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Instrument Landing System (ILS) & Microwave Landing System (MLS)


One of the basic system used for landing of an aircraft are visual flight rules (VFR), i.e. without any indication from instruments about the aircraft's position relative to the desired approach path. 

Although the VFR are still into existence a system called as precision approach radar (PAR) system was used, whereby the air traffic controller, having the aircraft 'on radar', can give guidance over the v.h.f.-r.t.

In order to increase the number of aircraft landing at an runways and for aircraft able to land under low visibility condition instrumentation provided in the cockpit provide steering information to the pilot which, if obeyed, will cause the aircraft to make an accurate and safe decent and touchdown. This system was known as Instrument Landing System (ILS).

The instrument landing system (ILS) is the most accurate used for precision approaches and landings. The system uses a combination of VHF and UHF radio waves to provide pilots with an accurate means of carrying out an instrument approach to a runway, giving guidance both in the horizontal and the vertical planes.

ILS is a precision approach system because it gives guidance in both the horizontal and the vertical plane. It even enables aircraft to carry out automatic landings.

As the technology is progressing a more modern approach called as Microwave Landing System (MLS) is being introduced it provided more accurate landing information.

Basic ILS Terminologies

There are few basic terms which are to be understood before understand Instrument Landing System (ILS), which are as follows:

Runway Markings

There are a number of runway marking as follows

  • The runway thresholds are markings across the runway that denote the beginning and end of the designated space for landing and takeoff under non-emergency conditions.

  • Runways are named by a number between 01 and 36, which is generally the magnetic azimuth of the runway's heading in decadegrees. A runway numbered 09 points east (90°), runway 18 is south (180°), runway 27 points west (270°) and runway 36 points to the north (360° rather than 0°). If there is more than one runway pointing in the same direction (parallel runways), each runway is identified by appending left (L), center (C) and right (R) to the end of the runway number to identify its position (when facing its direction)

  • A displaced threshold or DTHR is a runway threshold located at a point other than the physical beginning or end of the runway. The displaced portion of the runway may be used for takeoff but not for landing. After landing at the other end, the landing aircraft may use the displaced portion of the runway for roll out.

  • Touchdown Zone or TDZ is the portion of a runway, beyond the threshold, where it is intended landing aeroplanes first contact the runway. The TDZ is placed after the runway threshold and runway designation markings. This is done to ensure that the aircraft lands on the runway and provide a safety margin. The TDZ is marked by pairs of stripes symmetrically placed on the two sides of the runway centreline. The number of pairs depends on the runway length (e.g. one pair for runways that are shorter than 900 m, 6 if the length is 2 400 m or more, etc.).

  • The Runway centerline is the line indicating the centre of the runway.

Runway Markings.jpg


Runway Visual Range

The runway visual range (RVR) is the distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings delineating the runway or identifying its centre line. RVR is normally expressed in feet or meters. RVR is used to determine the landing and takeoff conditions for aircraft pilots, as well as the type of operational visual aids used at airport.

RVR is used as one of the main criteria for minima on instrument approaches, as in most cases a pilot must obtain visual reference of the runway to land an aircraft. The maximum RVR range is 2,000 metres or 6,000 feet, above which it is not significant and thus does not need to be reported.


RVRs are provided in METARs and are transmitted by air traffic controllers to aircraft making approaches to allow pilots to assess whether it is prudent and legal to make an approach.

The runway visual range (RVR ) can also be converted to statute mile (SM). 


A nautical mile used to be based on the curvature of the earth and was approximately equal to one minute (arc segment) of latitude along a meridian. It's a mathematical calculation based on degrees of latitude around the equator. However a statute mile, and it's based on paces.

While flying, distance is generally measured in nautical miles and visibility is usually stated or forecasted in statute miles.

Nautical Mile (NM):

  • 1 NM = 1,852 meters

  • 1NM = 6,076 feet

  • 1NM = 1.151 statute miles

Statute Mile (SM):

  • 1 SM = 1,609 meters

  • 1 SM = 5,280 feet

  • 1 SM = 0.869 NM

A "knot" is equal to one nautical mile per hour.

1 knot = 1.15 miles per hour.

