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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