Instrument landing system

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ILS functional diagram (GP = glide slope transmitter, L = landing course transmitter, σ = landing course level, λ = glide slope level)
Illustration of the transmission lobes of the landing course transmitter (localizer) and glide slope transmitter (glideslope)

The instrument landing system (engl. I instrument l anding s ystem , ILS ) is a ground-based system that the pilot of an aircraft in the phases of flight in front of the landing supported. The landing approach ends with the final approach .

Two guide beams, which determine the course (direction) and glide path ( height above ground ) for the landing, are processed by a special ILS receiver and displayed on a display device. In the event of deviations from the target values ​​based on the representation with vertical and horizontal pointers on the ILS display instrument, the pilot ( PIC ) is able to carry out precision approaches ( IMC ) even in poor visibility conditions . Two or three entry signs ( marker beacons ) are also available to signal the remaining distance to the runway , but these are gradually being replaced or supplemented by the newer DME technology.


In the German Reich of the Weimar Republic from 1931 the Deutsche Luft Hansa tested the ZZ procedure for landings in poor visibility. This first ground-based landing system was time-consuming and required a great deal of effort from the pilot and ground personnel. The airport's direction finding station had to transmit information to the pilot or navigator during the entire approach.

Lorenz beam method

In the early 1930s, the Berlin company C. Lorenz AG developed an automated process that worked without the help of ground staff. The first of these landing radio beacons (LFF), also known as the " Lorenzbake ", was set up by the company in 1933 at Berlin-Tempelhof Airport . The new “Lorenz landing procedure” only offered lateral guidance with the landing course transmitter. In addition, two entry signal transmitters (pre- and main entry signs, VEZ / HEZ) were set up to signal the runway at a certain distance in front of it. A representation of the glide path (vertical guidance) had not yet been realized. The next user was the Zurich-Dübendorf airfield and “Lorenzbaken” were soon in use abroad as well as in South Africa and Australia. At the end of the 1930s, the German air force equipped its air bases and the larger twin-engine machines with Lorenz systems (see also: Lorenz radio navigation and landing systems ). In the spring of 1941, the Askania plants in Germany carried out their first successful trials with a fully automatic landing system with a Junkers Ju 52 / 3m . The Second World War prevented further development.

The first tests with an instrument landing system began in the USA in 1929. The practicability of the ILS developed there was first proven on January 26, 1938, when a Boeing 247 landed after a flight from Washington, DC to Pittsburgh (Pennsylvania) during a snow storm and had to rely exclusively on the ILS.

From 1946, the system that is still used today was introduced worldwide.


The ILS ground system consists of a total of four, occasionally five transmitters: the localizer , which shows the lateral deviation, and the glideslope or glidepath , which is responsible for the vertical guidance of the approaching aircraft. There is also an automatic monitoring station (NFM - near field monitor for CAT I, additionally FFM - far field monitor for CAT II / III), which monitors the emitted signals and, if necessary, switches off the entire system if the signals are outside a specified tolerance.

Up to three approach signal transmitters ( marker beacon ) are used to signal the distance to the runway . These fore (outer marker, OM or LOM), main (middle marker, MM) and the inner marker (IM), which is no longer in use in Germany, are becoming increasingly rare and are being exchanged for DME systems. In the medium term, the ILS systems at all 16 international airports in Germany will be converted to ILS / DME systems, which no longer have marker beacons. The pre-entry sign (OM) and the main entry sign (MM) are replaced by a so-called DME reading. The frequency pairing of localizer and DME is mandatory in this case (as soon as the MM is no longer available).

Landing course transmitter

Landing course transmitter (back side) for runway (RWY) 27R of Hannover-Langenhagen airport ( EDDV / HAJ )

The antenna system of the landing course transmitter ( localizer , LOC or LLZ) is set up in the departure sector, approx. 300 m behind the end of the runway ( stop end of runway ) and consists of several directional antennas arranged in pairs (stretched λ / 2 dipoles ). The localizer informs the pilot about his lateral position in relation to the approach baseline (English centerline ) and thus shows the pilot whether he has to fly further to the right or left in order to touch down exactly in the center of the runway.

