MICROWAVE LANDING SYSTEM (MLS) Prepared by: Ranon, Johnatan Noveleon G. ECE 52 • all-weather, precision landing system originally intended to replace or supplement the Instrument Landing System (ILS) • provides azimuth, elevation, and distance, as well as “back azimuth“ • used for short-range communications with airport controllers Principle MLS employs 5GHz transmitters at the landing place which use passive electronically scanned rays to send scanning beams towards approaching aircraft.
An aircraft that enters the scanned volume uses a special receiver that calculates its position by measuring the arrival times of the beams. Operational Advantages ? wide selection of channels to avoid interference with other nearby airports weather performance in all ? excellent ? small “footprint” at the airports Advantages over ILS • antennas were much smaller • did not have to be placed at a specific point at the airport • signals covered a very wide fanshaped area off the end of the runway • uses a single frequency, broadcasting the azimuth and altitude information one after the other elimination of ILS/FM broadcast interference problems • provision of ail-weather coverage up to ±60 degrees from runway centerline, from 0. 9 degree to 15 degrees in elevation, and out of 20 nautical miles (NM) • capability to provide precision guidance to small landing areas such as roof-top heliports • continuous availability of a wide range of glide paths to accommodate STOL and VTOL aircraft and helicopters • accommodation of both segments and curved approaches; • availability of 200 channels – five times more than ILS • potential reduction of Category I (CAT l) minimums improved guidance quality with fewer flight path corrections required • provision of back-azimuth for missed approaches and departure guidance • elimination of service interruptions caused by snow accumulation • lower site preparation, repair, and maintenance costs. Functions • • • • • Approach azimuth Back azimuth Approach elevation Range communications Data communications Approach Azimuth ? analogous to ILS localizer but has a proportionally wider coverage ? transmits MLS angle and data ? ocated 1000 ft beyond the end of the runway ? coverage is reliable up to 20 nm from runway threshold and 20000ft of height ? laterally 40° on either side of the runway centerline ? frequency Range of 5031-5091 MHz The azimuth coverage extends: Laterally, at least 40 degrees on either side of the runway centerline in a standard configuration. In elevation, up to an angle of 15 degrees and to at least 20,000 feet (6 km), and in range, to at least 20 nautical miles (37 km) Volumes of the elevation station
Coverage volume of the Azimuth station Back Azimuth ? provides lateral guidance for missed approach and departure navigation ? operates on the same frequency as approach azimuth but at a different time in the transmission sequence An MLS azimuth guidance station with rectangular azimuth scanning antenna with DME antenna at left MLS Azimuth and Elevation Coverage figure, shows MLS azimuth coverage volumes. Approach Elevation ? transmits signals on the same frequency as the azimuth station ? ocated 400 ft from the side of the runway between runway threshold and touchdown zone ? elevation to at least 15° ? An MLS elevation guidance station The MLS Azimuth and Elevation Coverage figure, shows coverage volumes for the MLS elevation signal. It allows for a wide range of glide path angles selectable by the pilot. Range Communications ? provided by the MLS precision Distance Measuring Equipment DME(DME/P) ? provides continuous and highly accurate distance information Data Communications ? can include both the basic and auxiliary data words ?
MLS data are transmitted throughout the azimuth (and back azimuth when provided) coverage sectors ? Representative/Basic data ›Station identification ? four-letter designation starting with the letter M ›Exact locations of azimuth elevation and DME/P stations (for MLS receiver processing functions) ›Ground equipment performance level ›DME/P channel and status ? Auxiliary data content 3-D locations of MLS equipment Waypoint coordinates Runway conditions and Weather (e. g. , RVR, ceiling, altimeter setting, wind, wake vortex, wind shear)