Microwave landing system
[1] 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 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.
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.
An aircraft that enters the scanned volume uses a special receiver that calculates its position by measuring the arrival times of the beams.
The US version of MLS, a joint development between the FAA, NASA, and the United States Department of Defense, was designed to provide precision navigation guidance for exact alignment and descent of aircraft on approach to a runway.
MLS channels were also used for short-range communications with airport controllers, allowing long-distance frequencies to be handed over to other aircraft.
Most of this work was done jointly by the Federal Department of Civil Aviation (DCA), and the Radio Physics Division of the CSIRO.
An engineered version of the system, called MITAN, was developed by industry (AWA and Hawker de Havilland) under a contract with DCA's successor, the Department of Transport, and successfully demonstrated at Melbourne Airport in the late 1970s.
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.
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.
MLS also offered two hundred separate channels, making conflicts between airports in the same area easily preventable.
This allowed MLS to guide extremely accurate CAT III approaches, whereas this normally required an expensive ground-based high precision radar.
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.
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.
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.
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).
This occurs infrequently and only at outlying, low density airports where marker beacons or compass locators are already in place.
In the United States, the FAA suspended the MLS program in 1994 in favor of the GPS (Wide Area Augmentation System WAAS).