Runway Visual Range in Statute Mile or V


Decision Height (DH)

In a precision approach, the decision height (DH) or decision altitude (DA) is a specified lowest height or altitude in the approach descent at which, if the required visual reference to continue the approach (such as the runway markings or runway environment) is not visible to the pilot, the pilot must initiate a missed approach.


A decision height is measured AGL (above ground level) while a decision altitude is measured above MSL (mean sea level).


The specific values for DH and/or DA at a given airport are established with intention to allow a pilot sufficient time to safely re-configure an aircraft to climb and execute the missed approach procedures while avoiding terrain and obstacles.


A DH/DA denotes the altitude in which a missed approach procedure must be started, it does not preclude the aircraft from descending below the prescribed DH/DA.

Decision Height and Decision


Categories of ILS approach

Inorder for pilots to land without acquiring visual reference for a DH lower than 200 feet and RVR less than 550 meters. The ground and the aircraft needs to be equipped with the necessary equipments along with the equipment the pilots flying also needs to be suitably qualified. 

Hence Categories of ILS are defined in the ICAO.

ILS Facility Performance Categories (Ground Installation)


Category I

A category I ILS is one which provides guidance information from the coverage limit of the ILS to the point at which the localizer course line intersects the ILS glide path at a height of 200 ft (60 m) or less above the horizontal plane containing the threshold.


Category II

An ILS which provides guidance information from the coverage limit of the ILS to the point at which the localizer course line intersects the ILS glide path at a height of 50 ft (15 m) or less above the horizontal plane containing the threshold.


Category III

An ILS, which with the aid of ancillary equipment where necessary, provides guidance information from coverage limit of the facility to, and along, the runway surface.

Operational Performance Categories

The improvement in the ground installations allows guidance down to the surface of a runway and requires a corresponding improvement in the airborne equipment. An aircraft may be certified to operate to one of the following classifications:


Category I

An instrument approach and landing with :

  • a DH not lower than 60 m (200 ft) and

  • a Runway Visual Range (RVR) not less than 550 m.


Category II

A precision instrument approach and landing with

  • a DH lower than 60 m (200 ft) but not lower than 30 m (100 ft) and

  • a RVR not less than 300 m.


Category IIIA

A precision instrument approach and landing with:

  • a DH lower than 30 m (100 ft), or no DH; and

  • a RVR not less than 200 m.


Category IIIB

A precision instrument approach and landing with:

  • a DH lower than 15 m (50 ft), or no DH; and

  • a RVR less than 200 m but not less than 75 m.


Category IIIC

  • No DH and no RVR limitations.

The acceptance of category II or III operations will depend on whether the following criteria are met:

  • the aeroplane has suitable flight characteristics.

  • the aeroplane will be operated by a qualified crew in conformity with laid down procedures.

  • the aerodrome is suitably equipped and maintained.

  • it can be shown that the required safety level can be maintained.

Category of Landing.jpg


ILS overview

ILS provides the pilot with visual instructions in the cockpit to enable him to fly the aircraft down a predetermined glide path and extended runway centre line (localizer) to his Decision Height (DH). The ground installation has three distinct components  

  • Localizer 

  • Glide path / Glideslope

  • Marker beacons 

  • in some installations a back course may also be available.


The localizer (LLZ) guides the aircraft in the horizontal plane and is located about 300 m from the up-wind end of the runway. Localizer transmits in the VHF band between 108 and 111.975 MHz at 50 kHz spacing to provide 40 channels. Since the frequency band is shared with VHF Omnidirectional Range (VOR), the frequencies allocated are where the tenths of a megacycle count is odd, for example 108·10 and 108·15 MHz are localizer channels while 108·20 and 108·25 MHz are VOR.

Glide path / Glideslope 

The glide path (GP) or glide slope provides guidance in the vertical plane and is located 300 m in from the threshold and about 200 m from the runway edge abeam the touchdown point. It has a range of approximately 10 nm. The glide path (GP) transmitter operates in the UHF band between 329.15 and 335 MHz at 150 kHz spacing to provide 40 channels, for example 329.15, 329.3, 329.45, 329.6 - 335 MHz.


Marker beacon

A two or three marker beacons are located at key points on the extended runway centreline defined by the localizer and radiates directly upwards using a carrier frequency of 75 MHz. The modulating signal depends on the function of the marker.  These include the outer marker (OM), the middle marker (MM) and possibly an inner marker (IM). They are provided to enable the pilot to cross-check the aircraft’s height against ranges and timing to the runway threshold.


