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array plane (Area 1 of Figure G-21). A minimum distance of 3 m (10 ft) between the azimuth equipment and the localizer
array (end element) is preferable.
4.1.2.5.3 If siting the azimuth antenna abeam the localizer is not practical, the azimuth antenna may be sited behind the
localizer array plane (Area 2 of Figure G-21). The azimuth antenna offset has to provide at least a 3 m (10 ft) distance and
prohibit penetration of the azimuth proportional guidance region by the localizer array.
4.1.2.5.4 If siting the azimuth antenna ahead of the localizer array plane is required, degradation of the localizer signal
may result. The region where the least effect of the azimuth equipment on the localizer signal is expected is shown in Area 3
of Figure G-21. The azimuth antenna location can be verified using an azimuth equipment mock-up.
4.2 MLS siting within an approach lighting system
4.2.1 The presence of an approach lighting system serving the opposite end approach will affect the siting of an MLS
azimuth antenna. Factors to be considered in proper siting are coverage requirements (see 2.3.2), the need to avoid visual
blockage of lights, obstacle limitation requirements, and azimuth signal multipath from the light structures.
4.2.2 These criteria are applicable for typical installations where the approach lights are mounted at essentially a
constant height or rise with increasing distance from the runway.
4.2.3 The following guidance is based on MLS siting within existing lighting system structures. It may be more
practical to use light structures which do not affect the signal-in-space if these are available.
4.2.4 If the location of an MLS azimuth antenna on extended runway centre line 60 m (200 ft) beyond the far end of
the approach lighting system is not possible or practical, it may be sited within the light plane boundaries given the following
criteria:
23/11/06 ATT G-18
Attachment G Annex 10 — Aeronautical Communications
a) in the horizontal plane, the antenna is to be sited on extended runway centre line not closer than 300 m to the runway
stop end and as far as possible from the nearest light position toward runway stop end. (This places the back of the
azimuth equipment against a light position.)
b) the siting of the azimuth station is to be such that the shadowing of the lights of the approach lighting system is
minimized, particularly within decision height boundaries. The azimuth station should not shadow any light(s) other
than that located in a centre part of a cross bar or a centre line barrette (see Annex 14, Volume I, Attachment A,
Section 11.3 for further guidance).
4.2.4.1 If the spacing between adjacent light stations is 30 m (100 ft) or more, the phase centre should be at least
0.3 m (1 ft) above light centre line of the closest light station toward runway stop end. This could be relaxed to 0.15 m
(0.5 ft), if necessary, if the site is otherwise free of significant multipath problems. This may require the use of an elevated
azimuth station.
4.2.4.2 If the spacing between adjacent light stations is less than 30 m (100 ft), the phase centre should be at least 0.6 m
(2 ft) above light centre line of the closest light station toward runway stop end.
4.3 Critical and sensitive areas
4.3.1 The occurrence of interference to MLS signals is dependent on the reflection and shadowing environment around
the MLS antennas and the antenna beamwidths. Vehicles and fixed objects within 1.7 beamwidths of the receiver location are
considered “in-beam” and will cause main lobe multipath interference to the MLS guidance signals. Typically, the ground
equipment beamwidths are chosen such that no azimuth in-beam reflections exist along the final approach course and no
elevation in-beam multipath exists along the commissioned glide paths. However, movable objects may enter the in-beam
multipath regions and cause interfering reflections to or shadowing of the guidance signals to the extent that the quality
becomes unacceptable. The areas within which vehicles can cause degraded performance need to be defined and recognized.
For the purpose of developing protective zoning criteria, these areas can be divided into two types, i.e. critical areas and
sensitive areas:
a) The MLS critical area is an area of defined dimensions about the azimuth and elevation antennas where vehicles,
including aircraft, are excluded during all MLS operations. The critical area is protected because the presence of
vehicles and/or aircraft inside its boundaries will cause unacceptable disturbance to the guidance signals.
b) The MLS sensitive area is an area extending beyond the critical area where the parking and/or movement of
vehicles, including aircraft, is controlled to prevent the possibility of unacceptable interference to the MLS signals
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