Microwave Landing System
2. Back azimuth;
(MLS)
3. Approach elevation;
A. INTRODUCTION The Microwave Landing System (MLS) was
4. Range; and 5. Data communications.
communications with airport controllers, allowing long-distance frequencies to be handed over to other aircraft. In Australia, design work commenced on its version of an MLS in 1972. Most of this work was jointly done by the then Federal
designed to replace ILS with an advanced
With the exception of DME, all MLS signals
precision approach system that would
Radio Physics Division of the Commonwealth through time sharing. Two hundred channels Scientific and Industrial Research are available between 5031 and 5090.6 Organization (CSIRO). The project was called
overcome the disadvantages of ILS and also provide greater flexibility to its users. MLS is a precision approach and landing system that provides position information and various ground to air data. The position information is provided in a wide coverage sector and is determined by an azimuth angle measurement, an elevation measurement and a range measurement.
B. SYSTEM DESCRIPTION The time-referenced scanning beam Microwave Landing System (MLS) has been adopted by ICAO as the standard precision
are transmitted on a single frequency
Megahertz (MHz). By transmitting a narrow
Interscan, one of several microwave landing beam which sweeps across the coverage area systems under consideration internationally. at a fixed scan rate, both azimuth and Interscan was chosen by the FAA in 1975 elevation may be calculated by an airborne receiver which measures the time interval between sweeps. For the pilot, the MLS presentation will be similar to ILS with the use of a standard CDI or multi-function display,
C. HISTORY The US version of MLS was a joint
development between the FAA, NASA, and approach system to replace ILS. MLS provides the U.S. Department of Defense, was precision navigation guidance for alignment designed to provide precision navigation and descent of aircraft on approach to a
landing by providing azimuth, elevation and distance. The system may be divided into five functions: 1. Approach azimuth;
Department of Civil Aviation (DCA), and the
guidance for exact alignment and descent of aircraft on approach to a runway. It provides azimuth, elevation, and distance, as well as "back azimuth", for navigating from an aborted landing or missed approach. MLS channels were also used for short-range
and by ICAO in 1978 as the format to be adopted. An engineered version of the system, called MITAN, was developed by industry (Amalgamated Wireless Australasia Limited and Hawker de Havilland) under a contract with DCA's successor, the Department of Transport, and successfully demonstrated at Melbourne Airport (Tullamarine) in the late 1970s. The white antenna dishes could still be seen at Tullamarine up till 2003 before it was dismantled. This initial research was followed by the formation of Interscan International limited in Sydney, Australia in 1979 who manufactured MLS systems that were subsequently deployed in the US, EU, Taiwan,
China and Australia. The CAA in UK
airborne equipment, and the time between
developed a version of the MLS which is
each signal (less the guard time) relates to
installed at Heathrow and other airports due
the angle from the reference line, which is
to the greater incidence of instrument approaches and Cat II/III weather there. GPS has not yet solved the critical problems
the position of the beam when it starts its sweep. This can be seen in Fig 8-10. One
needed to match the MLS international
fan-shaped beam sweeps horizontally to
standard.
provide a position line in azimuth. At a different time, a horizontally orientated fan sweeps up and down in a similar fashion to
D. MLS OPERATION In 1972 ICAO published an operational requirement for a new type of non-visual approach and landing guidance system. This was to use a method called 'Time Referenced Scanning Beam’ (TRSB) in the SHF band. Signals at these frequencies are commonly called 'microwaves', so the system became known as the microwave landing system or MLS.
of approach is now known in both azimuth and elevation, and can be displayed in a similar fashion to that of ILS. The third part of the system consists of an accurate DME (precision DME or DME/P) signal to show the aircraft's position in from the station. The aircraft's position can thus be determined in three dimensions.
