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Navigation Systems - Level 3

Microwave Landing System (MLS)

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The time-referenced scanning beam Microwave Landing System (MLS) has been adopted by ICAO as the standard precision approach system to replace ILS. MLS provides precision navigation guidance for alignment and descent of aircraft on approach to a landing by providing azimuth, elevation and distance. The system may be divided into five functions:

  1. Approach azimuth;
  2. Back azimuth;
  3. Approach elevation;
  4. Range; and
  5. Data communications.

With the exception of DME, all MLS signals are transmitted on a single frequency through time sharing. Two hundred channels are available between 5031 and 5090.6 Megahertz (MHz). By transmitting a narrow beam which sweeps across the coverage area at a fixed scan rate, both azimuth and elevation may be calculated by an airborne receiver which measures the time interval between sweeps. For the pilot, the MLS presentation will be similar to ILS with the use of a standard CDI or multi-function display,


The Instrument Landing System (ILS) has served as the standard precision approach and landing aid for the last 40 years. During this time it has served well and has undergone a number of improvements to increase its performance and reliability. However, in relation to future aviation requirements, the ILS has a number of basic limitations:

  1. site sensitivity and high installation costs;
  2. single approach path;
  3. multi path interference; and
  4. channel limitations - 40 channels only.


As previously mentioned, ILS has limitations which prohibit or restrict its use in many circumstances. MLS not only eliminates these problems; but also offers many advantages over ILS including:

  1. elimination of ILS/FM broadcast interference problems;
  2. 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);
  3. capability to provide precision guidance to small landing areas such as roof-top heliports;
  4. continuous availability of a wide range of glide paths to accommodate STOL and VTOL aircraft and helicopters;
  5. accommodation of both segments and curved approaches;
  6. availability of 200 channels - five times more than ILS;
  7. potential reduction of Category I (CAT l) minimums;
  8. improved guidance quality with fewer flight path corrections required;
  9. provision of back-azimuth for missed approaches and departure guidance;
  10. elimination of service interruptions caused by snow accumulation; and
  11. lower site preparation, repair, and maintenance costs.


The approach azimuth antenna normally provides a lateral coverage of 40 either side of the center of scan (see MLS Azimuth and Elevation Coverage figure, below). Coverage is reliable to a minimum of 20 NM from the runway threshold and to a height of 20,000 feet (ft). The antenna is normally located about 1000 feet beyond the end of the runway.


The back azimuth antenna provides lateral guidance for missed approach and departure navigation. The back azimuth transmitter is essentially the same as the approach azimuth transmitter. However, the equipment transmits at a somewhat lower data rate because the guidance accuracy requirements are not as stringent as for the landing approach. The equipment operates on the same frequency as the approach azimuth bur at a different time in the transmission sequence. On runways that have MLS approaches on both ends, the azimuth equipment can be switched in their operation from the approach azimuth to the back azimuth and vice versa. The MLS Azimuth and Elevation Coverage figure, below, shows MLS azimuth coverage volumes.

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The elevation station transmits signals on the same frequency as the azimuth station. The elevation transmitter is normally located about 400 ft from the side of the runway between the threshold and the touchdown zone. The MLS Azimuth and Elevation Coverage figure, above, shows coverage volumes for the MLS elevation signal. It allows for a wide range of glide path angles selectable by the pilot.


Range guidance, consistent with the accuracy provided by the azimuth and elevation stations, is provided by the MLS precision DME (DME/P). DME/P has an accuracy of +100 ft as compared, with + 1200 ft accuracy of the standard. DME system. In the future it may be necessary to deploy DME/P with modes which could be incompatible with standard airborne DME receivers.


The azimuth ground station includes data transmission in its signal format which includes both basic and auxiliary data. Basic data may include approach azimuth track and minimum glide path angle. Auxiliary data may include additional approach information such as runway condition, wind-shear or weather.

The basis for this section is the Flight Training Manual by Transport Canada.   However, the text was modified for US users and readers by Dr. Claudius Carnegie of the ALLSTAR website.  Tables and/or pictures were unchanged.  Any questions should be directed to Dr. Carnegie at directly.

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Updated: March 12, 2004