There is no interference problem as the radiation pattern is a narrow fan-shaped vertical beam.

The GP frequency is paired with the localizer and selection of the frequency is automatic. The localizer and glide path transmissions are frequency paired in accordance with the list published at ICAO e.g. 108.1 MHz is paired with 334.7 MHz, and 111.95 MHz is paired with 330.95 MHz.


The advantages of this are:

  • One switch activates both receivers - this reduces the pilot’s workload.

  • Frequency selection is made easier and quicker as there is only one to consider.

  • The potential for a wrong frequency selection is reduced.

  • Only one identifier is needed.

The localizer and glide path frequencies are paired, hence selection of the localizer VHF frequency automatically energizes the glide path receiver circuits. Hence a single identification for ILS localizer and glide path transmissions is used.  The Ident on the localizer transmission is a 2 or 3 letter Morse signal at 7 groups/min. The first letter is usually “I”. 

Some ILS installations also have a co-located low powered NDB, called a locator (L), at the site of the OM beacon.

Distance Measuring Equipment (DME) that is frequency paired with the ILS frequencies are now increasingly provided to supplement or replace the range information provided by marker beacons.

Instrument Landing System.jpg


ILS Operations

The working of each of the above described is as follows



The localizer antenna is located at the far end of the runway and transmits two overlapping lobes on single VHF ILS frequency one to the left and other to right of the runway centreline or runway approach direction (QDM) modulated at 90 Hz and 150 Hz respectively.



Deviation from the centre line is given in d.d.m. (difference in depth of modulation), i.e. the percentage modulation of the larger signal minus the percentage modulation of the smaller signal divided by 100.

Either side of the centreline will produce a difference in depth of modulation (DDM); this difference is directly proportional to the deviation on either side of the extended centreline of the runway. On the extended runway centreline, the combined depth of modulation is equal.

The depth of modulation (DoM) increases away from the centre line i.e. the amplitude of the modulating signal increases away from the centre line.

The localizer also transmits a two- or three-letter Morse code identifier that the crew can hear on their audio panels.

The localizer coverage sector extends from the transmitter to distances of:

  • 25 NM (46.3 km) within plus or minus 10° from the centre line.

  • 17 NM (31.5 km) between 10° and 35° from the centre line.

  • 10 NM (18.5 km) outside ± 35° if coverage is provided.


These limits may be reduced to 18 NM within 10° sector and 10 NM within the remainder of the coverage when alternative navigational facilities provide satisfactory coverage within the intermediate approach area.

Localizer Range.jpg


Glide Path (GP) or Glideslope

The principle of glideslope operation is similar to that of localizer in that the carrier is modulated with 90 and 150 Hz tones. Above the correct glidepath the 90 Hz modulation predominates while below the corrected glidepath the 150 Hz modulation predominates and on the correct glidepath the d.d.m. is zero, both tones giving a 40 per cent depth of modulation.

When viewed from the side, the two lobes overlap and produce an approach path inclined at a fixed angle between 2.5 and 3.5 degrees.


Glide slope frequency is automatically selected when the crew tunes the localizer frequency.

Glide Slope.jpg


The glide path coverage extends from the transmitter to a distance of at least:

10 NM (18.5 km) in sectors of 8° in azimuth on each side of the centre line.

The vertical coverage is provided from 0.45θ up to 1.75θ above the horizontal where θ is the promulgated glide path angle. The lower limit may be reduced to 0.3θ if required to safeguard the promulgated glide path intercept procedure.

Glideslope Range.jpg


One of the problem associated with glide slope is false glide slope 

False Glide Slope(s) are defined as the paths of points, in the vertical plane, containing the runway centre line at which the DDM is zero; other than that path of points forming the ILS glide path. The twin lobes are repeated due to:

  • Metallic structures situated at the transmission point, and ground reflections.

  • The height and propagation characteristics of the aerial.


​The first false glide slope occurs at approximately twice the glide path angle, 6° above ground for a standard 3° glide path.


False glide slopes always occur above the true glide slope and should not constitute a danger but pilots should be aware of their presence. Normal flying practice is to establish on the localizer and intercept the glide slope from below.

