Microwave Landing System (MLS)

Wg Cdr Rajagopal

Microwave Landing System

Index

Introduction to Microwave Landing System (MLS)

  • MLS is an economical precision landing aid in SHF band frequencies between 5031.0 to 5090.7 MHz with a built in DME
  • Emission designator for MLS is N0X that indicates modulated carrier and data transmission
  • MLS includes Azimuth guidance, Elevation guidance, Distance measuring equipment, Missed approach system and Flare sub systems
  • MLS can be used in hilly terrain due to lack of site errors
  • MLS operates up to 200 channels in SHF band the system is not affected by civilian FM transmissions
  • Flexible approach direction and glide-path angle makes it suitable for all aircrafts including helicopters
  • MLS is capable of transmuting auxiliary information like weather and runway condition

Principle of MLS

  • MLS employs Time Referenced Scanning Beam (TRSB) and Time Division Multiplexing (TDM) techniques
  • Azimuth scanning beam scans from left to right at 13.5 scans per second provides horizontal position of aircraft
  • Elevation scanning beam scans from top to button at 40.5 scans per second provides vertical position of aircraft
  • Time difference between To and From scans in both azimuth and elevation scans provide position information
  • Time Division Multiplexing (TDM) technique enables the MLS to use the same frequency for all sub-systems
  • Distance from threshold is provided by a frequency paired precision DME

Coverage of MLS

  • MLS coverage extends up to 20 nm and 20,000 ft in both dimensions
  • Azimuth coverage extends to 40 deg either side of runway
  • Elevation coverage extends from 0.9 to 20 degrees from the horizontal
  • Offset approach provides guidance to aircraft even from 2 NM from touch down
  • Back beam coverage extends up to 10 NM and 10,000 ft
  • Azimuth coverage extends to 20 deg either side of runway
  • Elevation coverage extends from 0.9 to 15 degrees from the horizontal
  • Back beams of MLS can be used for missed approach as well as departures

Accuracy of MLS

  • MLS displays course, height and distance of aircraft with respect to any selected approach path
  • ICAO recommended accuracy for MLS is 13.5 ft at the datum reference which is 50 ft above the runaway threshold
  • Although, the maximum permissible error is 27 feet at its maximum range of 20 NM but normally the error does not exceed 20 ft
  • Error in vertical is limited to 2 feet at datum reference and maximum permissible error is 0.2 degrees on a 3 degree glide path

Best of luck