Technology allows every piece of information from each of the azimuth and elevation beams to be obtained from signals on the same frequency. Each piece of information requires a very short time to obtain it. After one piece has been received it is used and stored until it is replaced. Meanwhile, another piece of information can be received, and again used and stored, then another. The total time taken to receive every piece of information required for the MLS system to function in this fashion is about 84 milliseconds. This is divided into specific periods or bands in which the individual pieces of information are transmitted (and received). This is called multiplexing. In addition to guidance information, auxiliary information is also sent during the multiplex
D.1 Time Referenced Scanning Beams
Basic Principle.
give a position line in elevation. The angle
D. 2 Multiplexing
The idea is for a ground
station to sweep a narrow fan-shaped beam at a very accurate constant speed from one side of a sector to the other, then back again after a specific time interval (the 'guard time'). The signal will be received twice at the
transmission. This includes the station identification, safety information such as the minimum safe glideslope angle, and more sophisticated information such as system condition, weather and runway conditions which can be displayed on modern cockpit displays if fitted.
Beams can also scan in the opposite
Azimuth Coverage.
direction, away from the approach path, to
segment, the horizontal area scanned by the
specification for elevation coverage is
provide guidance to aircraft in the missed
guidance beams is +40° of the centerline,
slightly different. It only covers the approach
approach segment. These are also useful on
out to 22.5 nm from the station, although
sector, out to at least 20 nm, and within a
climb out after take-off. There are also test
proportional guidance may be restricted to
horizontal angle which at least corresponds
pulses, to check the serviceability of the
within 10° of the centerline. Vertically, the
to that within which the azimuth
system, and indicator pulses to give general
beams give guidance between 0.9° and 20°
proportional guidance is available. In
guidance in the area between the approach
above the horizontal, up to 20000 ft,
elevation, the minimum coverage is from 0.9°
and missed approach segments to guide the
although again proportional guidance may
to 7.5° above the horizontal, although it is
aircraft into the approach segment. Other
be restricted to a maximum elevation of 7.5°. recommended that it cover the whole of the
signals may be transmitted to give guidance
There is a region over the runway in which
at the flare on touchdown for Category III
coverage is provided from 8 ft up to 2000 ft
approaches. The time is not equally divided.
and out to 150 m either side of the
In the approach
Elevation Guidance Coverage. The
azimuth approach sector.
Three elevation signals are received for every centerline. The missed approach segment is azimuth signal. This indicates the greater
20° either side of the centre line, and from
danger of a rapid change in elevation angle
0.9° to 15° vertically, out to 10 nm from the
compared with a change in azimuth angle.
station and up to 10000’.
There are 40.5 elevation scans every second and 13.5 azimuth scans.
Frequencies. There are 200 allocated channels, spaced 300 kHz apart in the band between 5031.00 and 5090.70 MHz. Each station uses one channel for all its transmissions except the DME/P, which uses similar frequencies to a normal DME. The DME/P frequencies are automatically selected.
DME/P. Like the DME stations used for ILS ranges, the DME/P is electronically adjusted to give ranges from touchdown. Correct indications are available within the coverage of the guidance beams in the approach sector.
E. INSTALLATION
All signals (except DME) are sent on a
microwave beam that is transmitted towards
single frequency using time sharing.
the sector of approach and scans the sector
(each type of signal is sent in a
both in the horizontal as well as the vertical
specific order one after the other in quick succession; azimuth, elevation, data.)
F. MLS VS ILS
Azimuth transmitter at far end of
runway.
However, in relation to future aviation requirements, the ILS has a number of basic limitations:
interval between sweeps and
1. site sensitivity and high installation costs;
calculates azimuth and altitude.
2. single approach path;
aircraft CDI, HSI, or MFD. (similar to ILS)
and landing aid for the last 40 years. During
increase its performance and reliability.
information is displayed on the
A precision DME/P installation is incorporated into the system.
both in the horizontal direction of approach and the vertical plane, in whatever point of
space of approach in a given time and it’s
served as the standard precision approach
undergone a number of improvements to
Horizontal and lateral navigation
aircraft’s position is therefore determined
The Instrument Landing System (ILS) has
Elevation transmitter at near end of
Aircraft receiver measures the time
beam evaluates it’s location in space. The
reach of the scanning beam. Because the
this time it has served well and has
area at a fixed scan rate.
receives the signal and with the help of this
ILS LIMITATIONS
runway.
