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corresponding distribution of reply frequencies and signal amplitudes received at the interrogator.
7.1.4 Interrogator processor efficiency
The interrogator signal processor efficiency is the ratio of the number of replies processed by the interrogator to the
number of interrogations in the absence of garble and transponder dead time effects. This efficiency depends on the reply
pulse threshold level and the receiver noise level.
7.1.5 Relationship between aircraft served and transmission rate
7.1.5.1 Specification of the maximum transponder transmission rate establishes the maximum average transmitter
power level. Chapter 3, 3.5.4.1.5.5 recommends that the transponder have a transmission rate capability of 2 700 pulse pairs
per second if 100 aircraft are to be served. This represents typical transponder loading arising from 100 aircraft. To determine
the actual transmission rate capability that should be accommodated at a given facility during peak traffic conditions requires
that the maximum number of interrogators be estimated. To compute the interrogation loading on the transponder, the
following should be considered:
a) the number of aircraft that constitutes the peak traffic load;
ATT C-69 23/11/06
Annex 10 — Aeronautical Communications Volume I
b) the number of interrogators in use on each aircraft;
c) the distribution of operating modes of the interrogators in use (e.g. search, initial approach, final approach, ground
test);
d) the appropriate pulse repetition frequency as given in Chapter 3, 3.5.3.4.
7.1.5.2 Given the interrogation loading which results from the peak traffic as well as the reply efficiency of the
transponder in the presence of this load, the resulting reply rate can be computed, thereby establishing the required
transmitter capability. This reply rate is the level that, when exceeded, results in a reduction in receiver sensitivity (as
specified in Chapter 3, 3.5.4.2.4) in order to maintain the reply rate at or below this maximum level.
7.1.6 Siting of DME associated with ILS or MLS
7.1.6.1 The DME should, where possible, provide to the pilot an indicated zero range at touchdown in order to satisfy
current operational requirements.
7.1.6.2 The optimum site for a DME transponder is dependent upon a number of technical and operational factors.
DME/N may be installed with ILS or MLS where operational requirements permit. DME/P, which provides higher accuracy
and coverage throughout the entire runway region, is required to support the more flexible and advanced operations that are
available with MLS.
7.1.6.3 In the case of DME/N, the provision of zero range indication may be achieved by siting the transponder as
close as possible to the point at which zero range indication is required. Alternatively, the transponder time delay can be
adjusted to permit aircraft interrogators to indicate zero range at a specified distance from the DME antenna. When the
indicated DME zero range has a reference other than the DME antenna, consideration should be given to publishing this
information.
7.1.6.4 In the case of DME/P, in order to meet accuracy and coverage requirements, particularly in the runway region,
it is recommended that the DME/P be sited as closely as possible to the MLS azimuth facility, consistent with obstacle
clearance criteria. For aircraft equipped with a full MLS capability, the desired zero range indication can then be obtained
by utilizing MLS basic data. Note that the DME/P transponder time delay must not be adjusted for this purpose.
7.1.6.5 It is desirable that all users obtain indicated zero range at touchdown irrespective of the airborne equipment
fitted. This would necessitate location of the DME/P abeam the runway at the touchdown point. In this case accuracy
requirements for DME/P would not be met on the runway. It must be noted that MLS Basic Data Word 3 only permits the
coding of DME/P coordinates within certain limits.
7.1.6.6 If an MLS/DME/P and an ILS/DME/N serve the same runway, an aircraft equipped with a minimum MLS
capability can have a zero range indication at the MLS approach azimuth site when operating on MLS and a zero range
indication at the touchdown point when operating on ILS. As this is considered to be operationally unacceptable, specifically
from an ATC point of view, and if ILS/MLS/DME frequency tripling to prevent the relocation of the DME/N is not possible,
the implementation of DME/P is to be postponed until the DME/N is withdrawn.
7.1.6.7 The nominal location of the zero range indication provided by a DME/N interrogator needs to be published.
7.1.6.8 In considering DME sites, it is also necessary to take into account technical factors such as runway length,
profile, local terrain and transponder antenna height to assure adequate signal levels in the vicinity of threshold and along the
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