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An instrument landing system (ILS) is a radio beam transmitter that provides a direction for approaching aircraft that tune their receiver to the ILS frequency. It provides both lateral and a vertical signals. It is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions (IMC), such as low ceilings or reduced visibility due to fog, rain, or blowing snow.
An instrument approach procedure chart (or approach plate) is published for each ILS approach to provide the information needed to fly an ILS approach during instrument flight rules (IFR) operations. A chart includes the radio frequencies used by the ILS components or navaids and the prescribed minimum visibility requirements.
Radio-navigation aids must provide a certain accuracy (set by international standards of CAST/ICAO); to ensure this is the case, flight inspection organizations periodically check critical parameters with properly equipped aircraft to calibrate and certify ILS precision.
An aircraft approaching a runway is guided by the ILS receivers in the aircraft by performing modulation depth comparisons. Many aircraft can route signals into the autopilot to fly the approach automatically. An ILS consists of two independent sub-systems. The localizer provides lateral guidance; the glide slope provides vertical guidance.
A localizer is an antenna array normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. Two signals are transmitted on one of 40 ILS channels. One is modulated at 90 Hz, the other at 150 Hz. These are transmitted from co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other slightly to the right.
The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. The depth of modulation for each of the modulating frequencies is 20 percent. The difference between the two signals varies depending on the deviation of the approaching aircraft from the centerline.
If there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centerline. In the cockpit, the needle on the instrument part of the ILS (the omni-bearing indicator (nav indicator), horizontal situation indicator (HSI), or course deviation indicator (CDI)) shows that the aircraft needs to fly left or right to correct the error to fly toward the center of the runway. If the DDM is zero, the aircraft is on the LOC centerline coinciding with the physical runway centerline. The pilot controls the aircraft so that the indicator remains centered on the display (i.e., it provides lateral guidance).
A glide slope station is an antenna array sited to one side of the runway touchdown zone. The GS signal is transmitted on a carrier frequency using a technique similar to that for the localizer. The center of the glide slope signal is arranged to define a glide path of approximately 3° above horizontal (ground level). The beam is 1.4° deep (0.7° below the glide-path center and 0.7° above).
The pilot controls the aircraft so that the glide slope indicator remains centered on the display to ensure the aircraft is following the glide path to remain above obstructions and reach the runway at the proper touchdown point (i.e., it provides vertical guidance).
LOC and GS carrier frequencies are paired so that the navigation radio automatically tunes the GS frequency which corresponds to the selected LOC frequency.
LOC carrier 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). See Instrument Landing System (ILS) Frequencies on even-numbered TACAN channels from 18X to 56Y.
Due to the complexity of ILS localizer and glide slope systems, there are some limitations. Localizer systems are sensitive to obstructions in the signal broadcast area like large buildings or hangars. Glide slope systems are also limited by the terrain in front of the glide slope antennas. If terrain is sloping or uneven, reflections can create an uneven glidepath causing unwanted needle deflections. Additionally, since the ILS signals are pointed in one direction by the positioning of the arrays, glide slope supports only straight-line approaches with a constant angle of descent. Installation of an ILS can be costly because of siting criteria and the complexity of the antenna system.
ILS critical areas and ILS sensitive areas are established to avoid hazardous reflections that would affect the radiated signal. The location of these critical areas can prevent aircraft from using certain taxiways leading to delays in takeoffs, increased hold times, and increased separation between aircraft.
In addition to the previously mentioned navigational signals, the localizer provides for ILS facility identification by periodically transmitting a 1,020 Hz Morse code identification signal. For example, the ILS for runway 4R at John F. Kennedy International Airport transmits IJFK to identify itself, while runway 4L is known as IHIQ. This lets users know the facility is operating normally and that they are tuned to the correct ILS. The glide slope station transmits no identification signal, so ILS equipment relies on the localizer for identification.
It is essential that any failure of the ILS to provide safe guidance be detected immediately by the pilot. To achieve this, monitors continually assess the vital characteristics of the transmissions. If any significant deviation beyond strict limits is detected, either the ILS is automatically switched off or the navigation and identification components are removed from the carrier. Either of these actions will activate an indication ('failure flag') on the instruments of an aircraft using the ILS.
