What is the Microwave Landing System (MLS)?

MLS is a precision approach and landing system using scanning microwave beams to provide accurate azimuth, elevation, and distance guidance to aircraft.

How does MLS differ from ILS?

MLS uses microwave scanning beams and is less affected by terrain and obstacles, while ILS relies on VHF/UHF signals and is more sensitive to siting constraints and signal reflections.

What is the coverage area of the MLS landing system?

MLS provides azimuth coverage 40° left and right of the centerline and elevation from 0.9° to 20°, both out to 20 NM, with DME/P extending range information to 22 NM.

How does MLS enable segmented or curved approaches?

MLS, when integrated with a guidance computer or FMS, allows for programmed segmented or curved approach paths, unlike ILS which is limited to straight-in approaches.

Why is MLS less commonly used today?

MLS adoption was limited due to the rise of satellite-based navigation systems like GPS, leading to the phasing out of MLS installations in most regions.

MLS (Microwave Landing System) Overview

Medium4 min readRadio Navigation

Moderately Examined

Why this matters

Understanding MLS is essential for pilots operating at airports with challenging terrain or complex approach requirements, as it enables safe and precise landings where ILS may not be feasible. Knowledge of MLS also helps pilots interpret avionics displays and select appropriate navigation modes during critical phases of flight.

The Microwave Landing System (MLS) is a precision approach and landing aid that uses scanning microwave beams to provide highly accurate azimuth, elevation, and distance guidance to aircraft. Unlike the traditional ILS, MLS operates in the SHF band and can be installed at locations where terrain or buildings make ILS siting difficult. Its flexibility allows for both straight-in and complex curved approaches, making it especially useful at challenging aerodromes.

Quick Check

What is the main operational advantage of the MLS landing system compared to ILS in challenging terrain or built-up areas?

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In depth

Explanation

Principle of Operation

MLS operates in the Super High Frequency (SHF) band, specifically between 5031 MHz and 5090.7 MHz, offering 200 selectable channels. The system uses separate ground-based transmitters for azimuth (lateral) and elevation (vertical) guidance, each producing scanning beams. These beams sweep across the approach sector, and the aircraft's MLS receiver determines its position by measuring the timing of the received beams. A precision DME (DME/P) is integrated to provide exact distance information, enabling three-dimensional position fixes.

Coverage and Flexibility

MLS provides azimuth coverage 40° left and right of the runway centerline, and elevation coverage from 0.9° to 20° above the horizontal, both out to 20 NM. DME/P extends range information up to 22 NM. This wide coverage and reduced sensitivity to multipath effects allow MLS installations at sites unsuitable for ILS, such as those with significant terrain or obstacles.

Approach Types and Displays

With basic MLS equipment, aircraft can fly straight-in approaches similar to ILS. When paired with advanced avionics or a flight management system, MLS supports segmented and curved approaches, with the flight path displayed using crossbars on standard ADI/HSI or EFIS displays. The system can also provide additional data for automatic landing sequences.

MLS vs ILS and Modern Avionics

MLS offers several advantages over ILS, including more flexible siting, immunity to certain interference, and the ability to support complex approach paths. However, its adoption has been limited, with most regions now favoring satellite-based systems. Modern aircraft are typically equipped with multimode receivers (MMR) that can process MLS, ILS, and GPS signals, ensuring compatibility with all major precision approach systems.

The essentials

Key Points

MLS operates in the SHF band (5031–5090.7 MHz) with 200 channels.

Uses separate azimuth and elevation scanning beams for lateral and vertical guidance.

DME/P provides precise distance information for 3D position fixing.

Coverage is 40° left/right of centerline and 0.9°–20° elevation, up to 20 NM.

MLS is less affected by multipath and terrain interference than ILS.

Supports both straight-in and segmented/curved approaches with appropriate avionics.

Modern aircraft use multimode receivers to process MLS, ILS, and GPS signals.

Watch out

Exam Traps & Typical Mistakes

Confusing MLS coverage limits with those of ILS (MLS is 40° left/right, 0.9°–20° elevation, 20 NM).

Assuming MLS always requires DME/P for basic straight-in approaches (it's needed for 3D fixes and advanced procedures).

Believing MLS uses separate frequencies for azimuth and elevation (it uses multiplexing on a single frequency).

Thinking MLS is widely used today—most installations have been decommissioned in favor of satellite-based systems.

Mixing up the display indications for MLS with those of ILS—MLS can support more complex approach presentations with advanced avionics.

Test yourself

Example Exam Questions

Question 2Easy

Which frequency band does the Microwave Landing System (MLS) operate in, and how many channels are available?

Question 3Medium

Why are modern aircraft equipped with a multimode receiver (MMR)?

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