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to 40 dB.
2.6.3 The calculations are based on the assumption that the protection afforded to the wanted signal against
interference from the unwanted signal is 20 dB. This corresponds to a disturbance of not more than 15 microamperes at the
limit of the service distance of ILS.
ATT C-29 23/11/06
Annex 10 — Aeronautical Communications Volume I
2.6.4 In so far as the wanted and unwanted carriers may produce a heterodyne note, the protection ratio ensures that the
instrumentation is not affected. However, in cases where a voice facility is used, the heterodyne note may interfere with this
facility.
2.6.5 In general, when international use of ILS systems is confined to the pairings listed in Chapter 3, 3.1.6.1.1, the
criteria are such that, provided they are met for the localizer element, the glide path element is automatically covered. At
certain congested locations, where it is necessary to make assignments in both the first ten and the second ten sequence
pairings, it may be necessary to select certain pairings out of sequence in order to meet the minimum geographical separation
in 2.6.6.
Example: Referring to Chapter 3, 3.1.6.1.1, it will be noted that ILS Sequence Number 2 pairs the localizer frequency of
109.9 MHz with glide path frequency 333.8 MHz. Sequence Numbers 12 and 19, however, although providing wide
frequency separation from Sequence Number 2 in the case of the localizers, assign frequencies of 334.1 MHz and 333.5 MHz,
respectively, for the glide paths, both being first adjacent channels (300 kHz spacing) to the Sequence Number 2 glide path
channel. If selection of ILS channels is confined to either the first ten or the second ten pairings, then the minimum glide path
frequency separation will be 600 kHz.
2.6.6 Table of required distance separations [See Table C-1.]
2.6.7 The application of the figures given in Table C-1 will only be correct within the limitations set by the
assumptions which include that facilities are essentially non-directional in character, that they have similar radiated powers,
that the field strength is approximately proportional to the angle of elevation for angles up to 10 degrees, and that the aircraft
antenna is essentially omnidirectional in character. If more precise determination of separation distances is required in areas
of frequency congestion, this may be determined for each facility from appropriate propagation curves, taking into account
the particular directivity factors, radiated power characteristics and the operational requirements as to coverage. Where
reduced separation distances are determined by taking into account directivity, etc., flight measurements at the ILS protection
point and at all points on the approach path should be made wherever possible to ensure that a protection ratio of at least 20
dB is achieved in practice.
2.7 Localizers and glide paths achieving coverage
with two radio frequency carriers
2.7.1 Localizer coverage may be achieved by using two composite radiation field patterns on different carrier
frequencies spaced within the localizer frequency channel. One field pattern gives accurate course and displacement
indications within the front course sector; the other field pattern provides ILS indications outside the front course sector to
meet the coverage requirements in Chapter 3, 3.1.3.3 and 3.1.3.7. Discrimination between signals is obtained in airborne
receivers by the stronger signal capturing the receiver. Effectiveness of capture depends on the type of detector used but, in
general, if the ratio of the two signals is of the order of 10 dB or more, the smaller signal does not cause significantly large
errors in demodulated output. For optimum performance within the front course sector, the following guidance material
should be applied in the operation of two carrier frequency localizer systems.
2.7.2 The localizer should be designed and maintained so that the ratio of the two radiated signals-in-space within the
front course sector does not fall below 10 dB. Particular attention should be directed to the vertical lobe structure produced
by the two antenna systems which may be different in height and separated in distance, thus resulting in changes in ratio of
signal strengths during approach.
2.7.3 Due to the 6 dB allowance for the receiver pass-band filter ripple, localizer receiver response variations can occur
as the clearance frequency is displaced from the course frequency. To minimize this effect, particularly for Category III
operations, the course-to-clearance signal ratio needs to be increased from 10 dB to 16 dB.
23/11/06 ATT C-30
Attachment C Annex 10 — Aeronautical Communications
2.7.4 To minimize further the risk of errors if the ratio of the two radiated signals falls below 10 dB within the front
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