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Note about automatic landing: Automatic landing is not CAT III. An automatic landing system is only equipment providing automatic control of the aircraft during the approach and landing and is not related to particular weather conditions. This system is mandatory for all CAT III operations. However, it is a common practice to perform automatic landing in good visibility but in that case, the ILS performance must be sufficient and ILS signals protected (see Chapter 5).
As for CAT II operations, the desired level of safety is achieved with more stringent requirements.
CAT II / CAT III DEFINITIONS ACCORDING
TO ICAO, FAA, and JAA

ICAO  FAA  JAA 
CAT II  DH  100ft ≤ DH < 200ft  100ft ≤ DH < 200ft  100ft ≤ DH < 200ft 
RVR  350m ≤ RVR 1200ft ≤ RVR  350m ≤ RVR < 800m 1200ft ≤ RVR < 2400ft  300m ≤ RVR 1000ft ≤ RVR 
CAT III A  DH  No DH or DH < 100ft (1)  No DH or DH < 100ft (1)  DH < 100ft (1) 
RVR  200m ≤ RVR 700ft ≤ RVR  200m ≤ RVR 700ft ≤ RVR  200m ≤ RVR 700ft ≤ RVR 
CAT III B  DH  No DH or DH < 50ft  No DH or DH < 50ft  No DH or DH < 50ft 
RVR  50m ≤ RVR < 200m 150ft ≤ RVR < 700ft  50m ≤ RVR < 200m 150ft ≤ RVR < 700ft  75m ≤ RVR < 200m 250ft ≤ RVR < 700ft 
CAT III C  DH  No DH  No DH 
RVR  No RVR limitation  No RVR limitation 

(1)  DH ≥ 50ft if fail passive 
Acceptable operational correspondance meter/feet (according to ICAO) 
15m = 100m = 300m = 600m = 1600m =  50ft 300ft 1000ft 2000ft 5000ft  30m = 150m = 350m = 800m =  100ft 500ft 1200ft 2400ft  50m = 175m = 500m = 1000m =  150ft 600ft 1600ft 3000ft  75m = 200m = 550m = 1200m =  250ft 700ft 1800ft 4000ft 

2.3 DECISION HEIGHT AND ALERT HEIGHT
In CAT II / CAT III regulations, two different heights are defined:
. the Decision Height (DH),
. the Alert Height (AH).

2.3.1 DECISION HEIGHT DEFINITION
Decision height is the wheel height above the runway elevation by which a go-around must be initiated unless adequate visual reference has been established and the aircraft position and approach path have been assessed as satisfactory to continue the approach and landing in safety (JAA).
There are no significant differences in DH definitions according to other regulations.
In this definition, runway elevation means the elevation of the highest point in thetouchdown zone. According to the JAA, the DH recognition must be by means of height measured by radio-altimeter. But the FAA leaves the choice for CAT II DH recognition (radio-altimeter, inner markers or barometric altimeter).
Visual references at DH
Because the term of adequate visual reference could be differently interpreted, JAAhas defined criteria for CAT II and CAT III for visual reference at DH which are now commonly accepted.
　
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