Microwave Landing System


  • The Microwave Landing System (MLS) provides precision navigation guidance for exact alignment and descent of aircraft on approach to a runway. It provides azimuth, elevation, and distance
  • Both lateral and vertical guidance may be displayed on conventional course deviation indicators or incorporated into multipurpose cockpit displays. Range information can be displayed by conventional DME indicators and also incorporated into multipurpose displays
  • The MLS supplements the ILS as the standard landing system in the U.S. for civil, military, and international civil aviation. At international airports, ILS service is protected to 2010
  • The system may be divided into five functions:
    • Approach azimuth;
    • Back azimuth;
    • Approach elevation;
    • Range; and
    • Data communications
  • The standard configuration of MLS ground equipment includes:
    • An azimuth station to perform functions (a) and (e) above. In addition to providing azimuth navigation guidance, the station transmits basic data which consists of information associated directly with the operation of the landing system, as well as advisory data on the performance of the ground equipment
    • An elevation station to perform function (c)
    • Distance Measuring Equipment (DME) to perform range guidance, both standard DME (DME/N) and precision DME (DME/P)
  • MLS Expansion Capabilities:
    • The standard configuration can be expanded by adding one or more of the following functions or characteristics
      • Back azimuth:
        • Provides lateral guidance for missed approach and departure navigation
      • Auxiliary data transmissions:
        • Provides additional data, including refined airborne positioning, meteorological information, runway status, and other supplementary information
        • Expanded Service Volume (ESV) proportional guidance to 60 degrees
  • MLS identification is a four-letter designation starting with the letter M
    • It is transmitted in International Morse Code at least six times per minute by the approach azimuth (and back azimuth) ground equipment

Approach Azimuth Guidance:

  • The azimuth station transmits MLS angle and data on one of 200 channels within the frequency range of 5031 to 5091 MHz
  • The equipment is normally located about 1,000 feet beyond the stop end of the runway, but there is considerable flexibility in selecting sites. For example, for heliport operations the azimuth transmitter can be collocated with the elevation transmitter
  • The azimuth coverage extends:
    • Laterally, at least 40° on either side of the runway centerline in a standard configuration,
    • In elevation, up to an angle of 15° and to at least 20,000 feet, and
    • In range, to at least 20 NM

Elevation Guidance:

  • The elevation station transmits signals on the same frequency as the azimuth station. A single frequency is time-shared between angle and data functions
  • The elevation transmitter is normally located about 400 feet from the side of the runway between runway threshold and the touchdown zone
  • Elevation coverage is provided in the same airspace as the azimuth guidance signals:
    • In elevation, to at least +15 degrees;
    • Laterally, to fill the Azimuth lateral coverage; and
    • In range, to at least 20 NM

Range Guidance:

  • The MLS Precision Distance Measuring Equipment (DME/P) functions the same as the navigation Distance Measuring Equipment (DME), but there are some technical differences. The beacon transponder operates in the frequency band 962 to 1105 MHz and responds to an aircraft interrogator. The MLS DME/P accuracy is improved to be consistent with the accuracy provided by the MLS azimuth and elevation stations
  • A DME/P channel is paired with the azimuth and elevation channel. A complete listing of the 200 paired channels of the DME/P with the angle functions is contained in FAA Standard 022 (MLS Interoperability and Performance Requirements)
  • The DME/N or DME/P is an integral part of the MLS and is installed at all MLS facilities unless a waiver is obtained. This occurs infrequently and only at outlying, low density airports where marker beacons or compass locators are already in place

Data Communications:

  • The data transmission can include both the basic and auxiliary data words. All MLS facilities transmit basic data. Where needed, auxiliary data can be transmitted
  • Coverage limits. MLS data are transmitted throughout the azimuth (and back azimuth when provided) coverage sectors
  • Basic data content. Representative data include:
    • Station identification;
    • Exact locations of azimuth, elevation and DME/P stations (for MLS receiver processing functions);
    • Ground equipment performance level; and
    • DME/P channel and status
  • Auxiliary data content: Representative data include:
    • 3-D locations of MLS equipment;
    • Waypoint coordinates;
    • Runway conditions; and
    • Weather (e.g., RVR, ceiling, altimeter setting, wind, wake vortex, wind shear)
  • Operational Flexibility:
    • The MLS has the capability to fulfill a variety of needs in the approach, landing, missed approach and departure phases of flight. For example:
      • Curved and segmented approaches;
      • Selectable glide path angles;
      • Accurate 3-D positioning of the aircraft in space; and
      • The establishment of boundaries to ensure clearance from obstructions in the terminal area
    • While many of these capabilities are available to any MLS-equipped aircraft, the more sophisticated capabilities (such as curved and segmented approaches) are dependent upon the particular capabilities of the airborne equipment
    • Accuracy: The MLS provides precision three-dimensional navigation guidance accurate enough for all approach and landing maneuvers
    • Coverage: Accuracy is consistent throughout the coverage volumes
    • Environment: The system has low susceptibility to interference from weather conditions and airport ground traffic
    • Channels: MLS has 200 channels, enough for any foreseeable need
    • Data: The MLS transmits ground-air data messages associated with the systems operation
    • Range information: Continuous range information is provided with an accuracy of about 100'


  • Pilots should be aware of the possibility of momentary erroneous indications on cockpit displays when the primary signal generator for a ground-based navigational transmitter is inoperative
    • Pilots should disregard any navigation indication, regardless of its apparent validity, if the particular transmitter was identified by NOTAM or otherwise as unusable or inoperative
  • Remember, the FAA requests user reports on NAVAID outages
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