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operational significance. The achieved ILS reference datum can only be ascertained by flight check, i.e. the mean observed
position of that portion of the glide path typically between points 1 830 m (6 000 ft) and 300 m (1 000 ft) from the threshold
being represented as a straight line and extended to touchdown. The point at which this extended straight line meets the line
drawn vertically through the threshold at the runway centre line is the achieved ILS reference datum.
Note.— Further guidance on the measurement of the glide path angle and the achieved ILS reference datum is given in
Doc 8071.
23/11/06 ATT C-22
Attachment C Annex 10 — Aeronautical Communications
A
Glide path
antenna
Threshold
P
Horizontal
Runway
Straight line extension of glide path
D
Y
H
Line of glide path reflection plane
Figure C-5. Glide path siting for sloping runway
2.4.13 Chapter 3, 3.1.5.3.1 indicates the glide path coverage to be provided to allow satisfactory operation of a typical
aircraft installation. The operational procedures promulgated for a facility must be compatible with the lower limit of this
coverage. It is usual for descents to be made to the intercept altitude and for the approach to continue at this altitude until a
fly-down signal is received. In certain circumstances a cross-check of position may not be available at this point. Automatic
flight control systems will normally start the descent whenever a fly-up signal has decreased to less than about 10
microamperes.
2.4.14 The objective is, therefore, to provide a fly-up signal prior to intercepting the glide path. Although under
normal conditions, approach procedures will be accomplished in such a way that glide path signals will not be used below
0.45 θ, or beyond 18.5 km (10 NM) from the runway, it is desirable that misleading guidance information should not be
radiated in this area. Where procedures are such that the glide path guidance may be used below 0.45 θ, adequate precautions
must be taken to guard against the radiation of misleading guidance information below 0.45 θ, under both normal conditions
and during a malfunction, thus preventing the final descent being initiated at an incorrect point on the approach. Some
precautions which can be employed to guard against the radiation of misleading guidance include the radiation of a
supplementary clearance signal such as provided for in Chapter 3, 3.1.5.2.1, the provision of a separate clearance monitor and
appropriate ground inspection and setting-up procedures.
2.4.15 To achieve satisfactory monitor protection against below-path out-of-tolerance DDM, depending on the antenna
system used, the displacement sensitivity monitor as required in Chapter 3, 3.1.5.7.1 e) may not be adequate to serve also as a
ATT C-23 23/11/06
Annex 10 — Aeronautical Communications Volume I
clearance monitor. In some systems, e.g. those using multi-element arrays without supplementary clearance, a slight
deterioration of certain antenna signals can cause serious degradation of the clearance with no change or only insignificant
changes within the glide path sector as seen by the deviation sensitivity monitor. It is important to ensure that monitor alarm
is achieved for any or all possible deteriorated antenna and radiated signal conditions, which may lead to a reduction of
clearance to 0.175 DDM or less in the below-path clearance coverage.
2.5 Diagrams (Figures C-6 to C-12 illustrate certain of the Standards contained in Chapter 3)
90 Hz
90 Hz
150 Hz
150 Hz
20 degrees at 150 Hz
(370 microseconds)
10 degrees at 150 Hz
(185 microseconds)
P2 = 1.749
P2 = 1.806
P1 = 1.936
P1 = 1.900
= 0.903
= 0.951
+1
+1
0
0
-1
-1
+2
+2
+1
+1
0
0
-1
-1
-2
-2
Time
Time
Facility performance Categories I and II localizers and glide paths
Facility performance Category III localizers and glide paths
Amplitude Amplitude
The accompanying graphs illustrate a method that can be used to measure the relative phase relationship between the 90 Hz and 150 Hz
tones. The upper portion of each graph shows the individual waveforms and their relationship at the limit of phase differences allowed by
Chapter 3, 3.1.3.5.3.3 and 3.1.5.5.3. The lower portion shows the combined waveforms as would be seen on an oscilloscope. By taking the
ratio of P and P , which gives a value equal to or less than unity, it is possible to determine if the phasing is within tolerance. For Categories I
and II ILS the ratio should be greater than 0.903 and for Category III the ratio should be greater than 0.951.
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