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FAA Category III Instrument Landing System; A Ground Equipment Development Overview

Category 111 Instruænt Systae (ILS) of the conventional 
VHP/üHP type hag been introduced for actual operatim in the 
United States. Further inplæntation of •everal systas 18 
anticipated vithln the near future. The Category 111 ILS 
represents the latest existing ground electronic guidance syste 
designed to best dænds for high dependability during 
aircraft approach and landing In very limited visibility. 
nie continued developent of ILS over the years hag ude it 
possible for pilots during Inclemt weather to an every increasing 
extent depend on the ILS for safe and efficient guidance in airport 
approaches and landings. Categories of ILS operation aa defined 
in the Aeronautical Teleconunicationg Annex 10 to the Convention 
of Internatonal Civil Aviation (ICAO Annex 10) are used to describe 
operation under different visibility conditions. Category 111A 
operation represents at the present the minimam weather conditions 
during which co—ercial operations are possible. The ground equip— 
ment required for Category 111A operation, including Category Ill 
ILS and certain runway lighting aids and RVR equipentg vill permit 
"hands—off" landing by a qualified airczaft crev In forward visibility 
to 700 feet and no decision height. By comparison, u Category Il 
syeta provides landing capability under minima not lover than a 
100 foot decision height and forward visibility down to 1200 feet. 
Although the nominal radiated signal characteristics are the same 
for Category I, Il, or Ill, the difference in the ILS equipment 
itself cetrcsponding to higher categories of service is represented 
by extension of guidance required along the runway Itself and by 
refined accuracy and tighter tolerance capability In the ground 
equipnpnt ag described in the ICAO Annex 10.