The localizer has a transmission power of 25 to 50 watts and works in a frequency range from 108.10 MHz to 111.95 MHz. On the respective carrier frequency, two signals with 90 and 150 Hz are modulated in amplitude modulation, which are emitted by the antennas in such a way that a radiation maximum lies along the approach base line. It is called the CSB signal ( Carrier and Side Band ). Another signal in amplitude modulation with suppressed carrier , the so-called SBO signal ( Side Bands Only ), is emitted via the same antennas . Its radiation maxima lie on both sides of the approach baseline, while it becomes zero on this. This creates two modulation fields on the left and right of the web, which overlap in the middle. The localizer receiver in the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. On the approach baseline, the modulation depth for each modulated frequency is 20%, the difference becomes zero, the vertical needle on the display instrument is in the middle. Deviating to the left from the approach baseline, the degree of modulation of the 90 Hz signal increases while it simultaneously decreases for the 150 Hz signal, the vertical needle of the display instrument moves to the right and points the pilot to the right (fly into the needle) , to fly back to the center of the runway. Deviating from the approach baseline in the other direction, the degree of modulation of the 90 Hz signal decreases, while for the 150 Hz signal it increases at the same time, the vertical needle of the display instrument moves to the left and points the pilot to the left to return to the Fly to the middle of the runway.

The degree of modulation difference DDM between the two signals changes linearly depending on the position of the approaching aircraft up to the respective full deflection of the pointer instrument (cross pointer, cross pointer) at 5 points, which corresponds to a DDM of 15.5%. The approach baseline is thus formed as a line with a constant DDM = 0.

The localizer can also be used when approaching from the other side. This process is called backcourse , because the backbeam of the antennas, i.e. the radiation in the opposite direction, is used here. However, there is no glideslope support on a backcourse approach. Since there is no vertical guidance through the glide slope during a backcourse approach, such an approach is a purely non-precision approach with very high minimum descent altitudes MDA / MDH. Please also note that you now receive an inverted signal. If you receive a localizer display from too far to the right when approaching the backbeam, you have to steer in the opposite direction, i.e. to the right, in order to get on the correct course. This approach procedure is no longer permitted in Germany.

Glideslope transmitter

Glide path transmitter for RWY 09R at Hanover Airport

The signal of the Gleitwegsenders ( English glideslope , short GS or glide slope , short G / S, also glide path transmitter , short GP) is processed in the glideslope receiver and indicates to the pilot the vertical deviation from the optimal glide path ( glide path , GP) to . At an approach angle (approach profile / gradient ) of 3 degrees and a RDH ( Reference date Height ) of 50 ft (15 m) which is the touchdown point approximately 280 m behind the landing threshold ( threshold , THR ).

The transmitter is to the side of the runway at the level of the touchdown point and operates on a frequency in the 329–335 MHz range, which is significantly higher than the landing course transmitter . The two frequencies ( channels ) of the landing course transmitter and glide slope transmitter are permanently paired with one another; the pilot only needs to set the landing course transmitter and the frequency of the associated glide slope transmitter is automatically selected. The functional principle is analogous to the landing course transmitter, only the two radiation lobes of the glideslope transmitter are aligned vertically instead of horizontally as with the localizer. Two signals with 90 and 150 Hz with a modulation depth of 40% are modulated on the respective carrier frequencies in amplitude modulation, which are radiated by the antennas in such a way that a radiation maximum lies along the 3 ° approach path, known as the Carrier Side Band (CSB) . A further signal without a carrier component (double- side band / suppressed carrier - DSBSC), the so-called Side Band Only (SBO), is emitted via the same antennas, amplitude-modulated . Its radiation maxima are below and above the 3 ° approach path, while it becomes zero on this. This creates two modulation fields below and above the 3 ° approach path, which overlap in the middle. The glideslope receiver in the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. On the 3 ° approach gradient, the modulation depth for each modulated frequency is 40%, the difference becomes zero, the horizontal needle on the display instrument is in the middle.

