electronic guidance system designed to help airline pilots align their planes with the centre of a landing strip during final approach under conditions of poor visibility. The ground equipment of the ILS consists of two directional transmitters that send out radio beams, sometimes of microwave frequencies (i.e., frequencies of more than 1,000 MHz), from either side of the runway's centreline. The radio pulses are picked up by instruments on the plane and then processed and converted into precise directional and altitude information. These data are shown on an instrument display in the form of horizontal and vertical lines, which enable the pilot to determine his exact position in relation to the runway and maneuver his craft into proper alignment with it. The ILS can be tied into a plane's automatic pilot, whereby ground-based instruments guide the plane into position while those on the aircraft control airspeed by means of an automatic throttle. The instrument landing system was introduced in 1929 and was approved and adopted by the International Civil Aviation Organization (q.v.) in 1949.