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modulation, provided that these additional modulations do not materially affect the operational performance of the NDBs in
conjunction with currently used airborne direction finders, and provided their use does not cause harmful interference to
other NDB services.
3.4.6.1 Except as provided in 3.4.6.1.1, all NDBs shall radiate an uninterrupted carrier and be identified by on/off
keying of an amplitude modulating tone (NON/A2A).
3.4.6.1.1 NDBs other than those wholly or partly serving as holding, approach and landing aids, or those having an
average radius of rated coverage of less than 92.7 km (50 NM), may be identified by on/off keying of the unmodulated carrier
(NON/A1A) if they are in areas of high beacon density and/or where the required rated coverage is not practicable of
achievement because of:
a) radio interference from radio stations;
b) high atmospheric noise;
c) local conditions.
Note.— In selecting the types of emission, the possibility of confusion, arising from an aircraft tuning from a NON/A2A
facility to a NON/A1A facility without changing the radio compass from “MCW” to “CW” operation, will need to be kept in
mind.
3.4.6.2 For each NDB identified by on/off keying of an audio modulating tone, the depth of modulation shall be
maintained as near to 95 per cent as practicable.
3.4.6.3 For each NDB identified by on/off keying of an audio modulating tone, the characteristics of emission during
identification shall be such as to ensure satisfactory identification at the limit of its rated coverage.
Chapter 3 Annex 10 — Aeronautical Communications
3-35 23/11/06
Note 1.— The foregoing requirement necessitates as high a percentage modulation as practicable, together with
maintenance of an adequate radiated carrier power during identification.
Note 2.— With a direction-finder pass band of plus or minus 3 kHz about the carrier, a signal to noise ratio of 6 dB at
the limit of rated coverage will, in general, meet the foregoing requirement.
Note 3.— Some considerations with respect to modulation depth are contained in Attachment C, 6.4.
3.4.6.4 Recommendation.— The carrier power of an NDB with NON/A2A emissions should not fall when the identity
signal is being radiated except that, in the case of an NDB having an average radius of rated coverage exceeding 92.7 km
(50 NM), a fall of not more than 1.5 dB may be accepted.
3.4.6.5 Unwanted audio frequency modulations shall total less than 5 per cent of the amplitude of the carrier.
Note.— Reliable performance of airborne automatic direction-finding equipment (ADF) may be seriously prejudiced if the
beacon emission contains modulation by an audio frequency equal or close to the loop switching frequency or its second harmonic.
The loop switching frequencies in currently used equipment lie between 30 Hz and 120 Hz.
3.4.6.6 The bandwidth of emissions and the level of spurious emissions shall be kept at the lowest value that the state
of technique and the nature of the service permit.
Note.— Article S3 of the ITU Radio Regulations contains the general provisions with respect to technical characteristics
of equipment and emissions. The Radio Regulations contain specific provisions relating to necessary bandwidth, frequency
tolerance, spurious emissions and classification of emissions (see Appendices APS1, APS2 and APS3).
3.4.7 Siting of locators
3.4.7.1 Recommendation.— Where locators are used as a supplement to the ILS, they should be located at the sites
of the outer and middle marker beacons. Where only one locator is used as a supplement to the ILS, preference should be
given to location at the site of the outer marker beacon. Where locators are employed as an aid to final approach in the
absence of an ILS, equivalent locations to those applying when an ILS is installed should be selected, taking into account
the relevant obstacle clearance provisions of the PANS-OPS (Doc 8168).
3.4.7.2 Recommendation.— Where locators are installed at both the middle and outer marker positions, they should
be located, where practicable, on the same side of the extended centre line of the runway in order to provide a track between
the locators which will be more nearly parallel to the centre line of the runway.
3.4.8 Monitoring
3.4.8.1 For each NDB, suitable means shall be provided to enable detection of any of the following conditions at an
appropriate location:
a) a decrease in radiated carrier power of more than 50 per cent below that required for the rated coverage;
b) failure to transmit the identification signal;
c) malfunctioning or failure of the means of monitoring itself.
3.4.8.2 Recommendation.— When an NDB is operated from a power source having a frequency which is close to
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