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3.3.5.1 The radio frequency carrier as observed at any point in space shall be amplitude modulated by two signals as
follows:
a) a subcarrier of 9 960 Hz of constant amplitude, frequency modulated at 30 Hz and having a deviation ratio of
16 plus or minus 1 (i.e. 15 to 17):
1) for the conventional VOR, the 30 Hz component of this FM subcarrier is fixed without respect to azimuth and is
termed the “reference phase”;
2) for the Doppler VOR, the phase of the 30 Hz component varies with azimuth and is termed the “variable
phase”;
b) a 30 Hz amplitude modulation component:
1) for the conventional VOR, this component results from a rotating field pattern, the phase of which varies with
azimuth, and is termed the “variable phase”;
2) for the Doppler VOR, this component, of constant phase with relation to azimuth and constant amplitude, is
radiated omnidirectionally and is termed the “reference phase”.
3.3.5.2 The depth of modulation of the radio frequency carrier due to the subcarrier of 9 960 Hz shall be within the
limits of 28 per cent and 32 per cent.
3.3.5.3 The depth of modulation of the radio frequency carrier due to the 30 Hz or 9 960 Hz signals, as observed at any
angle of elevation up to 5 degrees, shall be within the limits of 28 to 32 per cent.
3.3.5.4 The variable and reference phase modulation frequencies shall be 30 Hz within plus or minus 1 per cent.
3.3.5.5 The subcarrier modulation mid-frequency shall be 9 960 Hz within plus or minus 1 per cent.
3.3.5.6
a) For the conventional VOR, the percentage of amplitude modulation of the 9 960 Hz subcarrier shall not exceed 5 per
cent.
b) For the Doppler VOR, the percentage of amplitude modulation of the 9 960 Hz subcarrier shall not exceed 40 per
cent when measured at a point at least 300 m (1 000 ft) from the VOR.
Annex 10 — Aeronautical Communications Volume I
23/11/06 3-30
3.3.5.7 Where 50 kHz VOR channel spacing is implemented, the sideband level of the harmonics of the 9 960 Hz
component in the radiated signal shall not exceed the following levels referred to the level of the 9 960 Hz sideband:
Subcarrier Level
9 960 Hz 0 dB reference
2nd harmonic –30 dB
3rd harmonic –50 dB
4th harmonic and above –60 dB
3.3.6 Voice and identification
3.3.6.1 If the VOR provides a simultaneous communication channel ground-to-air, it shall be on the same radio
frequency carrier as used for the navigational function. The radiation on this channel shall be horizontally polarized.
3.3.6.2 The peak modulation depth of the carrier on the communication channel shall not be greater than 30 per cent.
3.3.6.3 The audio frequency characteristics of the speech channel shall be within 3 dB relative to the level at 1 000 Hz
over the range 300 Hz to 3 000 Hz.
3.3.6.4 The VOR shall provide for the simultaneous transmission of a signal of identification on the same radio
frequency carrier as that used for the navigational function. The identification signal radiation shall be horizontally polarized.
3.3.6.5 The identification signal shall employ the International Morse Code and consist of two or three letters. It shall
be sent at a speed corresponding to approximately 7 words per minute. The signal shall be repeated at least once every
30 seconds and the modulation tone shall be 1 020 Hz within plus or minus 50 Hz.
3.3.6.5.1 Recommendation.— The identification signal should be transmitted at least three times each 30 seconds,
spaced equally within that time period. One of these identification signals may take the form of a voice identification.
Note.— Where a VOR and DME are associated in accordance with 3.5.2.5, the identification provisions of 3.5.3.6.4
influence the VOR identification.
3.3.6.6 The depth to which the radio frequency carrier is modulated by the code identification signal shall be close to,
but not in excess of 10 per cent except that, where a communication channel is not provided, it shall be permissible to
increase the modulation by the code identification signal to a value not exceeding 20 per cent.
3.3.6.6.1 Recommendation.— If the VOR provides a simultaneous communication channel ground-to-air, the
modulation depth of the code identification signal should be 5 plus or minus 1 per cent in order to provide a satisfactory
voice quality.
3.3.6.7 The transmission of speech shall not interfere in any way with the basic navigational function. When speech is
being radiated, the code identification shall not be suppressed.
3.3.6.8 The VOR receiving function shall permit positive identification of the wanted signal under the signal
conditions encountered within the specified coverage limits, and with the modulation parameters specified at 3.3.6.5, 3.3.6.6
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