Microwave Landing System (MLS) and its limitations

Microwave Landing System (MLS) Overview

• MLS aimed to improve upon and replace Instrument Landing Systems (ILS)
• There was very limited uptake of MLS primarily as only a minority of airports required its superior performance
• Airports like London Heathrow with high traffic density and frequent precision instrument approaches during bad weather would benefit
• MLS was superseded by GPS based precision landing systems
• The major MLS installation was at London Heathrow Airport, but it was decommissioned in 2017

Limitations of Instrument Landing Systems (ILS)

• US airlines began using ILS approaches regularly after World War II
• By 1949, the ILS had become the world standard for landing guidance systems
• By the late 1960s air traffic congestion and the need for more noise sensitive approach paths to airports increased the demand for a more capable and flexible landing system.
• The VHF/UHF frequencies in which ILS operates have a limited number of channels.
• As air transportation became more popular, planners foresaw a time when there would be more airports requesting ILS than the ILS frequency range could accommodate
• The relatively low frequency range of the ILS was susceptible to signal reflection, multipath errors from other ILS transmitters, high power FM interference and other distortions
• ILS only allowed one approach path to a runway.
• Airplanes would lock on to the ILS up to 10 miles away from the runway and fly a straight course in for a landing
• Two priorities identified by the Department of Transportation were to increase air traffic capacity of airports and to develop approaches that avoided noise sensitive areas
• Curved, segmented approach paths suggested by NASA and the FAA could accomplish both goals if a more flexible landing guidance system than the ILS was used
• ILS limitations include site sensitivity, high installation costs, single approach path, multi-path interference and channel limitations - 40 channels only

Introducing Microwave Landing Systems

• In 1968, a Special Committee was formed to develop specific requirements and specifications for a new aircraft approach and landing system
• The US Federal Aviation Administration decided that a microwave frequency based, scanning beam format would make the best landing system
• The higher frequency microwave band would alleviate the frequency allocation and some of the multipath signal problems
• A scanning beam technique provided broader signal coverage, allowing more flexible airplane approach paths

MLS Advantages over ILS

• MLS eliminates all the problems associated with ILS, and offers many advantages over ILS
• MLS eliminates ILS/FM broadcast interference problems
• MLS provides all-weather coverage up to ±60° from runway centerline, from 0.9° to 15° in elevation, and out of 20 NM
• MLS can provide precision guidance to small landing areas such as roof-top heliports and short final segments
• MLS allows continuous availability of a wide range of glide paths to accommodate STOL and VTOL aircraft and helicopters
• MLS accommodates curved approaches, which lead to noise reduced flight paths with curved decelerating approaches
• MLS has availability of 200 channels - five times more than ILS
• MLS has potential reduction of Category I (CAT I) minimums
• Improved guidance quality with fewer flight path corrections are required with MLS
• MLS can provide back-azimuth for missed approaches and departure guidance
• MLS eliminates service interruptions caused by snow accumulation
• MLS has lower site preparation, repair, and maintenance costs
• MLS provides more descent routes into airports and steep guided descents onto runways for STOL and VTOL aircraft

Azimuth, Elevation and Range

• MLS provides navigation guidance for aircraft alignment and descent during an approach to a selected runway
• MLS integrates Azimuth (AZ), Elevation angle (EL), and range (DME) information to provide aircraft positioning
• MLS components are similar to an ILS: Instead of a glideslope antenna, the MLS has an elevation station, and instead of a localiser antenna, it has an azimuth station
• The MLS has a precision DME (DME/P) transmitter with signals that are more accurate than traditional DME
• MLS operates 5.03 to 5.09 GHz
• The azimuth station transmits MLS angle and data on one of 200 channels within the frequency range of 5031 to 5090.7 MHz
• The azimuth station is normally located about 1000 feet (300 m) beyond the stop end of the runway and can be collocated with the elevation transmitter for heliport operations
• The azimuth coverage extends laterally, at least 40° (to a magimum of 60°) on either side of the runway centreline in a standard configuration, in elevation, up to an angle of 15° and to at least 20 000 feet (6 km), and in range, to at least 20 nautical miles (37 km)
• The elevation station transmits signals on the same frequency as the azimuth station and is located 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°; Laterally, to fill the Azimuth lateral coverage and in range, to at least 20 nautical miles (37 km)
• 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; the complete listing of the 200 paired channels of the DME/P with the angle functions is contained in FAA Standard 022
• 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, occurring only at outlying, low density airports where marker beacons or compass locators are already in place.

Ground Equipment Location

• MLS is normally installed in a configuration similar to ILS
• If necessary, all of the components of MLS can be installed together such as at a hospital's helipad
• In a standard airfield installation: The MLS azimuth transmitter is located between 1000 and 1500 feet beyond the departure end of the runway along the runway centreline; the elevation transmitter is located 400 feet from the runway centreline near the approach threshold; the DME, which provides range information, is collocated with the azimuth transmitter.
• The Time Reference Scanning Beam (TRSB) technique is used in an MLS to pinpoint the airplane's location based on two separate microwave beams with precisely timed scanning patterns
• The first beam swept back and forth across the runway centerline (plus or minus 60° on each side) at a rate of 13.5 times per second; the second swept up and down from the runway elevation to a position 20° above that at a rate of 40 times per second
• The airplane's horizontal and vertical position is determined by measuring the time difference in between each signal transmission received by the aircraft
• Integrating data with distance information from a conventional Distance Measuring Equipment (DME) transmitter on the airport, a receiver on board an airplane could accurately pinpoint the plane's location in relation to the runway
• A third beam, scanning up and down over a 7.5° arc from the runway elevation 40 times a minute, provides flare guidance for automatic landings
• The net result is a system that can provide precise manual or automatic landing guidance to an airplane anywhere in a wedge-shaped area stretching 120° wide and 20° high from the airport runway

Displays

• MLS displays are identical to the ILS, with lateral and vertical MLS guidance may be displayed on conventional Course Deviation Indicators (CDI) or incorporated into multipurpose cockpit displays

Principles of Operation

• TRSB principle: a radio beam scanned rapidly 'to-and-fro'; an appropriately tuned receiver on an aircraft within range of the beam source would receive two pulses in one complete scan
• The pulse spacing is related to the angle made between the centroid of the scanned sector and the line joining aircraft to beam source
• Knowing which is the 'to' and which is the 'fro' scan, or by knowing the scan cycle start time, removes any ambiguity
• Lateral and vertical guidance azimuth scanning and elevation scanning beams are required
• Preamble instructions identify the beams which are synchronized so that one-time difference for each beam is measured in a 150 ms frame.

Mobile Microwave Landing System (MMLS)

• Microwave landing systems provide the capability of mobility, and quick set-up
• MMLS compactness provides a significant advantage because they can be loaded on one pallet
• MMLS are easy to assemble, allowing operators to set it up very quickly: A trailerised version of can be setup in about 30 minutes to an hour; a non-trailerised version can be set up in three hours or less
• Ground equipment requires mobility primarily for military applications
• MLS ground equipment is much smaller and readily transportable than the ILS equivalent.