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results in an azimuth antenna siting which violates obstacle clearance requirements or a tower-mounted installation that is not
feasible, the following actions may be considered:
a) knowing the specific localizer and azimuth equipment involved, an analysis may be performed to determine the
height of the azimuth antenna phase centre. Generally, it is recommended that the azimuth antenna phase centre
height be selected so that the errors due to signal scattering from the localizer are limited to 0.03 degree. However,
that allocation may be increased after considering the contribution from other error sources such as ground and
airborne equipment errors, side lobe reflections from buildings, ground reflections, and errors due to interfering
aircraft (see Table G-10); and
b) a point on the line W – WN (Figure G-22) may be selected to determine the value for variable “X”. It is preferred
that the point selected be as close to point “W” as practical and it must be operationally acceptable for the procedure
concerned. Since the error allocation used in the development of this criteria represents a small portion of the total
propagation error budget, the azimuth signal might meet the accuracy requirement even below the plane which
contains the point selected and the azimuth antenna phase centre. The point to which acceptable azimuth signal
exists along the minimum glide path angle may be determined by flight measurements.
4.1.2.3.2 If a localizer near field monitor is present on the extended runway centre line, adjustment of the azimuth
antenna phase centre height (PCH) or the localizer monitor height may be required to minimize the effects of the localizer
monitor pole on the azimuth signal. However, it is expected that as long as the monitor pole is at or lower than the localizer
antenna element height no further adjustment due to the presence of the monitor pole will be required.
4.1.2.4 Integrated azimuth and localizer configuration
4.1.2.4.1 Azimuth antenna integrated under the localizer array
4.1.2.4.1.1 The first consideration for this configuration is to determine the height of the obstacle clearance surface at
the localizer array. The vertical distance between the ground and the obstacle clearance surface at this point should be at least
equal to the azimuth antenna height, including the pedestal, plus the required vertical spacing between the top of the azimuth
antenna and the localizer antenna element. If this condition is not observed an alternate collocation configuration has to be
considered.
4.1.2.4.1.2 Experimental results, from a 24-element log-period localizer, indicate that the vertical spacing between the
top of the azimuth antenna and the bottom of the localizer antenna elements has to be at least 0.5 m (1.6 ft) with a spacing of
greater than 0.7 m (2.3 ft) being preferred. For localizers with elements having relatively higher coupling, increased vertical
spacing is preferred.
ATT G-17 23/11/06
Annex 10 — Aeronautical Communications Volume I
4.1.2.4.2 Azimuth antenna integrated within the localizer array
4.1.2.4.2.1 For this configuration it may not be necessary to consider the height of the obstacle clearance surface since
the azimuth antenna is usually lower than the existing localizer antenna. When integrating the azimuth antenna, some
modifications at the localizer antenna are required which may influence the localizer signal-in-space. However, effects
depend very much on the type of localizer.
4.1.2.4.2.2 Experimental results, from a two-frequency localizer using dipole antennas, indicate that it is possible to
compensate these effects by minor on-site modifications at the localizer antenna. The feasibility of this integrated
configuration has to be confirmed for each type of localizer.
4.1.2.4.3 If an ILS near field monitor is present, it is necessary to determine the increase in azimuth antenna phase
centre height or decrease in the localizer monitor height required to minimize the effects of the monitor pole on the azimuth
signal. In general, satisfactory results may be obtained by siting the azimuth antenna phase centre approximately 0.3 m (1 ft)
above the monitor pole. This value is dependent on the localizer monitor design and location.
4.1.2.5 Offset azimuth
4.1.2.5.1 At some sites where ILS and MLS are to be collocated, it may be found impossible because of physical
restrictions to locate the MLS azimuth antenna in front of or back of the ILS localizer antenna or to integrate it with the ILS
localizer antenna. At those sites an advantageous solution could be to offset the MLS and DME/P antennas. The siting
information contained in auxiliary data would enable computation in the aircraft of an MLS centre line approach.
4.1.2.5.2 For this collocation configuration, the preferred siting is with the azimuth antenna radome in the localizer
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