The microwave landing system (MLS) has been developed to overcome some of the problems and limitations associated with the ILS system. The MLS system currently being deployed is called a time-reference scanning beam microwave landing system, or TRSB. The ILS provides one narrow flight path and operates at VHF/UHF frequencies, whereas the MLS provides a wide range of flexible flight paths on an approach to an airport. In addition, the MLS has the advantages inherent in operating at microwave frequencies (3 to 30 GHz).

Among the benefits of microwave frequencies are a much larger number of frequency channels, fewer problems with finding suitable sites for ground components, and elimination of severe multipath interference caused by signal reflections from buildings, hills, and other objects. With MLS, aircraft can approach a runway from a wide variety of angles rather than being required to be aligned with the runway for many miles on the approach. As explained previously, the ILS transmits signals that, when combined, provide a narrow beam rising from a point on the runway and extending for an indefinite distance along with the approach to a runway. This situation requires that all aircraft approaching a particular runway be “funneled” into the one approach path. With MLS, aircraft can approach the runway from many different angles, thus making it possible to accommodate more flights and shorten flight paths.


The principle of operation for the TRSB microwave landing system may be illustrated as shown in the figure bellow Two transmitters, one for azimuth and one for elevation, transmit fan scanning beams toward approaching aircraft.

The precise timing of the scanning beams provides exact information for the pilot regarding the position of the aircraft. Beams are scanned rapidly “to” and “fro” throughout the area shown in the drawing. In each complete scan cycle, two pulses are received by the aircraft.

One pulse is received during the “to” scan and the other during the “fro” scan. The aircraft receiver derives its position angle directly from the measurement of the time difference between the two pulses. The receiver processor computes the information and prepares it for display on a conventional course deviation indicator (CDI). In addition, a digital display of the information is presented on the control panel.

Distance information for the system is derived from conventional distance-measuring equipment (DME).

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