From Wikipedia
ILS categories
There are three categories of ILS which support similarly named categories of operation.
Category I - A precision instrument approach and landing with a decision height not lower than 60 m (200 ft) above touchdown zone elevation and with either a visibility not less than 800 m or a runway visual range not less than 550 m.
Category II - Category II operation: A precision instrument approach and landing with a decision height lower than 60 m (200 ft) above touchdown zone elevation but not lower than 30 m (100 ft), and a runway visual range not less than 350 m.
Category III is further subdivided
Category III A - A precision instrument approach and landing with: a) a decision height lower than 30 m (100 ft) above touchdown zone elevation, or no decision height; and b) a runway visual range not less than 200 m.
Category III B - A precision instrument approach and landing with: a) a decision height lower than 15 m (50 ft) above touchdown zone elevation, or no decision height; and b) a runway visual range less than 200 m but not less than 50 m.
Category III C - A precision instrument approach and landing with no decision height and no runway visual range limitations. A Category III C system is capable of using an aircraft's autopilot to land and guide the aircraft to the terminal if required.
In each case a suitably equipped aircraft and appropriately qualified crew are required. For example, Cat IIIc requires three autopilots, Cat I does not. Cat I only goes off of altimeter indications for decision height, the Cat II and Cat III approaches go off the radar altimeter for a decision height
(Reference ICAO Annex 10 AERONAUTICAL TELECOMMUNICATIONS Volume 1 RADIO NAVIGATION AIDS 2.1.1)
An ILS is required to shut down upon internal detection of a fault condition as mentioned in the monitoring section. With the increasing categories, ILS equipment is required to shutdown faster since higher categories require shorter response times. For example, a Cat I localizer must shutdown within 10 seconds of detecting a fault, but a Cat III localizer must shutdown in less than 2 seconds. [1]
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