Microwave landing system
A microwave landing system (MLS) is an all-weather, precision radio guidance system installed at large airports to assist aircraft in landing. It enables the approaching aircraft to determine when it's on the correct glidepath for a safe landing at a given runway. MLS was originally intended to replace or supplement the previous instrument landing systems (ILS). MLS has a number of operational advantages, including a wide selection of channels to avoid interference with other nearby airports, excellent performance in all weather, a small "footprint" at the airports, and wide vertical and horizontal "capture" angles that allowed approaches from wider areas around the airport.
Although some MLS systems became operational in the 1990s, the widespread deployment initially envisioned by its designers never became a reality. GPS-based systems, notably WAAS, allowed the expectation of the same level of positioning detail with no equipment needed at the airport. GPS/WAAS dramatically lowers the cost of implementing precision landing approaches, and since its introduction most existing MLS systems in North America have been turned off. GPS/WAAS-based LPV 'Localizer Performance with Vertical guidance' approaches provide vertical guidance comparable to ILS Category I and FAA-published LPV approaches currently outnumber ILS approaches at US airports.
Though initially MLS appeared to be of interest in Europe, where concerns over the availability of GPS were an issue, the widespread installation never occurred. The likely further deployment of the system is in doubt. NASA operated a similar system called the Microwave Scanning Beam Landing System to land the Space Shuttle orbiter.
MLS employs 5 GHz transmitters at the landing place which use passive electronically scanned arrays to send scanning beams towards approaching aircraft. An aircraft that enters the scanned volume uses a special receiver that calculates its position by measuring the arrival times of the beams.
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The US version of MLS, a joint development between the FAA, NASA, and the U.S. Department of Defense, was designed to provide precision navigation guidance for exact alignment and descent of aircraft on approach to a runway. It provides azimuth, elevation, and distance, as well as "back azimuth" for navigating away from an aborted landing or missed approach. MLS channels were also used for short-range communications with airport controllers, allowing long-distance frequencies to be handed over to other aircraft.
In Australia, design work commenced on a version of MLS in 1972. Most of this work was done jointly by the Federal Department of Civil Aviation (DCA), and the Radio Physics Division of the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The project was called Interscan, one of several microwave landing systems under consideration internationally. Interscan was chosen by the FAA in 1975 and by ICAO in 1978 as the format to be adopted. An engineered version of the system, called MITAN, was developed by industry (Amalgamated Wireless Australasia Limited and Hawker de Havilland) under a contract with DCA's successor, the Department of Transport, and successfully demonstrated at Melbourne Airport (Tullamarine) in the late 1970s. The white antenna dishes could still be seen at Tullamarine until 2003 when they were dismantled.
This initial research was followed by the formation of Interscan International limited in Sydney, Australia in 1979 which manufactured MLS systems that were subsequently deployed in the US, EU, Taiwan, China and Australia. The Civil Aviation Authority (United Kingdom) developed a version of MLS, which is installed at Heathrow Airport and other airports, because of the greater incidence of instrument approaches with Cat II/III weather.
Compared with the existing ILS system, MLS had significant advantages. The antennas were much smaller, using a higher frequency signal. They also did not have to be placed at a specific location at the airport, and could "offset" their signals electronically. This made placement easier compared with the physically larger ILS systems, which had to be placed at the ends of the runways and along the approach path.
Another advantage was that the MLS signals covered a very wide fan-shaped area off the end of the runway, allowing controllers to direct aircraft approaching from a variety of directions or guide aircraft along a segmented approach. In comparison, ILS could only guide the aircraft down a single straight line, requiring controllers to distribute planes along that line. MLS allowed aircraft to approach from whatever direction they were already flying in, as opposed to flying to a parking orbit before "capturing" the ILS signal. This was particularly valuable at larger airports, as it could allow the aircraft to be separated horizontally much closer to the airport. Similarly in elevation, the fan shaped coverage allows for variations in descent rate, making MLS useful for aircraft with steeper approach angles such as helicopters, fighters and the space shuttle.
Unlike ILS, which required a variety of frequencies to broadcast the various signals, MLS used a single frequency, broadcasting the azimuth and altitude information one after the other. This reduced the chance of frequency conflicts, as did the fact that the frequencies used were far away from FM broadcasts, another problem with ILS. MLS also offered two hundred separate channels, making conflicts between airports in the same area easily preventable.
Finally, the accuracy was greatly improved over ILS. For instance, standard DME equipment used with ILS offered range accuracy of only ±1200 feet. MLS improved this to ±100 ft in what they referred to as DME/P (for precision), and offered similar improvements in azimuth and altitude. This allowed MLS to guide extremely accurate CAT III approaches, whereas this normally required an expensive ground-based high precision radar.
Similar to other precision landing systems, lateral and vertical guidance may be displayed on conventional course deviation indicators or incorporated into multipurpose cockpit displays. Range information can also be displayed by conventional DME indicators and also incorporated into multipurpose displays.
