getting to grips with CAT II CAT III operations(50)

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5.7.2 ILS PROTECTION
In CAT II and III approaches, the ILS beams must be protected from unacceptable disturbance. For this purpose, two kinds of protection area are defined
. the critical area,
. the sensitive area.

ILS critical area: An area of defined dimensions about the localizer and glide path antennas where vehicles, including aircraft, are excluded during all ILS operations. The critical area is protected because the presence of vehicles and/or aircraft inside the boundaries will cause unacceptable disturbance to the ILS signal-in-space.
ILS sensitive area: An area extending beyond the critical area where the parking and/or movement of vehicles, including aircraft, is controlled to prevent the possibility of unacceptable interference to the ILS signal during ILS operations. The sensitive area is protected to provide protection against interference caused by large moving objects outside the critical area but still normally within the airfield boundary.
The dimensions of the critical area are contained in ICAO Annex 10, but there is no specification for the dimensions of the sensitive area. An example of both critical and sensitive areas, taken from ECAC doc n° 17, is given in Figure 5.6. Some states do not define the sensitive area but increase the critical area.
The ILS beam is also protected by longitudinal separation between aircraft on landing or take-off.
ILS protection is mandatory when low visibility procedures are in force.

Figure 5.6 Critical and sensitive areas (Edinburgh runway 25)

5.8 RVR
5.8.1 RVR MEASUREMENTS
The RVR measurements are provided by a system of calibrated transmissometers and take account of the effects of ambient background light and the intensity of runway lights.
Description of RVR measurement system
RVR measurement system includes:
.  One or more transmissometer(s): A transmissometer is a system providing the value of the opaqueness of the atmosphere on a reference distance through the ratio between the transmitted light flux and the received light flux.
There are two types of transmissometer commonly used as shown Figure 5.8.
5.8.2 LOCATION OF THE TRANSMISSOMETERS
The RVR measurements are basically provided for three parts of the runway.
. the touch-down zone (TDZ),
. the mid runway portion (MID),
. the roll-out portion or stop-end.

The required number of measurements depends on the type of operations.
Transmissometers should be placed at each zone in which they are intended to provide an RVR measurement. The location is supervised by the technical services of the authority. The transmissometer must be close enough to the runway to provide an acceptable value, but at the same time must be a non-hazardous obstacle for the aircraft. Generally, each transmissometer is at a distance of between 110m and 150m from the runway centerline. Moreover, to be representative of the vision of a pilot on the runway (see RVR definition paragraph 2.4.1), the transmissometer is installed at a height of between 5m and 10m above the ground. An example of the locations of two transmissometers (TDZ,MID) are shown in Figure 5.7.
　
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