In the United States, there are no plans for widespread deployment of MLS. However, the advent of GPS has placed many of the plans for deployment of MLS on hold. The microwave landing system is an ICAO-approved replacement for the current ILS system. Jablonski, in Reference Data for Engineers (Ninth Edition), 2002 MLS (Microwave 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 Then, the aircraft positions are adjusted accordingly to bring it back to the positions of exact cancellation ( Parkinson et al. Once the aircraft deviates from this path, one of the components exceeds the other and there appears a nontrivial resultant value of the difference signal. The pair of signals is spatially separated in such a way that exact cancellations of the modulating signal happen only along the recommended path of the movement of the aircraft.
This is done by comparing the modulation depth of the modulating signals within a particular pair through differencing. This signal is received by ILS receivers in the aircraft and is interpreted to obtain guidance information. Figure 1.9(a) and (b) illustrates the condition. A similar pair of signals is available in both lateral and vertical directions. Guidance is provided by transmitting a pair of amplitude-modulated signals from two spatially separated transmitters. The second, which gives vertical guidance, and hence restricts vertical deviation of the aircraft from the recommended path of descent, is called the glide slope or glide path. The first, which provides lateral guidance, restricting the aircraft approaching a runway to shift laterally from the recommended path, is called the localizer.
Thus, an ILS consists of two independent subsystems.
The ILS provides the aircraft with a recommended path it should follow so that it maintains its horizontal position at the center of the runway and the vertical position most appropriate for a smooth landing.
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