Instrument Landing System(ILS)

A. GENERAL DESCRIPTION
Instrument landing system (ILS) facilities are a highly accurate and dependable means of navigating to the runway in IFR conditions. When using the ILS, the pilot determines aircraft position primarily by reference to instruments. The ILS consists of:

1. the localizer transmitter;
2. the glide path transmitter;
3. the outer marker (can be replaced by an NDB or other fix);
4. the approach lighting system.

ILS is classified by category in accordance with the capabilities of the ground equipment. Category I ILS provides guidance information down to a decision height (DH) of not less than 200 ft. Improved equipment (airborne and ground) provide for Category II ILS approaches.
A DH of not less than 100 ft. on the radar altimeter is authorized for Category II ILS approaches.
The ILS provides the lateral and vertical guidance necessary to fly a precision approach, where glide slope information is provided. A precision approach is an approved descent procedure using a navigation facility aligned with a runway where glide slope information is given. When all components of the ILS system are available, including the approved approach procedure, the pilot may execute a precision approach.