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paragraph in the supporting text for Figure G-20. This paragraph provides a more detailed description of the factors to be
considered for that step.
4.1.2.1.4 The general regions for siting the azimuth antenna are shown in Figure G-21.
4.1.2.2 Azimuth antenna sited ahead of localizer antenna
4.1.2.2.1 The azimuth antenna is to be symmetrically sited on the localizer course line at least 30 m (100 ft) ahead of
the localizer antenna array. The limit for the maximum distance (variable “X” in Figure G-21) is determined by the
requirement to satisfy the obstacle limitation requirements set forth in Annex 14 for both the azimuth antenna structure and
azimuth monitor. This is the preferred location for the azimuth antenna. However, factors such as the presence of a localizer
near field monitor may require the location of the azimuth antenna to be modified. The azimuth antenna cannot be sited such
that it blocks line-of-sight between the localizer antenna and the localizer field monitor. Due to line-of-sight blockage of the
ILS ground check point by the azimuth station, the ILS ground check points may have to be reassessed.
4.1.2.2.2 It is desirable to collocate the DME/P antenna with azimuth antenna whenever possible. However, if the
DME/P antenna cannot be collocated with the azimuth antenna due to violation of obstacle limitation requirements, one may
consider an offset DME/P site or selecting an alternate collocation configuration (see Attachment C, Section 7.1.6 and
Section 5 below).
4.1.2.2.3 When possible, the azimuth antenna location can be adjusted to minimize the effect of the azimuth antenna
critical area on flight operations. In addition, it may be desirable to maximize the union of azimuth and localizer critical areas.
Due to the necessity to collocate the azimuth antenna in close proximity to the localizer antenna, normally one of the
antennas will have to be sited in the critical area of the other antenna. For the azimuth antenna critical area, see 4.3. For the
localizer critical areas see Attachment C, Section 2.1.10.
4.1.2.2.4 After a suitable site for the azimuth antenna has been determined, a location for the azimuth antenna field
monitor must be found. The azimuth antenna should be monitored as stated in 2.3.5. The preferred location for the field
monitor is on the extended runway centre line. However, the monitor pole can be a source of azimuth signal degradation.
Therefore, if this monitor location causes unacceptable signal degradation or unsatisfactory monitoring capabilities due to
the presence of light lane structures, ILS localizer, etc., an alternate field monitor location may be desirable. This second
procedure is only recommended if integral monitoring of the approach radial is available. The following considerations
may be helpful in determining a monitor location:
a) It is desirable to have the field monitor as close to the far field as practical to minimize near field effects on the
monitor. However, this distance should be limited to avoid false alarms due to vehicle and aircraft traffic between
the monitor and azimuth antenna.
b) It is desirable to minimize blockage and distortion of the azimuth signal by the field monitor in the final approach
region. The field monitor should be sited as far below the azimuth antenna phase centre as practical.
23/11/06 ATT G-16
Attachment G Annex 10 — Aeronautical Communications
c) The field monitor offset from the antenna boresight should be limited to maintain the appropriate monitor sensitivity
to mechanical stability.
d) The azimuth antenna field monitor should be sited to avoid affecting, or being affected by, the localizer monitor.
4.1.2.3 Azimuth antenna sited behind ILS localizer
4.1.2.3.1 The distance between the localizer and the MLS azimuth antenna will depend on obstacle limitation
requirements, availability of real estate, presence of a localizer back course, and the desirability of collocating the DME/P
antenna with the azimuth antenna. If a localizer back course is being utilized, a distance of at least 30 m (100 ft) between the
azimuth and localizer antennas is preferred, and the azimuth antenna must be symmetrically sited on the localizer course
centre line. For localizer antennas with a high front-to-back power ratio, it may be possible to reduce the 30 m (100 ft)
separation. Once the distance between the azimuth and localizer antennas is known, Figure G-22 can be used to determine the
height of the azimuth antenna phase centre relative to the localizer antenna array. To ensure that the azimuth guidance errors
due to signal scattering by the ILS localizer remain insignificant (≤0.03 degree) throughout the azimuth coverage volume,
point “W” (Figure G-22) is typically selected to determine the value for variable “X” of Figure G-22. If selection of that point
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