False Glide Slope.jpg


Marker beacons

The radiation patterns for ILS marker beacons is vertical and appears lens shaped or bone shaped in plan view. The signal is only received if the aircraft is flying within the fan; it is not a directional aid.


Two or three beacons are sited on the extended runway centreline at precise distances; these are specified in the approach charts for specific runways.


These beacons operate at 75 MHz and radiate approximately 3–4 W of power. The beacons provide visual and audible cues to the crew to confirm their progress on the ILS.


The outer marker is located between four and seven miles from the runway threshold; it transmits Morse code dashes at a tone frequency of 400 Hz and illuminates a blue light (or cyan ‘OM’ icon for electronic displays) when the aircraft passes over the beacon. The outer marker provides the approximate point at which an aircraft on the localizer will intercept the glide slope.


Some airfields use non directional beacons (NDBs) in conjunction with (or in place of) the outer marker. These are referred to as locator beacons.


The middle marker is located approximately 3500 feet from the runway threshold. When passing over the middle marker, the crew receive an alternating Morse code of dots/dashes modulated at 1300 Hz, and a corresponding amber light (or yellow ‘MM’ icon for electronic displays) is illuminated. The middle marker coincides with the aircraft being 200 feet above the runway touchdown point.


Runways that are used for low visibility approach and landings have a third inner marker. When passing over the inner marker, the crew receive Morse code dots modulated at 3000 Hz on the audio system, and a corresponding white light (or ‘IM’ icon for electronic displays) is illuminated.


The marker beacon system is currently being phased out with the introduction of DME and GPS approaches.

Marker Beacon.jpg


ILS Equipment

The airborne equipment comprises

  • Localizer Antenna

  • Glide slope Antennas

  • Marker beacon antenna

  • ILS Receiver

  • Controls Panel

  • Displays

Most aircraft are fitted with two or three independent ILS systems (typically named left, centre and right).


Localizer and Glide slope Antennas

The Localizer and Glide slope antenna are fitted in the radome of the aircraft and the marker beacon antenna is generally located in the belly of the aircraft 


ILS Receivers

ILS receivers are often combined with other radio navigation functions, e.g. VHF omnidirectional range (VOR); these are located in the avionic equipment bay.


ILS receivers are based on the super-heterodyne principle with remote tuning from the control panel.


The signal received from the localizer antenna is modulated with 90 and 150 Hz tones for left/right deviation; a 1020 Hz tone contains the navigation aid identification in Morse code. Filters in the ILS receiver separate out the 90 and 150 Hz tones for both localizer and glide slope. The identification signal is integrated with the audio system.

Localizer Block Diagram.jpg


The marker beacon function is often incorporated with other radio navigation receivers, e.g. a combined VOR and marker beacon unit. The marker beacon receiver filters out the 75 MHz tone and sends the signal to an RF amplifier.

Three bandpass filters are then employed at 400 Hz, 1300 Hz and 3000 Hz to identify the specific marker beacon. The resulting signals are sent to an audio amplifier and then integrated into the audio system. Discrete outputs drive the visual warning lights (or PFD icons).

Marker Beacon Receiver.jpg


Controls Panel

The NAV control panel located generally on the centre pedestal panel is used to select the ILS frequencies, since the localizer and glideslope frequencies are paired together only localizer frequency is tunes and the paired glideslope frequency is automatically tuned. The panel consist of the following controls

  • Active window displays the current frequency selected 

  • Standby window displays the standby frequency selected 

  • TRF is use to switch between the active and standby frequency

  • Mode select is to use select the different mode from ILS or VOR since modern airfacts uses a combined control panel

  • Frequency selection can be done by a rotating knob or number key depending on the type os system in use

  • Test switch is use to test the system

ILS Control Panel.jpg


ILS displays

The method of ILS display depends on the type of aircraft generally the following are used

  • Course Deviation Indicator (CDI)

  • Horizontal Situation Indicator (HSI)

  • Electronic Horizontal Situation Indicator (EHSI)


Course Deviation Indicator (CDI)

The omni-bearing selector is used to rotate the course card. This card is calibrated from 0 to 360° and indicates the selected runway heading, which is indicated by the triangle. The instrument consist of two deviation bar, horizontal bar indicates the deviation from localizer and the vertical bar indicates deviation from glideslope. 


