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status because the probability of cessation of signal consequent on failure of other equipment will be too high. This reversion
to a lower category is automatically indicated at remote locations.
2.8.3.1.2 An identical monitoring arrangement is used for the glide path facility.
2.8.3.1.3 To reduce mutual interference between the main and standby transmitters, any stray radiation from the latter
should be at least 50 dB below the carrier level of the main transmitter measured at the antenna system.
2.8.3.2 The highest order of protection is required against the consequence of undetected monitor system failures. This
should 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 design analysis.
2.8.4 Additional guidance material applicable to Categories II and III — ILS localizer and glide path ground equipment
is given below.
2.8.4.1 Reliability of equipment is governed by basic construction and operating environment. Equipment design
should employ the most suitable engineering techniques, materials and components, and rigorous inspection should be
applied in manufacture. Equipment should be operated in environmental conditions appropriate to the manufacturers’ design
criteria. It is expected that the equipment reliability be established by evaluation before introduction into Categories II and III
service. Design analysis should verify the predicted performance of the equipment.
2.8.5 Guidance relating to localizer far field monitors is given below.
2.8.5.1 Far field monitors are provided to monitor course alignment but may also be used to monitor course sensitivity.
A far field monitor operates independently from integral and near field monitors. Its primary purpose is to protect against the
risk of erroneous setting-up of the localizer, or faults in the near field or integral monitors. In addition, the far field monitor
system will enhance the ability of the combined monitor system to respond to the effects of physical modification of the
radiating elements or variations in the ground reflection characteristics. Moreover, multipath effects and runway area
23/11/06 ATT C-36
Attachment C Annex 10 — Aeronautical Communications
disturbances not seen by near field and integral monitors, and some occurrences of radio interferences may be substantially
monitored by using a far field monitoring system built around a suitable receiver(s), installed under the approach path.
2.8.5.2 A far field monitor is generally considered essential for Category III operations, while for Category II it is
generally considered to be desirable. Also for Category I installations, a far field monitor has proved to be a valuable tool to
supplement the conventional monitor system.
2.8.5.3 The signal received by the far field monitor will suffer short-term interference effects caused by aircraft
movements on or in the vicinity of the runway and experience has shown that it is not practical to use the far field monitor as
an executive monitor. When used as a passive monitor, means must be adopted to minimize such temporary interference
effects and to reduce the occurrence of nuisance downgrade indications; some methods of achieving this are covered in
2.8.5.4. The response of the far field monitor to interference effects offers the possibility of indicating to the air traffic control
point when temporary disturbance of the localizer signal is present. However, experience has shown that disturbances due to
aircraft movements may be present along the runway, including the touchdown zone, and not always be observed at the far
field monitor. It must not be assumed, therefore, that a far field monitor can provide comprehensive surveillance of aircraft
movements on the runway.
2.8.5.3.1 Additional possible applications of the far field monitor are as follows:
a) it can be a useful maintenance aid to verify course and/or course deviation sensitivity in lieu of a portable far field
monitor;
b) it may be used to provide a continuous recording of far field signal performance showing the quality of the far field
signal and the extent of signal disturbance.
2.8.5.4 Possible methods of reducing the occurrence of nuisance downgrade indications include:
a) incorporation of a time delay within the system adjustable from 30 to 240 seconds;
b) the use of a validation technique to ensure that only indications not affected by transitory disturbances are
transmitted to the control system;
c) use of low pass filtering.
2.8.5.5 A typical far field monitor consists of an antenna, VHF receiver and associated monitoring units which provide
indications of DDM, modulation sum, and RF signal level. The receiving antenna is usually of a directional type to minimize
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