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operate the ILS indicator in the aircraft in an erratic manner, giving rise to false indications.
d) The loss of the back course.
2.1.11.3.3 In respect to a), it is considered that orientation information is necessary but that practice has shown that
such information is preferably obtained in any event from an auxiliary aid such as a locator. Such an auxiliary aid would be
necessary if radiation from the localizer is confined to a narrow sector centred on the course line. In respect to b), it is
considered that the loss of a receiver check prior to entry into the sector centred on the course line could be operationally
accepted.
2.1.11.3.4 The disadvantage indicated in c) may, in some instances, be a serious drawback. In general, it is considered
that acceptance of this disadvantage will depend on the extent to which false indications will occur at a particular site and on
the procedures established or specified for the use of the ILS installation. In practice, it is possible to establish procedures so
that no use is made of the localizer signals until the aircraft is able to check that it is in the usable sector. Experience has
shown at one installation in operational use that, procedurally, no difficulty has arisen through the existence of erratic
indications in the off-course sector. It is considered that the question of whether or not the off-course signal characteristics
due to reduction of radiation in a narrow sector may be accepted operationally is a matter for individual assessment at each
location concerned.
2.1.11.3.5 The loss of the back course indicated in d) may have several disadvantages. At some locations, the back
course serves a useful function through intersection with other aids for facilitating procedures in the area concerned. Also, the
back course often provides a useful aid in missed approach procedures and can often be used to simplify approach for landing
when conditions require that the landing direction be opposite to the direction for which the ILS is primarily installed. Loss
of the back course will, in general, require the provision of a substitute aid or aids, and the principal disadvantage in
suppressing the back course may be considered in terms of the additional expense of a substitute aid or aids.
23/11/06 ATT C-14
Attachment C Annex 10 — Aeronautical Communications
2.1.11.4 Extent to which sector centred on course line may be narrowed. It is considered that a radiation sector 10
degrees each side of the localizer course line would be the minimum sector that could be accepted operationally. It is
desirable that the characteristics of the signal from the localizer be identical with those specified in Chapter 3 within the
region in the immediate vicinity (region from DDMs 0.155 to zero) of the course line and approximate closely to them out to
10 degrees, so that the indications of the ILS indicator and the signals fed to a coupling device, if used, will correspond to the
standard ILS throughout any manoeuvres necessary in the transition from the approach to the localizer to establishment on
course line.
It should be realized, however, that for an increased runway length, the localizer course sector wherein proportional
guidance is provided will be narrower as a result of adjusting the localizer to the sensitivity specified in Chapter 3, 3.1.3.7.1.
Although a proportional guidance signal is provided on each side of the course line up to a level of 0.180 DDM, the level
above 0.150 DDM may not be usable by the automatic airborne system during the intercept manoeuvre unless that system is
armed within the sector in which a minimum of 0.180 DDM is provided (e.g. plus or minus 10 degrees). It is advantageous to
permit the localizer capture mode of the automatic airborne system to be armed at off-course angles greater than 10 degrees;
consequently it is desirable to maintain a minimum DDM of 0.180 through a wider sector than plus or minus 10 degrees
wherever practical.
2.1.11.5 Further possibilities. If the disadvantages arising from the use of the restricted coverage and modified signal
characteristics discussed in 2.1.11.3 are unacceptable, possibilities exist through the use of two radio frequency carriers to
provide the coverage and signal characteristics that would maintain the essential information provided by a standard ILS in
the suppressed sector while, at the same time, maintaining in the regions about the course sector the objective of the restricted
coverage system. It may be necessary to employ this more elaborate system at aerodromes with high multipath environments.
Additional guidance on two radio frequency carrier coverage is provided in 2.7.
2.2 ILS airborne receiving equipment
Note.— The specified tolerances are those considered necessary to achieve the operational objective and include
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