![ILS localizer illustration](https://upload.wikimedia.org/wikipedia/commons/0/05/ILS_localizer_illustration.svg)
An instrument landing system localizer, or simply localizer (LOC),
is a system of horizontal guidance in the
instrument landing system
In aviation, the instrument landing system (ILS) is a precision radio navigation system that provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather. In its original form, it allows an aircraft to ...
, which is used to guide aircraft along the axis of the
runway
According to the International Civil Aviation Organization (ICAO), a runway is a "defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft". Runways may be a man-made surface (often asphalt concrete, as ...
.
Principle of operation
In aviation, a localizer is the lateral component of the ''instrument landing system'' (ILS) for the runway centerline when combined with the vertical
glide slope
Instrument landing system glide path, commonly referred to as a glide path (G/P) or glide slope (G/S), is "a system of vertical guidance embodied in the instrument landing system which indicates the vertical deviation of the aircraft from its o ...
, not to be confused with a
locator
Locator may refer to:
* One who locates, or is entitled to locate, a land or mining claim
* ''Lokator'' (in Latin ''locator''), a medieval servant in charge of organizing colonization and settlement
* Locator map
* Locator software, a type of e ...
, although both are parts of aviation navigation systems.
A localizer (like a glideslope) requires both a transmitting airport runway system and receiving
cockpit
A cockpit or flight deck is the area, usually near the front of an aircraft or spacecraft, from which a Pilot in command, pilot controls the aircraft.
The cockpit of an aircraft contains flight instruments on an instrument panel, and the ...
instruments. An older aircraft without an ILS receiver cannot take advantage of any ILS facilities at any runway, and much more importantly, the most modern aircraft have no use of their ILS instruments at runways which lack ILS facilities. In parts of Africa and Asia large airports may lack any kind of transmitting ILS system. Some runways have ILS only in one direction, this can however still be used for the opposite direction (with a lower precision) known as ''back beam'' or "Back Course" which is not associated with a
glide slope
Instrument landing system glide path, commonly referred to as a glide path (G/P) or glide slope (G/S), is "a system of vertical guidance embodied in the instrument landing system which indicates the vertical deviation of the aircraft from its o ...
.
Two signals are transmitted on one of 40 ILS channels. One is
amplitude modulated
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to t ...
at 90 Hz, the other at 150 Hz. These are transmitted from co-located
phased array
In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
antenna elements. Each antenna transmits a narrow beam. In addition, a ''clearing'' signal is transmitted at one tenth of the power with a wider beam to prevent receivers from picking up the
side lobe
In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the ''main lobe''.
The radiation pattern of most antennas shows a pattern of "''lobes'' ...
s of the main beam.
The signals' phases at the antenna elements are arranged such that the 150 Hz signal is more prominent (has a greater depth of modulation) at a receiver located to the left of centerline, and the 90 Hz signal is more prominent to the right. The cockpit instrument uses the difference between the modulation strengths of the two received signals to indicate left or right deviation from centerline.
Carrier frequency pairings
Localizer (LOC) and glide slope (G/S) carrier frequencies are paired so that the navigation radio automatically tunes the G/S frequency which corresponds to the selected LOC frequency. The LOC signal is in the 110 MHz range while the G/S signal is in the 330 MHz range.
LOC
carrier
Carrier may refer to:
Entertainment
* ''Carrier'' (album), a 2013 album by The Dodos
* ''Carrier'' (board game), a South Pacific World War II board game
* ''Carrier'' (TV series), a ten-part documentary miniseries that aired on PBS in April 20 ...
frequencies range between 108.10 MHz and 111.95 MHz (with the 100 kHz first decimal digit always odd, so 108.10, 108.15, 108.30, etc., are LOC frequencies and are not used for any other purpose).
Localizer in cockpit
![VMS Artificial Horizon](https://upload.wikimedia.org/wikipedia/commons/c/c5/VMS_Artificial_Horizon.jpg)
The localizer indicator is (on most aircraft manufactured from the late 1950s) shown ''below'' the Attitude Indicator, but is still a part of this instrument together with the
glideslope
Instrument landing system glide path, commonly referred to as a glide path (G/P) or glide slope (G/S), is "a system of vertical guidance embodied in the instrument landing system which indicates the vertical deviation of the aircraft from its o ...
indicator and the cross in the center of the instrument which is called
Flight Director
Flight controllers are personnel who aid space flight by working in such Mission Control Centers as NASA's Mission Control Center or ESA's European Space Operations Centre. Flight controllers work at computer consoles and use telemetry to mon ...
.
The glideslope scale is located to the right of the attitude sphere. On aircraft which have a mechanical gyro compass are both the localizer and glideslope indicated as a vertical and a horizontal arrow in the compass as well. But they are essentially read in the same way. On some aircraft is only the glideslope indicated on two main instruments, and the oldest version of ILS-instruments was an instrument of its own used instead. This used
two dangling bars, fixed in the middle of the top (localizer indicator) and in the middle of the left side (glideslope indicator), and if the aircraft was located on the intended glidepath, the dangling bars formed a cross. This is, in theory, however, more difficult to learn—but even for pilots experienced with using such indicators, it added another instrument they needed to focus on. With the indicators added to the artificial horizon (and to the compass), the pilot can theoretically watch the attitude simultaneously with the localizer and glideslope.
In modern cockpits, the localizer is seen as a colored dot (usually in the shape of a diamond) at the bottom of the
artificial horizon
The attitude indicator (AI), formerly known as the gyro horizon or artificial horizon, is a flight instrument that informs the pilot of the aircraft orientation relative to Earth's horizon, and gives an immediate indication of the smallest orien ...
gauge. It does not appear during cruise, but comes up during the descent and approach to the selected runway, provided that the
navigation radio
Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination.
