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two or more closely adjacent aerodromes are to be served by a single test facility, several watts of radio frequency energy
may need to be emitted.
1.5.2 Where there is a need to protect co-channel VORs, VOTs and ILS localizers from VOT interference, the radio
emission must be limited to that required to provide satisfactory operation and to ensure that interference with other
co-channel assignments does not occur.
1.6 Modulation
1.6.1 The radio frequency carrier as observed at any point in space should 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);
b) 30 Hz.
1.6.2 The depth of modulation due to the 9 960 Hz and the 30 Hz signals should be within the limits of 28 per cent for
each component.
1.6.3 The signal which frequency modulates the 9 960 Hz subcarrier and the signal which amplitude modulates the
radio frequency carrier should both be maintained at 30 Hz within plus or minus 1 per cent.
1.6.4 The frequency of the 9 960 Hz subcarrier should be maintained within plus or minus 1 per cent.
1.6.5 The percentage of amplitude modulation on the 9 960 Hz subcarrier present at the output of the transmitter should
not be greater than 5 per cent.
1.7 Identification
1.7.1 The VOT should transmit a 1 020 Hz identification signal. The identification code for a VOT installation should
be selected by the competent authority so as to be unmistakably distinctive as to the test function and, if necessary, as to the
location.
Note.— In one State, when the VOT coverage is confined to a single aerodrome, the identification consists of a
continuous series of dots.
1.7.2 The depth to which the radio frequency carrier is modulated by the identification signal should be approximately
10 per cent.
1.8 Monitoring
1.8.1 Basically, there is no need for continuous automatic monitoring of VOT provided the relative phase of the AM
and FM 30 Hz components are mechanically locked and facilities exist for periodic inspection and remote supervision of the
state of the VOT.
23/11/06 ATT E-2
Attachment E Annex 10 — Aeronautical Communications
1.8.2 Provision of automatic monitoring can double the cost of a VOT installation and, consequently, many competent
authorities are likely to employ only remote supervision at a control point. However, where, in the light of the operational use
to be made of a VOT, a State decides to provide automatic monitoring, the monitor should transmit a warning to a control
point and cause a cessation of transmission if either of the following deviations from established conditions arises:
a) a change in excess of 1 degree at the monitor site of the “bearing” information transmitted by the VOT;
b) a reduction of 50 per cent in the signal level of the 9 960 Hz or 30 Hz signals at the monitor.
Failure of the monitor should automatically cause a cessation of transmission.
2. Selection and use of VOR aerodrome check-points
2.1 General
2.1.1 When a VOR is suitably located in relationship to an aerodrome, the pre-flight checking of an aircraft VOR
installation can be facilitated by the provision of suitably calibrated and marked check-points at convenient parts of the
aerodrome.
2.1.2 In view of the wide variation in circumstances encountered, it is not practicable to establish any standard
requirements or practices for the selection of VOR aerodrome check-points. However, States wishing to provide this facility
should be guided by the following considerations in selecting the points to be used.
2.2 Siting requirements for check-points
2.2.1 The signal strength of the nearby VOR has to be sufficient to ensure satisfactory operation of a typical aircraft
VOR installation. In particular, full flag action (no flag showing) must be ensured.
2.2.2 The check-points should, within the limits of operating convenience, be located away from buildings or other
reflecting objects (fixed or moving) which are likely to degrade the accuracy or stability of the VOR signal.
2.2.3 The observed VOR bearing at any selected point should ideally be within plus or minus 1.5 degrees of the
bearing accurately determined by survey or chart plotting.
Note.— The figure of plus or minus 1.5 degrees has no direct operational significance in that the observed bearing
becomes the published bearing; however, where a larger difference is observed, there is some possibility of poor stability.
2.2.4 The VOR information at a selected point should be used operationally only if found to be consistently within plus
or minus 2 degrees of the published bearing. The stability of the VOR information at a selected point should be checked
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