# Microwave Landing System (MLS) in Aviation

> Study microwave landing system concepts, MLS aviation use, and key frequency band references.

- **Subject:** 062 — Radio Navigation
- **Section:** 062-02 — Radio Aids
- **Topic code:** 062-02-06
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/radio-navigation/radio-aids/microwave-landing-system-mls/

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**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

1.  **Azimuth Station:** Provides lateral guidance (±40° from centerline).
2.  **Elevation Station:** Provides vertical guidance (adjustable from 0.9° to 20°).
3.  **DME/P (Precision DME):** A more accurate DME than the standard one, essential for defining 3D paths.
4.  **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.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.