A localizer bar moves left/right over a deviation scale to display lateral guidance information. The glide slope deviation pointer moves up/down over a scale to indicate vertical deviation.


Each dot on the localizer scale represents a 0.5° deviation from the selected runway heading. Full scale deflection of the needle indicates that the aircraft is 2.5° or more left or right of the centre-line.


Each dot on the glideslope scale represents a 0.14° deviation from the glidepath. Full scale deflection indicates that the aircraft is 0.7° or more above or below the glide path.


The are two one for localizer and other for glideslope. The flags appear when 

  • localizer and/or glide slope signals are beyond reception range pilot has not selected an ILS frequency

  • ILS system is turned off, or is inoperative.



Electronic Horizontal Situation Indicator (ILS mode)

The Modern aircraft display the Horizontal Situation Indicator in the electronic form on the Navigation Display of the Electronics Flight Instrument System (EFIS), the display can be presented in two form

  • ILS Full Mode

  • ILS Expanded mode

ILS Full Mode

With an ILS frequency selected, the EHSI displays a full compass rose with the ILS source in the lower left and the frequency in the lower right.


Course selection (localizer) is displayed by the magenta course needle, the tip pointing to the selected course. Localizer deviation is shown by the traditional deviation bar moving across a two dot left and two dot right scale. This scale is exponential.

Glide slope deviation is shown by a magenta coloured triangle moving up and down the traditional scale on the right hand side.

DME distance is displayed in the top left corner.

Current heading is shown in the window and by the lubber line at the top of the compass rose (130), the current selection is Magnetic Heading as shown either side of the window.

Current track is shown by the white triangle on the inside edge of the compass rose.

Selected heading is shown by the magenta heading “bug” on the outer scale of the compass rose.

Wind speed and direction are shown in the lower left corner orientated to the display selection (Heading or Track, Magnetic or True).

ILS Expanded Mode

With an ILS frequency selected, the EHSI displays about 90° of compass rose with the ILS source in the lower left and the frequency in the lower right.

The white triangle at the bottom of the display is the aircraft symbol.

Selected course (track) is displayed by the magenta course needle, the tip pointing to the selected course. The course selectors are usually on either side of the autoflight main control panel (one for the Captain and one for the First Officer).


Localizer deviation is shown by the traditional deviation bar moving across a two dot left and two dot right scale. Glide slope deviation shown on the right again in the traditional fashion.

DME distance is displayed in the top left corner.

Current heading is shown in the window and by the lubber line at the top of the compass rose. In this case the heading is 130° Magnetic, as indicated by markings either side of the window. Current track is shown by the white line from the tip of the aircraft symbol to the inside edge of the compass rose.

Selected heading is shown by the magenta heading “bug” on the outer scale of the compass rose. Wind speed and direction are shown in the lower left corner orientated to the display selection (Heading or Track, Magnetic or True).


Weather radar displays are available, when selected “on”, range arcs are also visible. Weather radar is shown in three colours: green, yellow and red, green being the least turbulence, red being the worst. If TURBULENCE MODE is available, it is shown as magenta, the area of greatest activity in the cloud.

The range of the display can be selected on the control panel; half scale range is displayed (10 NM) so this display is selected to 20 NM. The outer arc of the compass rose is the furthest range from the aircraft.

EHSI in ILS mode.jpg


Low range radio altimeter

The LRRA incorporates an adjustable altitude bug that creates a visual or aural warning to the pilot when the aircraft reaches the selected altitude. Typically, the pilot will abort the approach if the runway is not visible when the decision height is reached.

ILS approach

The normal procedure is to capture the localizer first and then the glide slope. The crew select the ILS frequency on the navigation control panel, runway heading is also sent to the ILS receiver.

An aircraft approaching the runway centre line from the right will receive more of the 150 Hz signal than the 90 Hz modulation. This difference in depth of modulation (DDM) relates to the angular displacement of the aircraft from the centre line; it energizes the vertical needle of the ILS indicator, i.e. Go Left.

Similarly an aircraft approaching the runway centre line from the left will receive more of the 90 Hz signal than the 150 Hz modulation; the DDM energises the vertical needle, i.e. Go Right.

A DDM of zero indicates a balance between modulations, a zero needle-deflection and hence the runway centre line.

