The ILS is the ICAO approach international standard electronic landing aid and is installed at nearly all major aerodromes.
The ILS functions with equipment in the aircraft and on the ground. On the aerodrome a localizer transmits radio beam and another transmitter provides a glidepath at a fixed angle of approach. On the aircraft instrument panel an ILS indicator shows the aircraft's position in relation to the centre line of the runway and to the glidepath. The ILS can, therefore, guide the aircraft along the proper approach path down to a point where the pilot must be able to see the ground and be able to continue his approach to land. If he cannot see the ground at the point, he must decide to overshoot, go around and try to land again.
In addition to the ILS localizer and glidepath, an ILS installation also comprises two or three fan markers: one called the outer marker is situated from four to six miles from the threshold of the runway; a second called the middle marker is situated approximately half a mile from the threshold of the runway, and the third, called the inner marker (installed only when required) is about 1,000 feet from the threshold of the runway. These fan markers alert the pilot of his passing over these markers by causing a light on the aircraft instrument panel to flash on and off while he is over each marker.