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unwanted interference and should be at the greatest height compatible with obstacle clearance limits. For course line
monitoring, the antenna is usually positioned along the extended runway centre line. Where it is desired to also monitor
displacement sensitivity, an additional receiver and monitor are installed with antenna suitably positioned to one side of the
extended runway centre line. Some systems utilize a number of spatially separated antennas.
2.9 Localizer and glide path displacement sensitivities
2.9.1 Although certain localizer and glide path alignment and displacement sensitivities are specified in relation to the
ILS reference datum, it is not intended to imply that measurement of these parameters must be made at this datum.
2.9.2 Localizer monitor system limits and adjustment and maintenance limits given in Chapter 3, 3.1.3.7 and 3.1.3.11
are stated as percentage changes of displacement sensitivity. This concept, which replaces specifications of angular width in
earlier editions, has been introduced because the response of aircraft guidance systems is directly related to displacement
sensitivity. It will be noted that angular width is inversely proportional to displacement sensitivity.
ATT C-37 23/11/06
Annex 10 — Aeronautical Communications Volume I
2.10 Siting of ILS markers
2.10.1 Considerations of interference between inner and middle markers, and the minimum operationally acceptable
time interval between inner and middle marker light indications, will limit the maximum height marked by the inner marker
to a height on the ILS glide path of the order of 37 m (120 ft) above threshold for markers sited within present tolerances in
Annex 10. A study of the individual site will determine the maximum height which can be marked, noting that with a typical
airborne marker receiver a separation period of the order of 3 seconds at an aircraft speed of 140 kt between middle and inner
marker light indications is the minimum operationally acceptable time interval.
2.10.2 In the case of ILS installations serving closely spaced parallel runways, e.g. 500 m (1 650 ft) apart, special
measures are needed to ensure satisfactory operation of the marker beacons. Some States have found it practical to employ a
common outer marker for both ILS installations. However, special provisions, e.g. modified field patterns, are needed in the
case of the middle markers if mutual interference is to be avoided, and especially in cases where the thresholds are displaced
longitudinally from one another.
2.11 Use of DME as an alternative to ILS marker beacons
2.11.1 When DME is used as an alternative to ILS marker beacons, the DME should be located on the airport so that
the zero range indication will be a point near the runway. If the DME associated with ILS uses a zero range offset, this
facility has to be excluded from RNAV solutions.
2.11.2 In order to reduce the triangulation error, the DME should be sited to ensure a small angle (e.g. less than 20
degrees) between the approach path and the direction to the DME at the points where the distance information is required.
2.11.3 The use of DME as an alternative to the middle marker beacon assumes a DME system accuracy of 0.37 km
(0.2 NM) or better and a resolution of the airborne indication such as to allow this accuracy to be attained.
2.11.4 While it is not specifically required that DME be frequency paired with the localizer when it is used as an
alternative for the outer marker, frequency pairing is preferred wherever DME is used with ILS to simplify pilot operation
and to enable aircraft with two ILS receivers to use both receivers on the ILS channel.
2.11.5 When the DME is frequency paired with the localizer, the DME transponder identification should be obtained
by the “associated” signal from the frequency-paired localizer.
2.12 The use of supplementary sources of orientation guidance
in association with ILS
2.12.1 Aircraft beginning an ILS approach may be assisted by guidance information provided by other ground
referenced facilities such as VORs, surveillance radar or, where these facilities cannot be provided, by a locator beacon.
2.12.2 When not provided by existing terminal or en-route facilities, a VOR, suitably sited, will provide efficient
transition to the ILS. To achieve this purpose the VOR may be sited on the localizer course or at a position some distance
from the localizer course provided that a radial will intersect the localizer course at an angle which will allow smooth
transitions in the case of auto coupling. The distance between the VOR site and the desired point of interception must be
recognized when determining the accuracy of the interception and the airspace available to provide for tracking errors.
2.12.3 Where it is impracticable to provide a suitably sited VOR, a compass locator or an NDB can assist transition to
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