Microwave Landing System (MLS) in Aviation
MLS (Microwave Landing System) is a precision approach system designed to overcome the limitations of ILS and offer greater flexibility. Although technologically superior, its implementation has been limited due to the rise of satellite-based systems (GNSS).
Main Characteristics
- Frequency: Operates in the SHF band (5.031 - 5.091 GHz), with 200 channels available.
- Operating Principle: Uses the Time Reference Scanning Beam (TRSB). A beam sweeps the space laterally (azimuth) and vertically (elevation). The receiver measures the time between the "to" and "fro" sweeps to calculate the angular position.
- Immunity: By operating in SHF and using narrow beams, it is practically immune to reflections from buildings or terrain (multipath), allowing installation in locations where ILS is not viable.
System Components
- Azimuth Station: Provides lateral guidance (±40° from centerline).
- Elevation Station: Provides vertical guidance (adjustable from 0.9° to 20°).
- DME/P (Precision DME): A more accurate DME than the standard one, essential for defining 3D paths.
- Data Communications: Transmits station information to the aircraft.
Advantages over ILS
- Flexible Approaches: Allows curved and segmented paths, improving traffic management and reducing noise.
- Glide Path Selection: The pilot can select the desired glide path angle.
- Installation: Does not require leveling large areas of land in front of the antennas.
- Channels: Greater number of channels to avoid interference between nearby airports.
Airborne Use
- MMR (Multi-Mode Receiver): Modern aircraft use a single receiver capable of processing ILS, MLS, and GPS signals, simplifying the pilot interface.
- Display: Information is presented similarly to ILS (deviation bars), but computed to follow the programmed segment.
- DME/P: Without an operational DME/P, the MLS can only provide straight-in approaches (ILS-like), losing the ability to perform curved or segmented approaches.