It was originally intended that ILS would remain in operation until 2010 before being replaced by MLS. The system was only being installed experimentally in the 1980s when the FAA began to favor GPS. Even in the worst cases, GPS offered at least 300 ft accuracy, not as good as MLS, but much better than ILS. GPS also worked "everywhere", not just off the end of the runways. This meant that a single navigation instrument could replace both short and long-range navigation systems, offer better accuracy than either, and required no ground-based equipment.
The performance of GPS, namely vertical guidance accuracy near the runway threshold and the integrity of the system have not been able to match historical ICAO standards and practices. Greater GPS accuracy could be provided by sending out "correcting signals" from ground-based stations, which would improve the accuracy to about 10 m in the worst case, far outperforming MLS. Initially it was planned to send these signals out over short-range FM transmissions on commercial radio frequencies, but this proved to be too difficult to arrange. Today a similar signal is instead sent across all of North America via commercial satellites, in a system known as WAAS. However WAAS is not capable of providing CAT II or CAT III standard signals (those required for autolanding) and so a Local Area Augmentation System, or LAAS, must be used.
The system may be divided into five functions: Approach azimuth, Back azimuth, Approach elevation, Range and Data communications.
Approach azimuth guidance
The azimuth station transmits MLS angle and data on one of 200 channels within the frequency range of 5031 to 5090.7 MHz and is normally located about 1,000 feet (300 m) beyond the stop end of the runway, but there is considerable flexibility in selecting sites. For example, for heliport operations the azimuth transmitter can be collocated with the elevation transmitter.
The azimuth coverage extends: Laterally, at least 40 degrees on either side of the runway centerline in a standard configuration. In elevation, up to an angle of 15 degrees and to at least 20,000 feet (6 km), and in range, to at least 20 nautical miles (37 km) (See FIG 1-1-8.)
The elevation station transmits signals on the same frequency as the azimuth station. A single frequency is time-shared between angle and data functions and is normally located about 400 feet from the side of the runway between runway threshold and the touchdown zone.
Elevation coverage is provided in the same airspace as the azimuth guidance signals: In elevation, to at least +15 degrees; Laterally, to fill the Azimuth lateral coverage and in range, to at least 20 nautical miles (37 km) (See FIG 1-1-9.)
The MLS Precision Distance Measuring Equipment (DME/P) functions in the same way as the navigation DME, but there are some technical differences. The beacon transponder operates in the frequency band 962 to 1105 MHz and responds to an aircraft interrogator. The MLS DME/P accuracy is improved to be consistent with the accuracy provided by the MLS azimuth and elevation stations.
A DME/P channel is paired with the azimuth and elevation channel. A complete listing of the 200 paired channels of the DME/P with the angle functions is contained in FAA Standard 022 (MLS Interoperability and Performance Requirements).
The DME/N or DME/P is an integral part of the MLS and is installed at all MLS facilities unless a waiver is obtained. This occurs infrequently and only at outlying, low density airports where marker beacons or compass locators are already in place.
The data transmission can include both the basic and auxiliary data words. All MLS facilities transmit basic data. Where needed, auxiliary data can be transmitted. MLS data are transmitted throughout the azimuth (and back azimuth when provided) coverage sectors. Representative data include: Station identification, Exact locations of azimuth, elevation and DME/P stations (for MLS receiver processing functions), Ground equipment performance level; and DME/P channel and status.
MLS identification is a four-letter designation starting with the letter M. It is transmitted in International Morse Code at least six times per minute by the approach azimuth (and back azimuth) ground equipment.
Auxiliary data content: Representative data include: 3-D locations of MLS equipment, Waypoint coordinates, Runway conditions and Weather (e.g., RVR, ceiling, altimeter setting, wind, wake vortex, wind shear).
In the United States, the FAA suspended the MLS program in 1994 in favor of the GPS (Wide Area Augmentation System WAAS). The FAA's inventory of instrument flight procedures no longer includes any MLS locations; the last two were eliminated in 2008.
Due to different operational conditions in Europe many countries (particularly those known for low visibility conditions) were expected to embrace the MLS system as a replacement to ILS. However, in reality the only major installation was London Heathrow Airport, which was decommissioned on the 31st of May 2017. Other major airports, such as Frankfurt Airport which were expected to install MLS have instead published GBAS approaches. 
As more GBAS system are installed then the further installation of MLS or continued operation of existing systems must be in doubt.
- Instrument Landing System (ILS)
- Local Area Augmentation System (LAAS)
- Tactical air navigation system (TACAN)
- Transponder Landing System (TLS)
- Wide Area Augmentation System (WAAS)
- Microwave Scanning Beam Landing System
- Aeronautical Information Manual
- Aviation Today MLS: Back to the Future? April 1, 2003 article about new installation of MLS at London Heathrow Airport