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course sector, the difference in alignment of the radiation field patterns of the two signals should be kept as minimal as
practicable.
2.7.5 Glide paths which employ two carriers are used to form a composite radiation field pattern on the same radio
frequency channel. Special configurations of antennas and the distribution of antenna currents and phasing may permit siting
of glide path facilities at locations with particular terrain conditions which may otherwise cause difficulty to a singlefrequency
system. At such sites, an improvement is obtained by reducing the low angle radiation. The second carrier is
employed to provide coverage in the region below the glide path.
2.8 Integrity and continuity of service — ILS ground equipment
2.8.1 Introduction
2.8.1.1 This material is intended to provide clarification of the integrity and continuity of service objectives of ILS
localizer and glide path ground equipment and to provide guidance on engineering design and system characteristics of this
equipment. The integrity and continuity of service must of necessity be known from an operational viewpoint in order to
decide the operational application which an ILS could support.
2.8.1.2 It is generally accepted, irrespective of the operational objective, that the average rate of a fatal accident during
landing, due to failures or shortcomings in the whole system, comprising the ground equipment, the aircraft and the pilot,
should not exceed 1 × 10–7. This criterion is frequently referred to as the global risk factor.
2.8.1.3 In the case of Category I operations, responsibility for assuring that the above objective is not exceeded is
vested more or less completely in the pilot. In Category III operations, the same objective is required but must now be
inherent in the whole system. In this context it is of the utmost importance to endeavour to achieve the highest level of
integrity and continuity of service of the ground equipment. Integrity is needed to ensure that an aircraft on approach will
have a low probability of receiving false guidance; continuity of service is needed to ensure that an aircraft in the final stages
of approach will have a low probability of being deprived of a guidance signal.
2.8.1.4 It is seen that various operational requirements correspond to varied objectives of integrity and continuity of
service. Paragraph 2.14 identifies and describes four levels of integrity and continuity of service.
2.8.2 Guidance material concerning the achievement and retention of integrity and continuity of service levels
2.8.2.1 An integrity failure can occur if radiation of a signal which is outside specified tolerances is either
unrecognized by the monitoring equipment or the control circuits fail to remove the faulty signal. Such a failure might
constitute a hazard if it results in a gross error.
2.8.2.2 Clearly not all integrity failures are hazardous in all phases of the approach. For example, during the critical
stages of the approach, undetected failures producing gross errors in course width or course line shifts are of special
significance whereas an undetected change of modulation depth, or loss of localizer and glide slope clearance and localizer
identification would not necessarily produce a hazardous situation. The criterion in assessing which failure modes are
relevant must however include all those deleterious fault conditions which are not unquestionably obvious to the automatic
flight system or pilot.
2.8.2.3 It is especially important that monitors be designed to provide fail-safe operation through compliance with the
Standards of Chapter 3, 3.1.3.11.4 and 3.1.5.7.4. This often requires a rigorous design analysis. Monitor failures otherwise
may permit the radiation of erroneous signals. Some of the possible conditions which might constitute a hazard in
Operational Performance Categories II and III are:
ATT C-31 23/11/06
Annex 10 — Aeronautical Communications Volume I
a) an undetected shift of course line significantly outside the monitor limits for localizer and glide path;
b) an undetected fault that significantly changes the course width and glide path sensitivity;
c) an undetected fault causing slow cyclic movements of the course, producing apparent course bends as seen by the
approaching aircraft significantly exceeding in amplitude the figures specified in Chapter 3, 3.1.3.4.2 for the
localizer and Chapter 3, 3.1.5.4.2 for the glide path between ILS points “B” and “T”.
2.8.2.4 The highest order of protection is required against the risk of undetected failures in the monitoring and
associated control system. This would be achieved by careful design to reduce the probability of such occurrences to a low
level and by carrying out maintenance checks on the monitor system performance at intervals which are determined by the
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