In the case of deviations from the 3 ° approach gradient upwards (machine too high), the degree of modulation of the 90 Hz signal increases, while it decreases for the 150 Hz signal, the horizontal needle of the display instrument moves downwards and shows the pilot that he has to go deeper with the aircraft (“fly into the needle”) in order to return to the 3 ° approach gradient. If it flies too low, the degree of modulation of the 90 Hz signal decreases, while it increases at the same time for the 150 Hz signal, the horizontal needle of the display instrument moves upwards and shows the pilot that he has to gain altitude.

The approach angle for an ILS approach CAT I is typically between 2.5 and 3.5 degrees, ideally 3.0 degrees. In London City the GP is 5.5 °. During the ILS approach CAT II / III, the glide angle must be 3 degrees. The display device shows the pilot whether he has to steer up or down in order to reach the touchdown point of the runway. In almost all modern aircraft, the incoming signals from the instrument landing system can be used by the autopilot so that an approach can take place automatically. Depending on the approach category, the pilot takes over manual control and lands before landing, or after an automatic landing when the aircraft is already coasting.

Entry sign

Middle marker transmitter Hannover RWY EDDV 09L
Outer marker transmitter RWY EDLV 27 of Niederrhein Airport (Airport Weeze )

Entry signs or markers are radio beacons that radiate vertically upwards with a transmission power of 0.2 to 0.5 watts. They work in the frequency range 74.6 to 75.4 MHz and are usually four (Outer Marker, OM or LOM) or half a (Middle Marker, MM) nautical mile  - NM equals 7200 m or 1050 m - before the landing threshold ( English threshold , THR ). When flying over them, they trigger an acoustic signal and / or a flashing display.

During the final approach , pre-entry signs (Outer Marker, OM or LOM), main entry signs (Middle Marker, MM) and - the no longer used in Germany - space entry signs (Inner Marker, IM) are flown over one after the other. The corresponding sound signal becomes higher and higher.

More and more frequently, the approach signal transmitters are supplemented by distance beacons DME ( English Distance Measuring Equipment ), which enable a continuous display of the distance to the landing threshold. The transmitting and receiving antennas of the DME are also attached to the mast of the glide slope transmitter. The distance display in the cockpit is numerical in nautical miles.

Outer marker (OM or LOM)

The pre-entry sign, the outer marker , is located 7200 m ± 300 m in front of the landing threshold and is modulated in amplitude with a tone of 400 Hz (300 ms on, 100 ms off). In the cockpit, a low 400 Hz tone ("−−−") can be heard during the overflight and the blue "pre-entry sign" indicator lights up. The outer marker is used to check the altimeter ( barometric or additionally a radar altimeter ).

During an ILS approach, the aircraft must be on the OM on the glide path. For this reason, the height above ground ( height ; HGT) of the glide path required at the OM is indicated on every approach chart. If the OM is approx. 4 NM in front of the landing threshold and the approach angle is to be 3 degrees, the aircraft must still have a HGT of 1320 ft (approx. 400 m) when overflighting the OM  . The calculation is: 4 NM × 318 ft / NM + 50 ft threshold overflight height (RDH).

Blue outer marker light

Outer marker sound:

Middle marker (MM)

The main approach signal , the middle marker , is 1050 m ± 150 m in front of the landing threshold and is more highly modulated than the signal of the outer marker with a tone of 1300 Hz (300 ms on, 100 ms off, 100 ms on, 100 ms off) . Accordingly, a 1300 Hz tone (“- · - · - ·”) can be heard in the cockpit during the overflight and the yellow display “Main entry sign” lights up.

Yellow middle marker light

Middle marker sound:

Inner Marker (IM)

The inner marker is not used in Germany and is also rarely used in civil aviation worldwide . Inner markers are still used in military aviation . These are then directly at the landing threshold, are amplitude-modulated with 3000 Hz and accordingly generate a high 3000 Hz tone (100 ms on, 100 ms off). A white display lights up in the cockpit and a "···" tone can be heard.

White inner marker light

Inner marker sound:

Approach lights

The approach lights are a system of lights that allow the pilot to see the runway shortly before landing . There are different versions that differ in structure (non-precision approaches, CAT I or CAT II / III).

