The NASA 737 research aircraft on the Wallops runway in 1987 with the microwave landing system equipment in the foreground
The microwave landing system (MLS) is an all-weather, precision radio guidance system intended to be installed at large airports to assist aircraft in landing, including 'blind landings'. MLS enables an approaching aircraft to determine when it is aligned with the destination runway and on the correct glidepath for a safe landing. MLS was intended to replace or supplement the instrument landing systems (ILS). MLS has a number of operational advantages over ILS, including a wider selection of channels to avoid interference with nearby installations, 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 envisioned by some aviation agencies never became a reality. There were two reasons: (economic) while technically superior to ILS, MLS did not offer sufficiently greater capabilities to justify adding MLS receivers to aircraft equipage; and (potentially superior third system) GPS-based systems, notably WAAS, allowed the expectation of a similar level of positioning with no equipment needed at the airport. GPS/WAAS dramatically lowers an airport's cost of implementing precision landing approaches, which is particularly important at small airports. For these reasons, 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, widespread installation never occurred. Further deployment of the system is not likely. Rather, several European airports have implemented LPV approaches based on the EGNOS (WAAS-compatible) satellite system.
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