INSTRUMENT LANDING SYSTEM
The instrument landing system (ILS) was developed as a result of the need for a method whereby a pilot flying an airplane could locate and fly to an airport even when visibility was very poor. The systems now being operated by the FAA are the result of development work that has been conducted since 1928. The present system, except for the glideslope, was demonstrated in a commercial form in Indianapolis in 1939 and 1940. Commercial use of the ILS was greatly delayed by the war, however, because of manpower shortages, it was not possible to obtain the necessary equipment.
The ILS provides a horizontal directional reference and a vertical reference called the glideslope. The directional reference signal is produced by the runway localizer transmitter installed approximately 1000 ft [305 m] from the end of the runway and operating at frequencies of 108 to 112 MHz. The glideslope signal is produced by the glideslope transmitter, which is located near the side of the runway on a line perpendicular to the runway centerline at the point where airplane touchdown occurs. This point is generally about 15 percent of the runway length from the approach end of the runway. The glideslope transmitter operates on a frequency of 328.6 to 335.4 MHz.