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noise levels in different areas may be insufficient, because factors such as the frequency of occurrence of noise, its character
and effect on the airborne receiver and the nature of the air operation involved may all modify ratios determined in this way.
6.2.2.5 Values of diurnal and seasonal noise in various parts of the world have been published in Report 322 of the
former CCIR of the ITU.
6.2.2.5.1 Correlation of these values to actual local conditions and the derivation of required signal-to-noise ratios for
effective operational use of ADF equipment is not yet fully established.
6.2.3 Effective coverage
6.2.3.1 Effective coverage as defined in Chapter 3, 3.4.1, is the area surrounding an NDB, within which useful
information to the operator concerned can be obtained at a particular time. It is, therefore, a measure of NDB performance
under prevailing conditions.
6.2.3.2 The effective coverage is limited by the ratio of the strength of the steady (non-fading) signal received from the
NDB to the total noise intercepted by the ADF receiver. When this ratio falls below a limiting value, useful bearings cannot
be obtained. It should also be noted that the effective coverage of an NDB may in some cases be limited to the range of the
usable identification signal.
6.2.3.3 The strength of signal received from the NDB is governed by:
a) the power supplied to the antenna of the NDB;
23/11/06 ATT C-60
Attachment C Annex 10 — Aeronautical Communications
b) the radiation efficiency of the antenna, which varies according to the height of the antenna and other characteristics
of the radiating system;
c) the conductivity of the path between the NDB and the receiver, which may vary considerably as between one site
and another, and is always less over land than over seawater;
d) the operating radio frequency.
6.2.3.4 The noise admitted by the receiver depends on:
a) the bandwidth of the receiver;
b) the level of atmospheric noise, which varies according to the geographical area concerned, with the time of day and
the season of the year, and which may reach very high levels during local thunderstorms;
c) the level of the interference produced by other radio emissions on the same or on adjacent frequencies, which is
governed to a large extent by the NDB density in the area concerned and the effectiveness of regional planning;
d) the level of noise due to electrical noise in the aircraft or to industrial noise (generated by electric motors, etc.),
when the coverage of the NDB extends over industrial areas.
6.2.3.4.1 It has to be noted that the effect of noise depends on characteristics of the ADF receiver and the associated
equipment, and also on the nature of the noise (e.g. steady noise, impulsive noise).
6.2.3.5 A further factor which limits the effective coverage of an NDB is present at night when interaction occurs
between components of the signal which are propagated respectively in the horizontal plane (ground wave propagation) and
by reflection from the ionosphere (sky wave propagation). When there is interaction between these components, which arrive
at the ADF receiver with a difference of phase, bearing errors are introduced (night effect).
6.2.3.6 It will thus be seen that the effective coverage of an NDB depends on so many factors, some of which are
variable, that it is impossible to specify the effective coverage of an NDB in any simple manner. The effective coverage of
any NDB, in fact, varies according to the time of day and the season of the year.
6.2.3.6.1 Hence any attempt to specify an effective coverage, which would be obtainable at any time throughout the
day or throughout the year, would result either in a figure for coverage which would be so small (since this would be the
coverage obtained under the worst conditions of atmospheric noise, etc.) as to give quite a misleading picture of the
effectiveness of the NDB, or would involve such high power and costly antenna systems (to provide the required coverage
under the worst conditions), that the installation of such an NDB would usually be precluded by considerations of initial
and operating costs. No specific formula can be given in determining what rated coverage would be equivalent to a desired
effective coverage and the relation must be assessed regionally.
6.2.3.7 Those concerned with the operational aspects of NDB coverage will normally consider requirements in terms of
a desired operational coverage and, in regional planning, it will usually be necessary to interpret such requirements in terms
of a rated coverage from which may be derived the essential characteristics of the NDB required and which will also define
the area to be protected against harmful interference. No specific formula can be given in determining what rated coverage
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