Microwave Landing System (MLS)

The microwave landing system (MLS) was standardised by ICAO as the successor to the instrument landing system. Instead of forming one fixed course and one glide path, its ground stations sweep narrow microwave beams across wide sectors. The aircraft's receiver works out its own angle from the timing of the beam passing over it, and a precision DME supplies the range. The result is precision guidance throughout a large volume of airspace rather than along a single line.

MLS never displaced the ILS, and satellite guidance with augmentation, SBAS for LPV approaches and GBAS for GLS, now offers much of its flexibility without a dedicated transmitter at each runway (see SBAS and GBAS augmentation). It nevertheless remains in ICAO standards, in the definitions of precision approach categories, in the flight plan equipment codes and in airliner receivers such as the A320's, and it is still examined in ATPL radio navigation.

Why MLS was developed

The ILS has limitations that come from its technique:

• Only 40 channels, which leads to congestion where many airports lie close together.
• One fixed course and glide path. Every aircraft must join the same straight final and descend at the one published angle. A different, steeper path, wanted for noise abatement or needed by short take-off and landing aircraft and helicopters, cannot be offered from the same installation.
• Demanding sites. The glide path is formed with the help of reflection from the ground in front of its antenna, so the site must be flat and clear. Buildings, terrain and other aircraft reflect the signals and bend the course, which is why ILS critical and sensitive areas exist.
• No missed approach guidance, apart from the back course, which some states, the UK among them, do not permit for approaches.

MLS was designed to answer each point: 200 channels, wide coverage, selectable approach angles, frequencies high enough to give narrow beams that are less affected by reflections, so that an installation can be sited even in hilly terrain, and a back-azimuth function for the missed approach and departure.

Frequency and time-division multiplexing

MLS angle guidance works in the SHF band from 5031.0 to 5090.7 MHz, with 200 channels, five times as many as the ILS. The wavelength, about 6 cm, allows narrow beams from antennas of moderate size.

All the angle functions of one installation share a single channel by time-division multiplexing (TDM). Approach azimuth, elevation, back-azimuth and data are transmitted in turn, each in its own time slot, never at the same moment, so they do not interfere. An ILS, by contrast, needs a VHF localiser frequency and a paired UHF glide path frequency. Each MLS channel is also paired with a DME channel, so selecting the MLS selects its DME.

The time-reference scanning beam principle

MLS measures angles by the time-reference scanning beam (TRSB) technique. Each angle station radiates a narrow beam that sweeps at a constant rate across its coverage, first one way, the TO scan, then back, the FRO scan. An aircraft in the coverage receives two pulses as the beam passes over it, one in each direction. The time between them depends on the aircraft's angle from the reference line: the runway centreline for azimuth, the horizontal for elevation. The receiver measures the interval and converts it directly into an angle.

This differs fundamentally from the ILS, where the receiver infers its displacement by comparing the depth of modulation of the 90 Hz and 150 Hz signals. Because MLS measures the aircraft's actual angle, the receiver knows where it is anywhere within coverage. The course and the glide path angle to be flown become selections, and the receiver shows the deviation from them on ILS-style scales. Training material also notes that the time-reference principle is less affected than the ILS by reflections from buildings and terrain.

Azimuth, elevation and back-azimuth

The coverage figures quoted in ATPL training material are:

The approach azimuth gives lateral guidance across a sector at least 80° wide, compared with the ILS localiser's protected 10° either side of the course to 25 NM and 35° to 17 NM. An aircraft can therefore be guided well away from the extended centreline.

The elevation function scans vertically. Because the angle is measured rather than built into the beam, the glide path is a selection, not a property of the site, and the same installation can support a steep approach for noise abatement or for aircraft that need one.

The MLS back-azimuth covers the area beyond the far end of the landing runway, providing lateral guidance for the missed approach and for departures. The ILS has nothing equivalent. Its back course is a by-product of the localiser that some states do not allow for approaches, and it carries no glide path.

Precision DME (DME/P)

MLS uses no marker beacons. Range comes from a precision DME (DME/P) associated with the installation, accurate to about 100 ft (30 m), which is good enough for CAT II and III operations. The standard en-route DME, DME/N, is specified to ±0.25 NM plus 1.25% of range, as much as 1.5 NM at 100 NM (see DME). DME/P works on the same interrogator and ground transponder principle and uses the same frequency pairing.

With angles from the azimuth and elevation functions and range from DME/P, the airborne equipment knows the aircraft's three-dimensional position within coverage. That allows a flight management system to use MLS, like the ILS, as a precise position update on final approach, and, in principle, approaches that are not confined to one long straight final.

Advantages over ILS

MLS is not immune to interference from vehicles and aircraft, however. ICAO provisions refer to ILS and MLS critical and sensitive areas together, and taxiway centreline lights alternate green and yellow within either, as they do on high-speed exit taxiways.

MLS in current operations

MLS still runs through the rules. Precision approach categories were defined for approaches using ILS or MLS: CAT I with a decision height not below 200 ft and an RVR of at least 550 m, CAT II with a decision height below 200 ft but not below 100 ft and an RVR of at least 300 m, and the older CAT IIIA, IIIB and IIIC subdivisions. ICAO defines the final approach point for an ILS or MLS approach where the intermediate approach altitude meets the nominal glide path on the localiser course or the specified MLS azimuth. Precision approach runways are those served by ILS, MLS or PAR. Independent parallel approaches (Mode 1) are defined for aircraft on adjacent ILS or MLS approaches, each established on the localiser course or MLS final approach track before vertical separation is reduced below 1,000 ft. In Item 10 of the ICAO flight plan the letter K indicates MLS, and France's AIP lists a difference from the ICAO provisions for simultaneous ILS and/or MLS approaches to parallel runways.

Airliner equipment still includes it. On the A320, where multi-mode receivers are fitted, each houses ILS, GLS and MLS receivers, MLS1 in MMR1 and MLS2 in MMR2. The DME distance associated with the ILS, GLS or MLS appears on the PFDs. The limitations treat ILS and MLS alike: autoland for ILS or MLS CAT II and III is approved in CONF 3 and CONF FULL, with maximum winds of 30 kt headwind, 10 kt tailwind and 20 kt crosswind, and with CAT 1 displayed on the FMA the autopilot may be used down to 160 ft AGL on an ILS or MLS approach. As with an ILS, the FMGS NAV mode is not certified for the final approach of an MLS procedure.

In the United States, the FAA's AIM no longer contains an MLS paragraph among its navigation aids, and precision approach systems other than ILS and GLS require special instrument approach procedures, issued to an operator with specific authorisation. The ILS still supports the lowest minima in use (see low visibility operations), while satellite approaches, LPV with SBAS and GBAS landing system (GLS) approaches with GBAS, supply much of the flexibility MLS promised; a single GBAS station can serve several runway ends.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.