Microwave Landing System (MLS)

The Microwave Landing System (MLS) is a ground-based radio navigation aid that revolutionized precision approach and landing operations for aircraft. Operating in the 5 GHz microwave band, MLS was designed to overcome the limitations of legacy Instrument Landing Systems (ILS), offering enhanced flexibility, broader angular coverage, and higher resistance to signal reflections and interference. This makes MLS especially suitable for complex airport environments needing multiple, flexible approach paths—including straight-in, offset, curved, and segmented procedures.

Standardized by the International Civil Aviation Organization (ICAO) under Annex 10, MLS was initially intended to replace ILS globally, particularly at airports where ILS siting was impractical due to terrain, obstacles, or urban development. While satellite-based navigation systems have since eclipsed MLS in widespread use, the technical innovations and operational concepts of MLS remain foundational in modern navigation system development.

MLS Ground Segment

MLS ground infrastructure is composed of high-precision microwave transmitters and supporting equipment, strategically sited to optimize coverage and flexibility for a given runway. The modular design allows for adaptation to specific airport layouts, and optional transmitters can be added for specialized operations.

MLS transmitters operate in the 5031–5091 MHz microwave band, with up to 200 discrete channels available for approach guidance and data. DME/P (Distance Measuring Equipment, Precision) operates in the 962–1105 MHz UHF band. All transmitters use time division multiplexing (TDM) to broadcast both angular guidance and operational data, ensuring efficient spectrum utilization and system integrity.

ICAO requires each MLS installation to transmit a unique four-letter Morse code identifier beginning with “M” at least six times per minute, ensuring positive system identification.

Siting flexibility is one of MLS’s signature strengths. Unlike ILS, which demands clear and precise antenna placement, MLS transmitters can be positioned to accommodate airport constraints, terrain, and development, while still maintaining broad coverage—typically at least ±40° (expandable to ±60°) from the runway centerline, up to 15° in elevation, and coverage to over 20 nautical miles.

MLS Airborne Segment

Onboard the aircraft, MLS capability is delivered through a suite of specialized avionics:

• MLS Receiver: Decodes azimuth, elevation, and data signals from the ground.
• MLS Antennas: Typically nose or belly mounted for optimal microwave reception.
• DME Interrogator: Transmits pulse-pair signals and measures round-trip delay for precise range.
• MLS Indicator/Display: Presents real-time guidance on course deviation, glide path, and distance, integrated with cockpit displays (EFIS, MFD, or dedicated indicators).
• FMS Integration: Modern aircraft may integrate MLS directly with the Flight Management System, enabling automated execution of complex approaches and reduced pilot workload.

Rigorous safety standards (RTCA DO-178C for software, DO-254 for hardware) govern all MLS avionics. Redundancy, real-time diagnostics, and cross-verification with other navigation sources (e.g., GPS, inertial systems) are standard, ensuring resilient and fail-safe operation.

Operational Workflow

During an MLS approach, the crew selects the desired channel, verifies the Morse identifier, and the MLS receiver continuously decodes lateral, vertical, and range guidance, displaying it in real time. For complex approach types, the FMS uses MLS data to guide the autopilot or flight director along pre-programmed curved or segmented paths.

Azimuth Transmitter (AZ)

The MLS Azimuth Transmitter is the primary source of lateral guidance, using a precisely controlled, scanning microwave beam to define the approach corridor.

• Coverage: At least ±40°, extendable to ±60° from runway centerline, up to 15° in elevation, and >20 NM in range.
• Location: Typically 1,000 feet beyond runway end, with flexible siting to suit airport constraints.
• Signal: Time-multiplexed beam alternates with data bursts; unique Morse code identifier included.

Compared to ILS localizer antennas, the AZ transmitter is less sensitive to ground reflections and environmental obstructions, providing reliable guidance even in complex airport environments.

Elevation Transmitter (EL)

The Elevation Transmitter supplies vertical glide path guidance via a scanning beam, defining the optimal descent angle for approach.

• Coverage: Up to 15° above horizontal, lateral coverage matching AZ transmitter.
• Location: ~400 feet lateral from the threshold, placement adjustable as needed.
• Function: Supports selectable glide slopes (commonly 2.5°–3.5°, up to 15° for special approaches).

The scanning beam technique allows aircraft to precisely measure vertical deviation, supporting both standard and customized approach profiles for terrain, obstacle, or noise abatement needs.

