path to be near the tolerance limits for the category of operation, much larger sensitive areas may
need to be established. This is because the effect of moving objects, which the sensitive areas are designed to protect the ILS
against, has to be added to the static beam bends caused by fixed objects. However, direct addition of the maximum bend
amplitudes is not considered appropriate and a root sum square combination is felt to be more realistic. Examples are as
follows:
a) localizer course bends due to static objects equals plus or minus 1½μA. Limit plus or minus 5μA. Therefore
allowance for moving objects to define localizer sensitive area is
52−1.52=4.77μA
b) localizer course bends due to static objects equals plus or minus 4μA. Limit plus or minus 5μA. Therefore allowance
for moving objects to define localizer sensitive area is
52−42=3μA
In case b) the sensitive area would be larger, thus keeping interfering objects further away from the runway so that they
produce 3μA or less distortion of the localizer beam. The same principle is applied to the glide path sensitive area.
2.1.11 Guidance on operational aspects of improving the performance of the ILS localizer in respect to bends
2.1.11.1 Introduction. Owing to site effects at certain locations, it is not always possible to produce with simple
standard ILS installations localizer courses that are sufficiently free from troublesome bends or irregularities. At such
installations, it will often be possible to reduce bends and irregularities in the localizer course to a satisfactory extent by
various methods, most of which require acceptance of some deviation from the specification for ILS set forth in this Annex,
together with possible penalties from an operational aspect.
2.1.11.2 Methods of effecting improvement. In general, improvements in localizer courses from the aspect of bends or
irregularities may be effected by restriction of radiation in particular directions so as to avoid or minimize reflection from
ATT C-13 23/11/06
Annex 10 — Aeronautical Communications Volume I
objects that give rise to the bends. In the majority of instances where special treatment is required, this may be achieved by
screens placed and designed to reduce the radiation in the direction of the object. Where reflecting objects are numerous or of
large dimensions, however, it may be necessary to restrict almost all the radiation from the localizer to a narrow sector
centred on the course line. Each method introduces certain disadvantages which should be weighed for the individual
installation in the light of the specific operational application to be made of the installation and the following considerations.
2.1.11.3 Disadvantages of methods of effecting improvements mentioned above
2.1.11.3.1 The use of screens limiting radiation in selected directions will, in general, give rise to a reduction of the
clearance between the two modulation signals of the ILS in some other direction, with the consequence that the ILS indicator
needle may move towards the centre when the aircraft is passing through areas in that direction. It is considered however that,
in general, such deviations are not operationally significant or may be overcome by suitable procedures. In certain
applications including the use of screens or reflectors to reinforce signals in the course sector, the use of screens or reflectors
will modify the range and characteristics of the back course of the localizer. Here again, it is considered that the effects are
unlikely to be operationally significant unless operational use is being made of the back course. In this latter case, it may be
necessary to provide an additional facility to supplement or replace the back course.
2.1.11.3.2 Where it is necessary to limit radiation from the localizer over a wide sector and confine most of it to a
sector centred on the front course of the localizer in order to reduce bends sufficiently, the disadvantages will, in general, be
as follows:
a) Orientation information from the localizer in the sector in which radiation is limited will no longer be available or
will be unreliable.
b) It will not be practicable to carry out a preliminary check of the performance of the aircraft receiver through the flag
system until the aircraft is within the sector centred on the course line.
c) In the area outside the sector centred on the course line, sufficient radiation may occur in particular directions to
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