Narrow beams sweep across coverage
plane. An aircraft in the approach sector
3. multi path interference; and 4. channel limitations - 40 channels only. The MLS provides an accurate landing approach for an aircraft in the area of the final approach, where the path of the final
microwave technology is radiated into the not spread out over different directions, no signal interruption results from various obstacles or terrain protrusions as it was with the ILS system. The MLS system can thus be situated also in developed areas, where an ILS system couldn’t be set up. An onboard computer enables to solve the approach manoeuvre from a random direction, for variously oriented runways, even along a curved of bend landing trajectory. The MLS system is approved by the ICAO for every three categories of an accurate landing approach. ILS DISADVANTAGES - There are only 40 channels available worldwide.
approach isn’t identical with the enlonged
- The azimuth and glideslope beams are
runway’s axis. The system works with a
fixed and narrow. As a result, aircraft have to
be sequenced and adequately separated
guidance ± 20° of runway direction up to 15°
which causes landing delays.
in elevation to a range of 10 nm and a height operate to CAT III criteria.
-There are no special procedures available
of 10,000 ft.
for slower aircraft, helicopters, and Short
- It operates in the SHF band, 5031 - 5090
Take Off and Landing (STOL) aircraft.
MHZ. This enables it to be sited in hilly
- ILS cannot be sited in hilly areas and it requires large expanses of flat, cleared land to minimize interference with the localizer and glideslope beams.
areas without having to level the site. Course deviation errors (bending) of the localizer and glidepath caused by aircraft, vehicles and buildings are no longer a problem because the MLS scanning beam can
-Vehicles, taxying aircraft, low-flying aircraft be interrupted and therefore avoids the and buildings have to be kept well away from reflections. the transmission sites to minimize localizer and glideslope course deviations (bending of the beams). THE MLS SYSTEM The Microwave Landing System (MLS) has the
-Because of its increased azimuth and elevation coverage aircraft can choose their own approaches. This will increase runway utilization and be beneficial to helicopters and STOL aircraft.
following features:
-The MLS has a built-in DME.
-There are 200 channels available
- MLS is compatible with conventional
worldwide. - The azimuth coverage is at least ± 40° of the runway on-course line (QDM) and glideslopes from .9° to 20° can be selected. The usable range is 20-30 nm from the MLS site; 20nm in the UK. - There is no problem with back-course transmissions; a secondary system is provided to give overshoot and departure
localizer and glide path instruments, EFIS, auto- pilot systems and area navigation equipment. - MLS gives positive automatic landing indications plus definite and continuous on/off flag indications for the localizer and glideslope needles. -The identification prefix for the MLS is an ‘M’ followed by two letters.
- The aim is for all MLS equipped aircraft to
PRINCIPLE MLS employs 5GHz transmitters at the landing place which use passive electronically scanned rays 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. MLS ADVANTAGES As previously mentioned, ILS has limitations which prohibit or restrict its use in many circumstances. MLS not only eliminates these problems; but also offers many advantages over ILS including: 1. elimination of ILS/FM broadcast interference problems; 2. provision of ail-weather coverage up to ±60 degrees from runway centerline, from 0.9 degree to 15 degrees in elevation, and out of 20 nautical miles (NM); 3. capability to provide precision guidance to small landing areas such as roof-top heliports;
4. continuous availability of a wide range of glide paths to accommodate STOL and
MLS will be limited to specialized
configuration of an Azimuth Transmitter (AZ)
applications, such as military MMLS.
with an added DME rangefinder, perhaps even a more precise DME/P, in close distance
VTOL aircraft and helicopters;
of a course transmitter and near an elevation
5. accommodation of both segments and
transmitter, see Fig. 1. A scaled up
curved approaches;
configuration is supplemented with a course transmitter for an unsuccessful approach
6. availability of 200 channels - five times
and a flare transmitter.