Optical signaling of the glide path (VASI / PAPI)

Optical systems can also be available as a further option for checking the correct glide path, especially for approaches in the dark. These are VASI (Visual Approach Slope Indicator) and PAPI (Precision Approach Path Indicator), which are characterized primarily by their ease of use, but of course both are dependent on sufficient flight visibility.

Board components

AFN 2: "Display device for radio navigation", make Siemens Apparate und Maschinen (SAM), cross pointer instrument of a Lorenz radio landing system , 1943

There are at least receivers on board for the landing course signal and the glide slope signal. A cross pointer instrument, or cross pointer for short, is used to display the landing course and glide slope together . In addition to two pointers that are at right angles on an ideal landing course, it has markers that provide information on whether the display is based on correctly received signals or a display error. The pointer deviations from the center show the direction of the required approach correction.

ILS categories

As with the ILS approach to achieve the is on each instrument approach decision height (engl. Height decision , DH or decision altitude , DA) of the moment in which the cockpit crew of the approaching aircraft will decide on the further implementation of the approach. If the (visibility) conditions (the pilot must recognize the runway or parts of the approach lights) for continuing the approach are not given when the decision height is reached, the approach must be aborted and restarted ( go around ). After deciding for start, the aircraft follows the missed approach procedure (engl. Missed approach procedure ), can be carried out after the end of a renewed approach. Precision approaches, which also include the ILS approach, are divided into different categories depending on various factors:

Cross pointer instrument (Soviet Union approx. 1964). The markers are located to the right or bottom of the pointer ends.
Easiest category with a decision height of 200 ft (60 m) above ground or more and a runway visual range (engl. Runway visual range , RVR) of at least 550 m, or a bottom view of 800 m (the bottom view is determined by an authorized by the authority person )
Medium category with a decision height between 100 ft and less than 200 ft above ground (30–60 m) and an RVR of at least 300 m.

Depending on the technical equipment and the freedom from obstacles of the airfield, CAT III is divided into CAT IIIa, CAT IIIb, and CAT IIIc:

Decision height between 0 ft and less than 100 ft above ground and RVR at least 175 m
Decision height less than 50 ft above ground and RVR less than 175 m, but at least 50 m
No decision height (0 ft) and no RVR (0 m). Not yet approved as a minimum visual range is required on the taxiways .

Technical equipment

The failure of certain components of the aircraft in flight (for example the radar altimeter) directly reduces the aircraft's ability to carry out approaches of higher categories, which in marginal weather conditions makes it necessary for the aircraft to move from the actual destination airport to an alternative destination. The ILS signals from the airfields have to be checked periodically by means of measurement flights, as buildings and construction cranes can interfere with them, for example.

Higher ILS categories also require a greater graduation of the aircraft on approach, which significantly reduces the capacity of the airfields. The reason is that the signals can be disturbed by taxiing and flying aircraft. An example of such a disruption is an incident at Munich Airport in 2011. The approaches were carried out under ILS CAT I. An aircraft taking off interfered with the ILS signals. The autopilot then steered the Boeing 777 , which was flying under CAT IIIB (simulated), to the left before touching down; the plane came off the runway.

For carrying landing in CAT I need the cockpit crew an instrument rating (Engl. Instrument rating to be equipped, I / R) own the aircraft for IFR flights and approved (which are now the most airplanes). The landing as such may, however, be carried out manually by the pilot, i.e. controlled by hand. Pilots with such a rating must regularly perform such flights in accordance with instrument flight rules (IFR), otherwise they lose their authorization for CAT-I approaches, among other things.
Special training or authorization of the crew is necessary. There is no need for an autopilot, but the approach is rarely flown by hand. Instruments must be present in duplicate (an independent display of the landing course / glide path for the captain and first officer). A radar altimeter is also necessary.
Landings according to CAT III must be controlled by multiple autopilots of the aircraft ( auto coupled landing ). These control the aircraft independently of one another using ILS signals that are received independently of one another ( redundancy ). The cockpit crew and the airline must have special authorization. The autopilot must, among other things by radar altimeter to be able, the aircraft upon landing automatically to flare (Engl. Flare ) to intercept and set up, from CAT IIIb should he after touchdown during braking and coasting by nose wheel steering the Localizer follow to the Keep airplane on the runway center.