Localizer Navigation.jpg


When you are aligned to the centre of the runway and  the glideslope is intercepted from underneath, so you don't intercept a false glideslope. After you intercept the glideslope, you start a gradual, (typically) 3 degree descent toward the runway.

Deviation from the localizer and glide slope is monitored throughout the approach together with confirmation of position from the marker beacons. The ILS can be used to guide the crew on the approach using instruments when flying in good visibility.

As you intercept the glideslope and start descending toward the runway, deviation from the localizer and glide slope is monitored throughout the approach that you need to fly left/right to stay on course, or increase/decrease descent rate to stay on glideslope; together along with indications from the marker beacons.

As you get close to the runway, the localizer and glideslope signals become more sensitive, because the course width of both decreases the closer you get to the runway.

Localizer back-beam or back-course (BC) Approach

All localizer antennas actually transmit in two directions; with

  • Primary Lobes (forward-course)

  • Secondary Lobes (back course)


The "front course" is the LOC navigation used to fly a standard ILS or LOC approach. When flying standard approaches, the localizer is situated at the departure end of the runway you're landing on.

When you're using LOC BC approach, your receiver references signals emitting from opposite side of the localizer antenna.

The advantage is that same ILS systems can be used for each runway direction, back course approaches utilise the same localizer beam. Note that most back course approaches do not have glide slopes.


As the back course approach are the reciprocal of the main precision approach runway, the CDI needle will give reverse indications hence when flying inbound on the back course it is necessary to steer the aircraft in the direction opposite the needle deflection when making corrections from off-course to on-course.


Hence when you're established on the LOC BC approach, you will always "reverse sense" using a CDI with OBS. If you're in this situation, try saying out loud "fly away from the needle."

An HSI will give correct indications provided that the front course QDM has been selected.

Localizer Back Course.jpg



In the event that visibility is not good, then the approach is flown using the AFCS. The crew select localizer and glide slope as the respective roll and pitch modes on the AFCS mode control panel (MCP).


Automatic approaches are usually made by first capturing the localizer (LOC) and then capturing  the glide slope (GS).


The localizer is intercepted from a heading hold mode on the AFCS, with LOC armed on the system.


The active pitch mode at this point will be altitude hold, with the GS mode armed. Once established on the localizer, the glide slope is captured and becomes the active pitch mode.


The approach continues with deviations from the centreline and glide slope being sensed by the ILS receiver; these deviations are sent to roll and pitch channels of the AFCS, with sensitivity of pitch and roll modes being modified by radio altitude.


The auto throttle controls desired airspeed. Depending on aircraft type, two or three AFCS channels will be engaged for fully automatic landings, thus providing levels of redundancy in the event of channel disconnects.


Although the glide slope antenna is located adjacent to the touchdown point on the runway, it departs from the straight-line guidance path below 100 feet. The approach continues with radio altitude/descent rate being the predominant control input into the pitch channel.


At approximately 50 feet, the throttles are retarded and the aircraft descent rate and airspeed are reduced by the ‘flare’ mode, i.e. a gradual nose-up attitude that is maintained until touchdown.


The final pitch manoeuvre is to put the nose of the aircraft onto the runway. Lateral guidance is still provided by the localizer at this point until such time as the crew take control of the aircraft.


The Microwave Landing System (MLS) was designed to replace ILS with an advanced precision approach system that would overcome the disadvantages of ILS and also provide greater flexibility to its users.


MLS is based on the principle of time referenced scanning beams and provides precision navigation guidance for approach and landing. The system provides three-dimensional approach guidance, i.e. azimuth, elevation and range; it also provides multiple approach angles for both azimuth and elevation guidance.

The ILS system has the following disadvantages

ILS has the following disadvantages:

  • There are only 40 channels available worldwide.

  • The azimuth and glide slope beams are fixed and narrow. As a result, aircraft have to be sequenced and adequately separated which causes landing delays.

  • There are no special procedures available for slower aircraft, helicopters, and Short Take-off and Landing (STOL) aircraft.

  • ILS cannot be sited in hilly areas and it requires large expanses of flat, cleared land to minimize interference with the localizer and glide slope beams.