more than ILS;
An MLS azimuth guidance station with
7. potential reduction of Category I (CAT l)
rectangular azimuth scanning antenna with
minimums; 8. improved guidance quality with fewer flight path corrections required; 9. provision of back-azimuth for missed approaches and departure guidance; 10. elimination of service interruptions caused by snow accumulation; and 11. lower site preparation, repair, and maintenance costs. THE MLS DISADVANTAGES
I. OTHER INFORMATION BASIC ELEMENTS
pieces of equipment that are divided into the
frequency signal. They also did not have to
protractor components, rangefinder
be located at a specific point at the airport,
components, and the onboard hardware. The and could "offset" their signals electronically. This made placement at the airports much information about the angles of the approach course, descent, flare and the
simpler compared to the large ILS systems,
course of an unsuccessful approach are
which had to be placed at the ends of the
acquired through an onboard antenna or the
runways and along the approach path.
aircraft itself by measuring the time between
infrastructure set up and aircraft equipment
frequency signal. The distance is determined
installation.
with the help of an ancillary device, the DME
With market forces driving GPS usage it is destined to be the system of the future.
had significant advantages. The antennas were much smaller, due to using a higher
two passages of an oscillating lobe of a high
mainstream RNAV system for civil aviation.
Compared to the existing ILS system, MLS
The MLS system is comprised of ground
Expensive: the initial expense of the
New alternatives: GNSS is emerging as the
DME antenna at left.
rangefinder. The MLS system further sends with the help of phase modulation and timedivision multiplexing additional data, as identification, system status and so on. The ground equipment consists in the basic
Another advantage was that the MLS signals covered a very wide fan-shaped area off the end of the runway, allowing controllers to vector aircraft in from a variety of directions or guide aircraft along a segmented approach. In comparison, ILS could only guide the aircraft down a single straight line, requiring controllers to distribute planes along that line. MLS allowed aircraft to
approach from whatever direction they were
referred to as DME/P (for precision), and
already flying in, as opposed to flying to a
offered similar improvements in azimuth and not been able to match historical ICAO
parking orbit before "capturing" the ILS
altitude. This allowed MLS to guide the
standards and practices. Additional GPS
signal. This was particularly interesting to
extremely accurate CAT III approaches,
accuracy could be provided by sending out
larger airports, as it potentially allowed the
whereas this normally required expensive
"correcting signals" from ground-based
aircraft to be separated horizontally until
ground-based high precision radar.
stations, which would improve the accuracy
much closer to the airport. Similarly in elevation, the fan shape coverage allows for variation in approach angle making MLS particularly suited to aircraft with steep approach angles such as helicopters, fighters and the space shuttle. An MLS elevation guidance station. Unlike ILS, which required a variety of
Similar to other precision landing systems, lateral and vertical guidance may be displayed on conventional course deviation indicators or incorporated into multipurpose cockpit displays. Range information can also be displayed by conventional DME indicators and also incorporated into multipurpose displays.
frequencies to broadcast the various signals,
It was originally intended that ILS would
MLS used a single frequency, broadcasting
remain in operation until 2010 before being
the azimuth and altitude information one
replaced by MLS. The system was only being
after the other. This reduced frequency
installed experimentally in the 1980s when
contention, as did the fact that the
the FAA began to favor GPS. Even in the
frequencies used were well away from FM
worst cases, GPS offered at least 300 ft
broadcasts, another problem with ILS.
accuracy, not as good as MLS, but much
Additionally, MLS offered two hundred
better than ILS. Additionally, GPS worked
channels, making the possibility of
"everywhere", not just off the end of the
contention between airports in the same area runways. This meant that a single navigation extremely remote. Finally, the accuracy was greatly improved over ILS. For instance, standard DME equipment used with ILS offered range
instrument could replace both short and long-range navigation systems, offer better accuracy than either, and required no ground-based equipment.
accuracy of only +/- 1200 feet. MLS
The major issues with GPS, namely 2 feet
improved this to +/- 100 ft in what they
vertical guidance accuracy near the runway
threshold and the integrity of the system has
to about 10 m in the worst case, far outperforming MLS. 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. Today a similar signal is instead sent across all of North America via commercial satellites, in a system known as WAAS. However WAAS is not capable of providing CAT II or CAT III standard signals (those required for autolanding) and so a Local Area Augmentation System, or LAAS, must be used.
