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I want to know the characteristics of the different categories of Instrument Landing System (ILS): CAT I, CAT II, CAT IIIa, CAT IIIb and CAT IIIc. What are the differences between them?

(This question only covers CAT III).

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The category specs are easily found on Wikipedia. Here is a concise view of the ILS approach minimums, as far as the crew is concerned.

Cat I

  • DH: 200 ft (61 m) or more
  • RVR: 1,800 ft (550 m); at some airports 1,210 ft (370 m) is approved. For single crew operations, increased to 2,600 ft (790 m) or
    VISIBILITY 800 m (2,600 ft)

Cat II

  • DH: less than 200 ft and more than 100 ft (30 m)
  • RVR: 1,000 feet (300 m)

Cat IIIa

  • DH: less than 100 ft and more than 50 ft (15 m)
  • RVR: 600 feet (180 m)

Cat IIIb

  • DH: less than 50 ft (15 m) or none
  • RVR: 150 feet (46 m)

Cat IIIc

  • DH: No limitations
  • RVR: None

They are based on decision height and RVR (or visibility for cat I). Note that there are no public transport aircraft certified for Cat IIIc as there is no use for landing without visibility and then staying on the runway because you cannot visually taxi to the gate.

The requirement for equipment certification accuracy itself can be found here.

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