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5.5.3 CROSSBAR LIGHTS
The crossbar provided at 150m from the threshold fills in the gap between the centerline and the side row lights. The crossbar provided at 300m is extended on both sides of the centerline lights to a distance of 15m from the centerline. The lights forming the two crossbars are fixed lights showing variable white.
5.6 OBSTACLE CLEARANCE AREA
5.6.1 INTRODUCTION
Due to the very low visibility in CAT II and CAT III operations, each airfield must meet stringent criteria concerning obstacle clearance to avoid any aircraft on approach, landing or go-around touching obstacles on the ground. The basis of those criteria are fully included in ICAO Annex 14 and PANS-OPS Doc 8168 and in other national documents. In CAT II and III operations, two important concepts are often mentioned in the regulations.
. the Obstacle Free Zone (OFZ),
. the Obstacle Clearance Height (OCH).

5.6.2 DEFINITIONS
Provided below are the definitions of the OCH and OFZ as defined in ICAO.
OCA/OCH: The lowest altitude (OCA), or alternatively the lowest height above the elevation of the relevant runway threshold or above the aerodrome elevation as applicable (OCH), used in establishing compliance with appropriate obstacle clearance criteria.
Sometimes, the term OCL (obstacle clearance limit) can be found in the documentation, but this term is being phased out.
When an operator establishes his operating aerodrome minima, he must take into account the OCH only for CAT II. The minimum DH for CAT II is always equal to or higher than any OCH mentioned in the aerodrome chart. This OCH is a function of the category of aircraft (A to E).
OFZ: The airspace above the inner approach surface, inner transitional surfaces, and balked landing surfaces and that portion of the strip bounded by these surfaces, which is not penetrated by any fixed obstacle other than a low-mass and frangibly mounted and required for air transportation purposes.
5.7 ILS FACILITY
5.7.1 DESCRIPTION
Nowaydays, all CAT II and III approaches are based on ILS facilities. The ILS installation must conform to the appropriate specifications contained in ICAO Annex 10, Volume 1, Part 1, Chapters 2 and 3, and be designed and operated in accordance with the guidance material contained in attachments C to part 1 of Annex 10.
There are three categories of ILS, providing guidance down to a height higher or equal to
. 60m(200ft) for CAT I, . 15m(50ft) for CAT II, . runway surface and along the runway for CAT III.
Generally, the authority requires a CAT II ILS facility for the performance of CAT II approaches, and a CAT Ill ILS facility for the performance of CAT III approaches. However, it is acceptable to use a category II ILS facility for the performance of CAT III approaches with the highest minima (for example CAT III A or CAT III with DH not less than 50ft). Generally, a special agreement from the authority should be obtained. Mainly, the authority will take account of the continuity of service objective and the integrity objective of those installations.
　
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