Exceptions to this are some aircraft with head-up displays , e.g. B. the Canadair Regional Jet (CRJ) , which are also approved for manually controlled CAT III approaches.

First aircraft with CAT III capabilities

Due to the prevailing weather conditions in Europe, but especially in Great Britain, the development of CAT III-capable aircraft initially took place in Europe. In the 1980s, for example, CAT III capability was not yet part of the standard equipment of American aircraft. In contrast, Boeing and Alaska Airlines took the pioneering role in CAT III landings with head-up displays.

The Sud Aviation Caravelle was approved for CAT III approaches in December 1968. This was followed by the Hawker-Siddeley Trident (IIIa 1972, IIIb 1975). The first manual CAT III landing on a passenger flight took place in 1989 ( Alaska Airlines Boeing 727 ).

special cases

The ILS flight path does not necessarily have to lead to a runway. In the case of the former Kai Tak Airport (Hong Kong) , the so-called IGS (Instrument Guidance System) led to a marked and illuminated hill (“Checkerboard Hill”), whereupon the pilots turned off for a visual approach to runway 13.

Even aircraft carrier equipped with ILS systems. To prevent aircraft carriers from being identified based on the ILS signals, all larger warships emit such signals. Regardless of the guidance system used (optical, laser, ILS), the last ¾ nautical miles (1.4 km) must be flown visually.

ILS in Germany

International airports

The DFS has to 16 German rightful international airports 49 ILS systems. Of these, 15 systems meet CAT I and 34 systems CAT II or CAT III.

Regional airports

In Germany there are other commercial airports and airfields that are equipped with an instrument landing system. There are a total of 35 ILS systems. Of these, 27 systems meet CAT I and 8 systems CAT II or CAT III.

Military airfields

The Bundeswehr operates an ILS at eleven of its 28 airfields. All systems comply with CAT I.

An ILS CAT I installed in Celle was dismantled in 1992.

However, instrument approaches of Bundeswehr aircraft are mostly carried out via non-directional radio beacons ( helicopters ) or TACAN or ARA approaches (Airborn Radar Approach) (in each case combat aircraft ) or with the help of the precision approach radar on the airfield . Only transport aircraft or civil co-users of the airfields predominantly use the ILS.

Other airfields with ILS

Further landing aids

In addition to ILS, there are other types of approach . In the military sector, the precision approach radar , a system on par with the ILS (CAT I / CAT II for helicopters), is still in use.

MLS is more accurate than ILS. It will probably not spread further in favor of the EGNOS satellite system . In the USA a landing approach of the category LPV200 is already possible with the satellite system WAAS (in Germany there are already LPV approaches at some airports). The term LPV stands for Lateral Precision with Vertical Guidance . It belongs to the ICAO category APV (Approach with Vertical Guidance), a landing approach without ground support. LPV200 corresponds to CAT I at ILS. The LAAS system, expanded to include ground stations, increases the precision to CAT II and CAT III. The military instrument landing system PRMG is still partly in use in Russian aircraft .


The German Air Traffic Control (DFS) checks (as of mid-2014) new approach procedures for. B. the "continuous descent approach" (continuous glide path). Airplanes approaching an airport ( descent ) would save fuel and emit significantly less aircraft noise than if they descend in stages (on horizontal sections they always have to fly with higher power).

Web links

Commons : ILS  - collection of images, videos and audio files
  • Bernd Büdenbender ( DFS employee): Instrument landing systems, on, accessed on March 11, 2016 (PDF; 0.7 MB)

Individual evidence

  1. Bernd Büdenbender: Instrumentenlandesysteme
  2. ^ Federal Bureau of Aircraft Accident Investigation : "Investigation Report: Serious incident, Munich, November 3, 2011". Retrieved March 24, 2019 .
  3. ^ Military Aviation Handbook Germany. (pdf) In: Bundeswehr Air Traffic Control Office, October 23, 2008, accessed on October 31, 2008 (English).
  4. August 11, 2014: Do airliners have to be that loud?