Distance Measuring Equipment (DME/P)

DME/P provides highly accurate slant range distance, essential for three-dimensional approach guidance.

• Operation: The aircraft sends pulse-pair interrogations; the ground station replies. The airborne system measures round-trip delay for real-time distance.
• Accuracy: Within ±100 feet (30 meters), compliant with ICAO precision standards.
• Integration: DME/P is paired to the MLS channel and co-located with the AZ transmitter.

DME/P range is crucial for identifying approach fixes, step-down points, and missed approach locations, and supports the execution of complex, distance-defined procedures.

Back Azimuth Transmitter

An optional component, the Back Azimuth Transmitter provides reciprocal lateral guidance for missed approaches or departures in the opposite runway direction. Using the same scanning beam and TDM techniques as the primary AZ, it is sited at the far end of the runway or another strategic point to ensure continuous, high-integrity guidance for both arrivals and departures.

Auxiliary Data Transmitters

These optional transmitters broadcast supplementary data—such as real-time weather, runway status, and operational advisories—on the MLS data channel. Pilots receive and view this information on cockpit displays, enhancing situational awareness and reducing reliance on separate voice or datalink communications.

MLS Scanning Beam Principle

MLS achieves precision by employing a scanning beam technique for both azimuth and elevation transmitters. Each transmitter sweeps a narrow, high-frequency microwave beam across its sector at a constant rate. The airborne receiver detects the timing of the “TO” and “FROM” lobes as they pass, and calculates its angular position from the time interval—delivering highly accurate, interference-resistant guidance.

• Advantages: Wide angular coverage, precision unaffected by multipath interference, and simultaneous transmission of guidance and data.

Time Division Multiplexing (TDM) in MLS

MLS uses Time Division Multiplexing to transmit azimuth, elevation, range, and data over a single frequency channel. Each function is allocated a specific time slot, eliminating interference and enabling up to 200 discrete channels per airport. This innovation allows MLS to provide robust, simultaneous guidance and information services, even in dense operational environments.

MLS Guidance Types

Approach Azimuth Guidance

Precise lateral alignment, supporting straight-in, offset, and curved approaches—even simultaneous parallel operations at complex airports.

Elevation Guidance

Vertical descent path definition with selectable glide slopes from standard (2.5–3.5°) to steep (up to 15°), accommodating terrain, obstacles, or noise restrictions.

Range Guidance

Continuous, precision slant range (distance) from DME/P, essential for identifying fixes, step-down points, and missed approach locations.

Data Communications

Digital messages embedded in the MLS signal transmit station identification, operational status, and optional weather/runway data in real time.

Back Azimuth Guidance

Precise lateral guidance for reciprocal (opposite direction) approaches, departures, and missed approaches.

MLS Approach Types and Operational Flexibility

• Straight-In Approaches: Aligned with runway centerline, like ILS but with wider, more robust coverage.
• Offset Approaches: Guidance for paths offset from the centerline to avoid obstacles or sensitive areas; made possible by wide azimuth sector.
• Curved/Segmented Approaches: MLS supports non-linear paths, including segmented and continuous curves, programmed into the FMS for advanced arrival routing.

MLS vs. ILS

Historical Context and Current Status

Despite its technical superiority and flexibility, MLS was overtaken by advances in satellite-based navigation (e.g., GPS, WAAS, GBAS), which provide global coverage and require less ground infrastructure. Most civil MLS installations have been decommissioned, but the system remains relevant in military and specialized roles, and its innovations underpin the evolution of modern precision approach technologies.

Summary

The Microwave Landing System (MLS) was a leap forward in ground-based precision navigation, offering robust, flexible, and interference-resistant guidance for aircraft approach and landing. Its legacy endures in today’s satellite-based systems, and its technical contributions continue to shape the future of aviation navigation.

References

• ICAO Annex 10, Volume I – Aeronautical Telecommunications, Radio Navigation Aids
• FAA Order 8200.1 – United States Standard for Terminal Instrument Procedures (TERPS)
• RTCA DO-178C (Software), DO-254 (Hardware)
• Eurocontrol – The Microwave Landing System (MLS): Principles, Implementation and Transition

See Also

• Instrument Landing System (ILS)
• Global Navigation Satellite System (GNSS)
• Ground-Based Augmentation System (GBAS)
• Distance Measuring Equipment (DME)
• Flight Management System (FMS)