The basic principles a ...
is set to the ILS frequency of that specific runway. If the transmitted localizer beam, which usually, but not always, is directed in the heading of the runway extension. (exceptions exist, for instance, in Innsbruck, Austria and in Macao)
If the aircraft is located on this line, the localizer dot will appear in the middle of the scale. But if the aircraft is located a little ''left'' of the beam, the marker will appear to the ''right'' on the localizer gauge scale in cockpit. The pilot then knows he or she must adjust the heading ''towards'' the dot.
In older cockpits, the localizer scale below the artificial horizon is rather short. But in older style cockpit instrumentation, the localizer also appears as an arrow in the
gyro compass
A gyrocompass is a type of non-magnetic compass which is based on a fast-spinning disc and the rotation of the Earth (or another planetary body if used elsewhere in the universe) to find geographical direction automatically. The use of a gyroc ...
below the artificial horizon. The top and bottom of this arrow "is one unit", which shows current heading. But the middle part of this arrow is moving independently of the aircraft's heading. The middle of that arrow could be described as being "stand alone", and moves to the left if the aircraft is located to the right of localizer beam and to the right if the aircraft is located to the left of the localizer beam. When the arrow is "united" to a straight line, then the aircraft is following the localizer beam. (This second "arrow-indicator" is omitted in modern cockpits, but the main compass is still located below the artificial horizon.)
The very first generation of localizer gauges had a different cockpit interface, and were not included in the artificial horizon nor any compass, but at a gauge of its own. The localizer was then represented as a dangling stick hanging from a fixed point at the top of a separate gauge, and the glideslope was represented by a similar, but horizontal, dangling stick, fixed at one of the sides of the gauge. When the aircraft was located exactly at the ILS-beam (or glidepath) the two sticks formed a cross. This interface resembles the
flight director
Flight controllers are personnel who aid space flight by working in such Mission Control Centers as NASA's Mission Control Center or ESA's European Space Operations Centre. Flight controllers work at computer consoles and use telemetry to mon ...
, which also forms a cross, but on the artificial horizon.
This older ILS instrumentation system was omitted around the same time as jet airliners like
Boeing 707
The Boeing 707 is an American, long-range, narrow-body airliner, the first jetliner developed and produced by Boeing Commercial Airplanes.
Developed from the Boeing 367-80 prototype first flown in 1954, the initial first flew on December 20, ...
and
DC 8
The Douglas DC-8 (sometimes McDonnell Douglas DC-8) is a long-range narrow-body airliner built by the American Douglas Aircraft Company.
After losing the May 1954 US Air Force tanker competition to the Boeing KC-135, Douglas announced in Ju ...
were introduced.
The expression "catch the localizer" refers to runway approaches with the
autopilot
An autopilot is a system used to control the path of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator' ...
engaged. The angle between the aircraft heading and localizer beam should be less than 30 degrees, and the
indicated airspeed
Indicated airspeed (IAS) is the airspeed of an aircraft as measured by its pitot-static system and displayed by the airspeed indicator (ASI). This is the pilots' primary airspeed reference.
This value is not corrected for installation error, inst ...
at least below 250 knots (for jet airliners), then by pushing a button marked "APP" or "ILS", then the autopilot presumably will turn and then follow the localizer. The autopilot will then also automatically descend according to the
glideslope
Instrument landing system glide path, commonly referred to as a glide path (G/P) or glide slope (G/S), is "a system of vertical guidance embodied in the instrument landing system which indicates the vertical deviation of the aircraft from its o ...
. Normal procedure is to capture the localizer first and then follow the glideslope as well. If the angle is too large or the airspeed too high, capturing the localizer may be unsuccessful.
Modern aircraft can land "themselves", provided the runway ILS is of sufficient standard (class III C) and the cross wind component is low. Autolandings are mostly done when the visibility is low.
The cockpit ILS indicators are not to be confused with the
flight director
Flight controllers are personnel who aid space flight by working in such Mission Control Centers as NASA's Mission Control Center or ESA's European Space Operations Centre. Flight controllers work at computer consoles and use telemetry to mon ...
, which also places vertical and horizontal lines on the artificial horizon. A flight director only shows how the autopilot would fly. If the localizer dot (or arrow) indicate runway is to be found to the left, but the flight director suggests a right turn, and the runway isn't visible, then the pilot in command is having difficulties.
Localizer at runways
![Localizer](https://upload.wikimedia.org/wikipedia/commons/0/08/Localizer.png)
When the glideslope is unserviceable, the localizer element can often be conducted as a separate non-precision approach, abbreviated to 'LOC'. A standalone
instrument approach
In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landi ...
installation without an associated glidepath carries the abbreviation 'LLZ'.
See also
*
AN/MRN-1 The AN/MRN-1 was an instrument approach localizer used by the Army Air Force during and after World War II. It was standardized on 3 July 1942. It replaced the SCR-241, and was a component of SCS-51.
Use
The transmitter provides a signal to guide ...
*
Andrew Alford
Andrew Alford (5 August 1904 – 25 January 1992) was an American electrical engineer and inventor.
Born in Samara, Russia, Alford invented and developed antennas for radio navigation systems, now used for VHF omnidirectional range and instrument ...
*
Localizer type directional aid
A localizer type directional aid (LDA) or Instrument Guidance System (IGS) is a type of localizer-based instrument approach to an airport. It is used in places where, due to terrain and other factors, the localizer antenna array is not aligned wit ...
(LDA)
*
Simplified directional facility
Simplified directional facility (SDF) is a localizer-based instrument non-precision approach to an airport, which provides final approach course similar to instrument landing system (ILS) and localizer type directional aid (LDA) approaches, altho ...
References
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Aeronautical navigation systems
Aircraft landing systems
Radio navigation
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