Instrument Landing System (ILS) is defined as a precision runway approach aid based on two radio beams which together provide pilots with both vertical and horizontal guidance during an approach to land. An Instrument Landing System is a precision runway approach aid employing two radio beams to provide pilots with vertical and horizontal guidance during the landing approach. The localiser (LOC)provides azimuth guidance, while the glideslope (GS) defines the correct vertical descent profile. Marker beacons and high intensity runways lights may also be provided as aids to the use of an ILS, although the former are more likely nowadays to have been replaced by a DME integral to the ILS or one otherwise located on the aerodrome, for example with a VOR