Microwave landing system (MLS)

(navigation) A system of ground equipment which generates guidance beams at microwave frequencies for guiding aircraft to landings; it is intended to replace the present lower-frequency instrument landing system. Abbreviated MLS.

An aircraft landing-guidance system that operates at microwave frequencies and provides deviations from the landing runway centerline using time-referenced scanning beam (TRSB) technology. The MLS was standardized in 1988 and approved for use in international civil aviation. The instrument landing system (ILS) is also standardized internationally and approved for use. Standards for a third landing system, based on Global Positioning System (GPS) technology, are expected. See also Instrument landing system (ILS); Satellite navigation systems.

The operating frequencies for MLS lie in a portion of the C-band (5030–5091 MHz) designated for use in aeronautical telecommunications. This frequency choice allows a 12-ft (3.6-m) antenna to generate the 1° beamwidth pattern needed to exclude most reflections.

As with ILS, the MLS equipment is sited near the primary runway, with the azimuth transmitter and distance-measuring equipment (DME) transponder located near the runway stop end, and the elevation transmitter located alongside the runway near landing threshold. With this geometry, the approach course and glide path, generated by the ground equipment, are monitored at the landing runway. Also, the aircraft lateral and vertical displacements due to guidance errors become vanishingly small as the runway is approached and the angular guidance converges to its origin. Unlike ILS, the 50-times higher frequency of the MLS allows generation of narrow beams by relatively small equipment. Because of this 50:1 scale factor, a 1° beamwidth antenna for MLS requires a 12-ft (3.6-m) antenna, while for ILS a 600-ft (180-m) antenna would be required. See also Antenna (electromagnetism); Distance-measuring equipment.

The large coverage volume of MLS is provided by scanning the narrow beams clockwise then counterclockwise for azimuth functions and up then down for elevation functions. This scanning is electronically controlled at a precise rate of 20,000°/s and fills a lateral sector of 60° (maximum) on each side of the runway center line and a vertical sector of 30° (maximum). The angular position of the aircraft is decoded by the airborne receiver, which measures the time elapsed between successive passages of azimuth or elevation beams.

The antennas typically used are phased arrays where beam scanning is accomplished by a stored set of commands which, at the appropriate time in the transmission sequence, are directed to variable signal-delay devices (phase shifters) associated with each radiating element of the array. See also Air navigation; Electronic navigation systems.