When the aircraft is on the center-line, the CDI vertical needle will be centered. A 150-Hz signal will drive the CDI vertical needle to the left when the aircraft is right of the centerline course. A 90-Hz signal detected by the aircraft receiver will cause the course deviation indicator (CDI) to deviate to the right. Marker beacons operating along the extension of the runway centerline at 75 MHz are used to indicate decision height points for the approach or distance to the threshold of the runway.Īzimuth guidance provided by the localizer is accomplished by use of a 90-Hz-modulated left-hand antenna pattern and a 150-Hz-modulated right-hand pattern as viewed from the aircraft on an approach. The glide slope is normally positioned 1000 feet after the approach end of the runway and operates in the 328.6–335.4 MHz band. The localizer, operating in the 108–112 MHz band, is normally located 1000 feet beyond the stop end of the runway. However, approval of GPS as the sole means of navigation for precision landing has not yet occurred, and ILS remains in widespread use as the system of choice.Īn ILS normally consists of two or three marker beacons, a localizer, and a glide slope to provide vertical and horizontal guidance information. However, in the United States, development of the Microwave Landing System was deferred in most cases in favor of the long-term adoption of GPS as a precision landing system. An alternate system, the microwave landing system (MLS) was scheduled to replace the ILS by the year 2000. Jablonski, in Reference Data for Engineers (Ninth Edition), 2002 ILS (Instrument Landing System)Īt present, the instrument landing system operating in the 108–112 MHz band is the primary worldwide, ICAO approved precision landing system. Signal standards for ILS are established internationally, and about 1500 ILS's are operational at airports throughout the world. In the ILS airborne equipment, a Morse-Code identification signal is received audibly in the cockpit on the localizer band, and a voice transmission from the airport's control tower may also be provided.
In “auto land” systems, such deviation signals are “hard-wired” to the AFCS (see Section IV). The receiver, then, displays information to the pilot that only is “nulled” when the aircraft is “on course” to landing, and these information signals grow with the level of deviation to either side or above/below the proper course. Their radiation patterns in space and in specific frequency channels provide signals to an aircraft receiver indicating deviations from the desired height as a function of range from the end of, and lateral displacement from the centerline of, a particular runway. What is known as the Instrument Landing System (ILS) consists of (1) “localizer” transmitters, located at the centerline of and off the ends of runways, which provide lateral guidance to aircraft approaching to land (2) “glide slope” transmitters located beside runways near the end of the runway over which the aircraft first passes in landing (the “threshold”), which provide vertical guidance and (3) marker beacons reporting progress along the glide-path to the pilot of the landing aircraft.Īll three, localizer, glide slope, and marker beacon transmitters, radiate continuous wave EM energy at radio frequencies. Loewy, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 III.F Instrument Landing Systems