  • Vehicles, taxiing aircraft, low-flying aircraft and buildings have to be kept well away from the transmission sites to minimize localizer and glide slope course deviations (bending of the beams).

Microwave Landing System (MLS)

The principle of MLS allows curved, or segmented approaches in azimuth together with selectable glide slope angles. All of these features are beneficial in mountainous regions, or for environmental reasons, e.g. over residential areas of a town or city. MLS installations are not affected by ground vehicles or taxiing aircraft passing through the beam as with the localizer.

The Microwave Landing System (MLS) has the following features:

  • There are 200 channels available worldwide.

  • The azimuth coverage is at least ± 40° of the runway on-course line (QDM) and glide slopes from 0.9° to 20° can be selected. The usable range is 20-30 NM from the MLS site; 20 NM in the UK.

  • There is no problem with back course transmissions; a secondary system is provided to give overshoot and departure guidance ± 20° of runway direction up to 15° in elevation to a range of 10 NM and a height of 10000 ft.

  • It operates in the SHF band, 5031 - 5090.7 MHz. This enables it to be sited in hilly areas without having to level the site. Course deviation errors (bending) of the localizer and glide path caused by aircraft, vehicles and buildings are no longer a problem because the MLS scanning beam can be interrupted and therefore avoids the reflections.

  • 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.

  • The MLS has a built-in DME.

  • MLS is compatible with conventional 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 glide slope needles.

  • The identification prefix for the MLS is an ‘M’ followed by two letters.

  • The aim is for all MLS equipped aircraft to operate to CAT III criteria.

MLS Operation

The system is based on the principle of time referenced scanning beams and operates in the C-band at 5 GHz. Two directional fan-shaped beams are used for azimuth and elevation guidance. The azimuth approach transmitter is located at the stop end of the runway; the elevation transmitter is located near the threshold of the runway. Azimuth scanning is through ±40° either side of the runway centreline with a range of 20 nm.


An expansion capability can extend azimuth coverage to ±60°, but with a reduced range of 14 nm. Elevation scanning sweeps over an angle of 15 degrees (with 20 degrees as an option) providing coverage up to 20,000 feet.


At the aircraft receiver, a pulse is detected each time the respective beams sweep past the aircraft.

Aircraft equipment

The aircraft is fitted with two antennas located on the nose and aft centrelines. An MLS receiver (often incorporated into a multi-mode receiver with ILS, marker beacon and VOR capability) is tuned into one of 200 channels and calculates azimuth and elevation guidance as described. The receiver operates in the frequency range 5031 MHz to 5091 MHz with 300 kHz spacing.


An integral part of the MLS is a distance measuring equipment (DME) system to provide range; this can either be a conventional DME system or a dedicated system operating in the 962 MHz to 1105 MHz frequency range. DME frequencies are automatically tuned with the azimuth and elevation beams to provide range information.

Ground equipment

The items of ground equipment needed for MLS are the azimuth and elevation transmitters and a DME navigation aid. This basic system can be expanded to provide lateral guidance for missed approaches. Both azimuth and elevation transmissions are radiated on the same frequency with a time-sharing arrangement.


In addition to guidance, the MLS also transmits data to system users. Basic data includes runway identification (four-letter Morse code), together with locations and performance levels of the azimuth, elevation transmitters and the DME transponder. The expanded data transmission provides runway conditions and meteorological data, e.g. visibility, cloud base, barometric pressures, wind speed/direction and any wind shear conditions.


Locations of the ground equipment are not as critical as with ILS; this is particularly useful in mountainous regions. Military users of MLS take advantage of this by having mobile systems that can be deployed within hours. The azimuth transmitter has an accuracy of 4 meters at the runway threshold. The elevation transmitter has an accuracy of 0.6 meters. The dedicated DME navigation aid has a range accuracy of 100 feet. 

MLS summary

Despite the advantages of MLS, it has not yet been introduced on a worldwide basis for commercial aircraft. The advent and development of global navigation satellite systems have led to the reality of precision approaches and automatic landings being made under the guidance of satellite navigation systems during low visibility; however, this is not likely to be available for some time.


Since MLS technology is already available, a number of European airlines have been lobbying for MLS; ground equipment has been installed at a number of airports, including London Heathrow and Toulouse Blagnac, for development purposes. The reader is encouraged to monitor the industry press for developments of this subject.

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