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during MLS operations. The sensitive area provides protection against interference caused by large objects outside
the critical area but still normally within the airfield boundary.
Note 1.— Where disturbance to the guidance signal can occur only at some height above the ground the terms “critical
volume” or “sensitive volume” are used.
Note 2.— The objective of defining critical and sensitive areas is to afford adequate protection of the MLS guidance
signals. The manner in which the terminology is applied may vary between States. In some States, the term “critical area” is
also used to describe the area that is referred to herein as the sensitive area.
4.3.2 Typical examples of critical and sensitive areas that need to be protected are shown in Figure G-23 and Figure
G-24. The tabled values associated with Figure G-23 and Figure G-24 apply to approach procedures with elevation angles
of three degrees or higher. To assure the signal quality, it is necessary normally to prohibit all entry of vehicles and the
taxiing or parking of aircraft within this area during all MLS operations. The critical area determined for each azimuth and
elevation antenna should be clearly designated. Suitable signal devices may need to be provided at taxiways and roadways
which penetrate the critical area in order to restrict the entry of vehicles and aircraft.
ATT G-19 23/11/06
Annex 10 — Aeronautical Communications Volume I
4.3.3 Computer modelling techniques can be employed to calculate the magnitude and duration of signal disturbances
caused by structures or by aircraft of various sizes and orientation at differing locations. Typically, the parameters required to
operate such a model are the antenna beamwidths and the size, location and orientation of reflecting and shadowing objects.
Taking into account the maximum allowable multipath degradation of the signal due to aircraft on the ground, the
corresponding critical and sensitive areas can be determined. Such a method has been used in developing Figures G-23 and
G-24, after validation of computer models which included comparisons at selected points of computed results with actual
field and flight data on parked aircraft interference to the MLS guidance signals.
4.3.4 Control of critical areas and the designation of sensitive areas on the airport proper generally will be sufficient to
protect MLS signals from multipath effects caused by large, fixed ground structures. This is particularly significant when
considering the size of new buildings. Structures outside the boundaries of the airport generally will not cause difficulty to
the MLS signal quality as long as the structures meet obstacle limitation criteria.
4.3.5 The boundary of the protected zone (i.e. the combined critical and sensitive areas) is defined such that
interference caused by aircraft and vehicles outside that boundary will not cause errors in excess of typical allowances for
propagation effects. The derivation of error allowances to protect centre line approach profiles, as shown in Tables G-10 and
G-11 for a “clean” and “complex” propagation environment, proceed as follows. Allowances for equipment errors are
subtracted (on a root sum square basis (RSS)) from the system error limits at the approach reference datum (ARD) and the
resulting balance of the error budget is available for propagation anomalies. The ground reflection is accommodated at both
clean and complex sites, while in complex environments, a margin is reserved to accommodate additional error sources such
as support structure vibration, diffracted signals from, for example, approach lighting system (ALS) lights and supports or
more intense lateral reflections. Finally, 70 per cent of the remaining error balance is allocated to define the protected zone
boundary. Thus, error balances are available to define protected zone boundaries for the extreme cases of a very clean
propagation environment with only ground reflections and for a very complex environment with several significant sources
of propagation errors.
4.3.6 The MLS critical areas are smaller than the ILS critical areas. Where MLS antennas are located in close
proximity to the ILS antennas, the ILS critical areas in most cases will protect the MLS for similar approach paths.
Note.— A reduction of the MLS critical and sensitive areas may be obtained by measurements or analysis which
consider the specific environment. It is recommended that samples be taken at least every 15 m (50 ft).
4.3.7 Azimuth. For an azimuth antenna supporting an aligned approach along the zero degree azimuth, the region
between the azimuth antenna and runway stop end is to be designated as a critical area. The sensitive area of Figure G-23
provides additional signal protection when low visibility landing operations are in progress. In general, the azimuth sensitive
　
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