The Instrument Landing System (ILS) is an internationally normalized system for navigation of aircrafts upon the final approach for landing. It was accepted as a standard system by the ICAO, (International Civil Aviation Organization) in 1947.
Since the technical specifications of this system are worldwide prevalent, an aircraft equipped with a board system like the ILS, will reliably cooperate with an ILS ground system on every airport where such system is installed.
The ILS system is nowadays the primary system for instrumental approach for category I.-III-A conditions of operation minimums and it provides the horizontal as well as the vertical guidance necessary for an accurate landing approach in IFR (Instrument Flight Rules) conditions, thus in conditions of limited or reduced visibility.The accurate landing approach is a procedure of permitted descent with the use of navigational equipment coaxial with the trajectory and given information about the angle of descent.
The equipment that provides a pilot instant information about the distance to the point of reach is not a part of the ILS system and therefore is for the discontinuous indication used a set of two or three marker beacons directly integrated into the system. The system of marker beacons can however be complemented for a continuous measurement of distances with the DME system (Distance measuring equipment), while the ground part of this UKV distance meter is located co-operatively with the descent beacon that forms the glide slope. It can also be supplemented with a VOR system by which means the integrated navigational-landing complex ILS/VOR/DME is formed.