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Regulations.
Note 2.— The selection of locations and times at which the field strength is measured is important in order to avoid
abnormal results for the locality concerned; locations on air routes in the area around the beacon are operationally most
significant.
Chapter 3 Annex 10 — Aeronautical Communications
3-33 23/11/06
3.4.2.2 All notifications or promulgations of NDBs shall be based upon the average radius of the rated coverage.
Note 1.— In classifying radio beacons in areas where substantial variations in rated coverage may occur diurnally and
seasonally, such variations should be taken into account.
Note 2.— Beacons having an average radius of rated coverage of between 46.3 and 278 km (25 and 150 NM) may be
designated by the nearest multiple of 46.3 km (25 NM) to the average radius of rated coverage, and beacons of rated
coverage over 278 km (150 NM) to the nearest multiple of 92.7 km (50 NM).
3.4.2.3 Recommendation.— Where the rated coverage of an NDB is materially different in various operationally
significant sectors, its classification should be expressed in terms of the average radius of rated coverage and the angular
limits of each sector as follows:
Radius of coverage of sector/angular limits of sector expressed as magnetic bearing clockwise from the beacon.
Where it is desirable to classify an NDB in such a manner, the number of sectors should be kept to a minimum and preferably
should not exceed two.
Note.— The average radius of a given sector of the rated coverage is equal to the radius of the corresponding circlesector
of the same area. Example:
150/210° – 30°
100/30° – 210°.
3.4.3 Limitations in radiated power
The power radiated from an NDB shall not exceed by more than 2 dB that necessary to achieve its agreed rated coverage,
except that this power may be increased if coordinated regionally or if no harmful interference to other facilities will result.
3.4.4 Radio frequencies
3.4.4.1 The radio frequencies assigned to NDBs shall be selected from those available in that portion of the spectrum
between 190 kHz and 1 750 kHz.
3.4.4.2 The frequency tolerance applicable to NDBs shall be 0.01 per cent except that, for NDBs of antenna power
above 200 W using frequencies of 1 606.5 kHz and above, the tolerance shall be 0.005 per cent.
3.4.4.3 Recommendation.— Where two locators are used as supplements to an ILS, the frequency separation between
the carriers of the two should be not less than 15 kHz to ensure correct operation of the radio compass, and preferably not
more than 25 kHz in order to permit a quick tuning shift in cases where an aircraft has only one radio compass.
3.4.4.4 Where locators associated with ILS facilities serving opposite ends of a single runway are assigned a common
frequency, provision shall be made to ensure that the facility not in operational use cannot radiate.
Note.— Additional guidance on the operation of locator beacons on common frequency channels is contained in
Volume V, Chapter 3, 3.2.2.
3.4.5 Identification
3.4.5.1 Each NDB shall be individually identified by a two- or three-letter International Morse Code group transmitted
at a rate corresponding to approximately 7 words per minute.
Annex 10 — Aeronautical Communications Volume I
23/11/06 3-34
3.4.5.2 The complete identification shall be transmitted at least once every 30 seconds, except where the beacon
identification is effected by on/off keying of the carrier. In this latter case, the identification shall be at approximately
1-minute intervals, except that a shorter interval may be used at particular NDB stations where this is found to be
operationally desirable.
3.4.5.2.1 Recommendation.— Except for those cases where the beacon identification is effected by on/off keying of
the carrier, the identification signal should be transmitted at least three times each 30 seconds, spaced equally within that
time period.
3.4.5.3 For NDBs with an average radius of rated coverage of 92.7 km (50 NM) or less that are primarily approach and
holding aids in the vicinity of an aerodrome, the identification shall be transmitted at least three times each 30 seconds,
spaced equally within that time period.
3.4.5.4 The frequency of the modulating tone used for identification shall be 1 020 Hz plus or minus 50 Hz or 400 Hz
plus or minus 25 Hz.
Note.— Determination of the figure to be used would be made regionally, in the light of the considerations contained in
Attachment C, 6.5.
3.4.6 Characteristics of emissions
Note.— The following specifications are not intended to preclude employment of modulations or types of modulations
that may be utilized in NDBs in addition to those specified for identification, including simultaneous identification and voice
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