We have been reading a hell a lot of stuff about Airports in Delhi and northern part of India being affected because of fog and bad weather, making it very difficult to ground the aircraft and need for airlines with CAT III based instruments.
Did some research on CAT III and here I go with my findings:
From the basics of Aviation technology, for a flight to land, it has to take care of the vertical movement(technically lateral guidance provided by a device called localizer) as well as the angle at which it comes down(Vertical guidance provided by glidescope).The communication happens through the concept of frequency modulation taken care by the Localizer and glidescope.
Generally landing is supported by instruments of different categories CAT I,II,III. Classification is based on the decesion height at which the pilot has the freedom to land. To make it simple, let us say the airport is completely covered in fog and only at 50m height, you realise that you are going to collide on the ground. In this case, CAT I or CAT II doesnot help in saving the lives of passengers.You need a CAT III system.
"""Category I - A precision instrument approach and landing with a decision height not lower than 200 feet (61 m) above touchdown zone elevation and with either a visibility not less than 2,625 feet (800 m) or a runway visual range not less than 1,800 feet (550 m). An aircraft equipped with an Enhanced Flight Vision System may, under certain circumstances, continue an approach to CAT II minimums.
Category II - Category II operation: A precision instrument approach and landing with a decision height lower than 200 feet (61 m) above touchdown zone elevation but not lower than 100 feet (30 m), and a runway visual range not less than 1,150 feet (350 m).
Category III is further subdivided
Category III A - A precision instrument approach and landing with:
a) a decision height lower than 100 feet (30 m) above touchdown zone elevation, or no decision height; and
b) a runway visual range not less than 655 feet (200 m).
Category III B - A precision instrument approach and landing with:
a) a decision height lower than 50 feet (15 m) above touchdown zone elevation, or no decision height; and
b) a runway visual range less than 2,625 feet (800 m) but not less than 165 feet (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 the aircraft and can also provide guidance along the runway surface. """" -- excerpts from Wikipedia
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