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service objectives of Level 3 would be described as class II/D/3.
2.14.4 ILS classes are appropriate only to the ground ILS element. Consideration of operational categories must also
include additional factors such as operator capability, critical and sensitive area protection, procedural criteria and ancillary
aids, such as transmissometers and lights.
2.15 ILS carrier frequency and phase modulation
2.15.1 In addition to the desired 90 Hz and 150 Hz AM modulation of the ILS RF carriers, undesired frequency
modulation (FM) and/or phase modulation (PM) may exist. This undesired modulation can cause centring errors in ILS
receivers due to slope detection by ripple in the intermediate frequency (IF) filter pass-band.
2.15.2 For this to occur, the translated RF carrier frequency must fall on an IF frequency where the pass-band has a
high slope. The slope converts the undesired 90 Hz and 150 Hz frequency changes to AM of the same frequencies. Similarly,
any difference in FM deviation between the undesired 90 Hz and 150 Hz components is converted to DDM, which in turn
produces an offset in the receiver. The mechanism is identical for PM as for FM, since PM causes a change in frequency
equal to the change in phase (radians) multiplied by the modulating frequency.
2.15.3 The effect of the undesired FM and/or PM is summed by vector addition to the desired AM. The detected FM is
either in phase or anti-phase with the AM according to whether the pass-band slope at the carrier’s IF is positive or negative.
The detected PM is in quadrature with the AM, and may also be positive or negative according to the pass-band slope.
23/11/06 ATT C-40
Attachment C Annex 10 — Aeronautical Communications
2.15.4 Undesired FM and/or PM from frequencies other than 90 Hz and 150 Hz, but which pass through the 90 Hz and
150 Hz tone filters of the receiver, can also cause changes to the desired 90 Hz and 150 Hz AM modulation of the ILS RF
carrier, resulting in a DDM offset error in the receiver. Thus, it is essential that when measuring undesired FM and PM levels,
audio band-pass filters with a pass-band at least as wide as that of the tone filters of ILS receivers be used. These filters are
typically inserted in commercial modulation meter test equipment between the demodulation and metering circuits, to ensure
that only spectral components of interest to ILS applications are measured. To standardize such measurements, the filter
characteristics are recommended as shown below:
Frequency
(Hz)
90 Hz band-pass
filter attenuation,
dB
150 Hz band-pass
filter attenuation,
dB
≤45 –10 –16
85 –0.5 (no specification)
90 0 –14
95 –0.5 (no specification)
142 (no specification) –0.5
150 –14 0
158 (no specification) –0.5
≥300 –16 –10
2.15.5 The preferred maximum limits, as shown below, are derived from ILS receiver centring error limits specified in
EUROCAE documents ED-46B and ED-47B, based on the worst-case-to-date observed correlation between undesired
modulation levels and centring errors:
Facility type
90 Hz peak
deviation,
FM Hz/PM
radians
(Note 1)
150 Hz peak
deviation,
FM Hz/PM
radians
(Note 2)
Deviation
difference, Hz
(Note 3)
Localizer, Cat I 135/1.5 135/0.9 45
Localizer, Cat II 60/0.66 60/0.4 20
Localizer, Cat III 45/0.5 45/0.3 15
Glide path, Cat I 150/1.66 150/1.0 50
Glide path, Cat II or III 90/1.0 90/0.6 30
Note 1.— This column applies to the peak frequency or phase deviation as measured with the 90 Hz tone filter specified
in 2.15.4.
Note 2.— This column applies to the peak frequency or phase deviation as measured with the 150 Hz tone filter specified
in 2.15.4.
Note 3.— This column applies to the difference in peak frequency deviation between the separate measurements of the
undesired 90 Hz FM (or equivalent PM) and the 150 Hz FM (or equivalent PM) obtained with the filters specified in the
table in 2.15.4. The equivalent deviation for 90 Hz and 150 Hz measured PM values is calculated by multiplying each peak
PM measurement in radians by its corresponding modulating frequency in Hz.
ATT C-41 23/11/06
Annex 10 — Aeronautical Communications Volume I
3. Material concerning VOR
3.1 Guidance relating to VOR effective radiated power (ERP) and coverage
3.1.1 The field strength specified at Chapter 3, 3.3.4.2, is based on the following consideration:
Airborne receiver sensitivity
–117 dBW
Transmission line loss, mismatch loss,
antenna polar pattern variation with
respect to an isotropic antenna
+7 dBW
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