Microwave Landing System
The Microwave Landing System (MLS) is an all-weather, precision landing system originally intended to replace or supplement the Instrument Landing System (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, and a small "footprint" at the airports.
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. However, GPS has not yet provided the vertical guidance accuracy needed for precision approach, contrary to the MLS system. Additionally the integrity of the GPS system for precision approach, namely Cat II and Cat III, has been an issue that has not been resolved, in spite of continued efforts by FAA, Mitre and others since the early 1990s. The integrity and continuity of service of the MLS signal-in-space does possess the necessary characteristics to support Cat II and Cat III, as does the ILS.
MLS continues to be of some interest in Europe, where concerns over the availability of GPS continue to be an issue. A widespread installation in the United Kingdom is currently underway, which included installing MLS receivers on most British Airways aircraft, but the continued deployment of the system is in doubt. NASA operates a similar system called the Microwave Scanning Beam Landing System to land the Space Shuttle.
Radio-navigation aids must keep a certain degree of accuracy (given by international standards, FAA, ICAO...); to assure this is the case, Flight inspection organizations check periodically critical parameters with properly equipped aircraft to calibrate and certify MLS precision.