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be added whenever, in the opinion of the Competent Authority, an additional beacon is required because of
operational procedures at a particular site.
b) The marker beacons shall conform to the requirements prescribed in 3.1.7. When the installation comprises only two
marker beacons, the requirements applicable to the middle marker and to the outer marker shall be complied with.
c) The marker beacons shall produce radiation patterns to indicate predetermined distance from the threshold along the
ILS glide path.
3.1.7.1.1 When a marker beacon is used in conjunction with the back course of a localizer, it shall conform with the
marker beacon characteristics specified in 3.1.7.
Chapter 3 Annex 10 — Aeronautical Communications
3-23 23/11/06
3.1.7.1.2 Identification signals of marker beacons used in conjunction with the back course of a localizer shall be
clearly distinguishable from the inner, middle and outer marker beacon identifications, as prescribed in 3.1.7.5.1.
3.1.7.2 Radio frequency
3.1.7.2.1 The marker beacons shall operate at 75 MHz with a frequency tolerance of plus or minus 0.005 per cent and
shall utilize horizontal polarization.
3.1.7.3 Coverage
3.1.7.3.1 The marker beacon system shall be adjusted to provide coverage over the following distances, measured on
the ILS glide path and localizer course line:
a) inner marker (where installed): 150 m plus or minus 50 m (500 ft plus or minus 160 ft);
b) middle marker: 300 m plus or minus 100 m (1 000 ft plus or minus 325 ft);
c) outer marker: 600 m plus or minus 200 m (2 000 ft plus or minus 650 ft).
3.1.7.3.2 The field strength at the limits of coverage specified in 3.1.7.3.1 shall be 1.5 millivolts per metre
(minus 82 dBW/m2). In addition, the field strength within the coverage area shall rise to at least 3.0 millivolts per metre
(minus 76 dBW/m2).
Note 1.— In the design of the ground antenna, it is advisable to ensure that an adequate rate of change of field strength
is provided at the edges of coverage. It is also advisable to ensure that aircraft within the localizer course sector will receive
visual indication.
Note 2.— Satisfactory operation of a typical airborne marker installation will be obtained if the sensitivity is so adjusted
that visual indication will be obtained when the field strength is 1.5 millivolts per metre (minus 82 dBW/m2).
3.1.7.4 Modulation
3.1.7.4.1 The modulation frequencies shall be as follows:
a) inner marker (when installed): 3 000 Hz;
b) middle marker: 1 300 Hz;
c) outer marker: 400 Hz.
The frequency tolerance of the above frequencies shall be plus or minus 2.5 per cent, and the total harmonic content of each
of the frequencies shall not exceed 15 per cent.
3.1.7.4.2 The depth of modulation of the markers shall be 95 per cent plus or minus 4 per cent.
3.1.7.5 Identification
3.1.7.5.1 The carrier energy shall not be interrupted. The audio frequency modulation shall be keyed as follows:
Annex 10 — Aeronautical Communications Volume I
23/11/06 3-24
a) inner marker (when installed): 6 dots per second continuously;
b) middle marker: a continuous series of alternate dots and dashes, the dashes keyed at the rate of 2 dashes per second,
and the dots at the rate of 6 dots per second;
c) outer marker: 2 dashes per second continuously.
These keying rates shall be maintained to within plus or minus 15 per cent.
3.1.7.6 Siting
3.1.7.6.1 The inner marker, when installed, shall be located so as to indicate in low visibility conditions the imminence
of arrival at the runway threshold.
3.1.7.6.1.1 Recommendation.— If the radiation pattern is vertical, the inner marker, when installed, should be
located between 75 m (250 ft) and 450 m (1 500 ft) from the threshold and at not more than 30 m (100 ft) from the extended
centre line of the runway.
Note 1.— It is intended that the inner marker pattern should intercept the downward extended straight portion of the
nominal ILS glide path at the lowest decision height applicable in Category II operations.
Note 2.— Care must be exercised in siting the inner marker to avoid interference between the inner and middle markers.
Details regarding the siting of inner markers are contained in Attachment C, 2.10.
3.1.7.6.1.2 Recommendation.— If the radiation pattern is other than vertical, the equipment should be located so as
to produce a field within the course sector and ILS glide path sector that is substantially similar to that produced by an
antenna radiating a vertical pattern and located as prescribed in 3.1.7.6.1.1.
3.1.7.6.2 The middle marker shall be located so as to indicate the imminence, in low visibility conditions, of visual
approach guidance.
3.1.7.6.2.1 Recommendation.— If the radiation pattern is vertical, the middle marker should be located 1 050 m
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