Modern localizer antennas are highly directional. However, usage of older, less directional antennas allows a runway to have a non-precision approach called a localizer back course. This lets aircraft land using the signal transmitted from the back of the localizer array. Highly directional antennas do not provide a sufficient signal to support a back course. In the United States, back course approaches are typically associated with Category I systems at smaller airports that do not have an ILS on both ends of the primary runway. Pilots flying a back course should disregard any glide slope indication.
On some installations, marker beacons operating at a carrier frequency of 75 MHz are provided. When the transmission from a marker beacon is received it activates an indicator on the pilot's instrument panel and the tone of the beacon is audible to the pilot. The distance from the runway at which this indication should be received is published in the documentation for that approach, together with the height at which the aircraft should be if correctly established on the ILS. This provides a check on the correct function of the glide slope. In modern ILS installations, a DME is installed, co-located with the ILS, to augment or replace marker beacons. A DME continuously displays the aircraft's distance to the runway.
The outer marker is normally located 7.2 kilometres (3.9 nmi; 4.5 mi) from the threshold, except that where this distance is not practical, the outer marker may be located between 6.5 and 11.1 kilometres (3.5 and 6.0 nmi; 4.0 and 6.9 mi) from the threshold. The modulation is repeated Morse-style dashes of a 400 Hz tone (--) ("M"). The cockpit indicator is a blue lamp that flashes in unison with the received audio code. The purpose of this beacon is to provide height, distance, and equipment functioning checks to aircraft on intermediate and final approach. In the United States, a NDB is often combined with the outer marker beacon in the ILS approach (called a Locator Outer Marker, or LOM). In Canada, low-powered NDBs have replaced marker beacons entirely.
The middle marker should be located so as to indicate, in low visibility conditions, the missed approach point, and the point that visual contact with the runway is imminent, ideally at a distance of approximately 3,500 ft (1,100 m) from the threshold. The modulation is repeated alternating Morse-style dots and dashes of a 1.3 kHz tone at the rate of two per second (·-·-) ("Ä" or "AA"). The cockpit indicator is an amber lamp that flashes in unison with the received audio code. In the United States, middle markers are not required so many of them have been decommissioned.
The inner marker, when installed, shall be located so as to indicate in low visibility conditions the imminence of arrival at the runway threshold. This is typically the position of an aircraft on the ILS as it reaches Category II minima. Ideally at a distance of approximately 1,000 ft (300 m) from the threshold. The modulation is repeated Morse-style dots at 3 kHz (····) ("H"). The cockpit indicator is a white lamp that flashes in unison with the received audio code.
Distance measuring equipment (DME) provides pilots with a slant range measurement of distance to the runway in nautical miles. DMEs are augmenting or replacing markers in many installations. The DME provides more accurate and continuous monitoring of correct progress on the ILS glide slope to the pilot, and does not require an installation outside the airport boundary. When used in conjunction with an ILS, the DME is often sited midway between the reciprocal runway thresholds with the internal delay modified so that one unit can provide distance information to either runway threshold. For approaches where a DME is specified in lieu of marker beacons, DME Required is noted on the Instrument Approach Procedure and the aircraft must have at least one operating DME unit to begin the approach.
Some installations include medium- or high-intensity approach light systems. Most often, these are at larger airports but many small general aviation airports in the U.S. have approach lights to support their ILS installations and obtain low-visibility minimums. The approach lighting system (abbreviated ALS) assists the pilot in transitioning from instrument to visual flight, and to align the aircraft visually with the runway centerline. Pilot observation of the approach lighting system at the Decision Altitude allows the pilot to continue descending towards the runway, even if the runway or runway lights cannot be seen, since the ALS counts as runway end environment. In the U.S., an ILS without approach lights may have CAT I ILS visibility minimums as low as 3/4 mile (runway visual range of 4,000 feet) if the required obstacle clearance surfaces are clear of obstructions. Visibility minimums of 1/2 mile (runway visual range of 2,400 feet) are possible with a CAT I ILS approach supported by a 1,400-to-3,000-foot-long (430 to 910 m) ALS, and 3/8 mile visibility 1,800-foot (550 m) visual range is possible if the runway has high-intensity edge lights, touchdown zone and centerline lights, and an ALS that is at least 2,400 feet (730 m) long (see Table 3-5a in FAA Order 8260.3b). In effect, ALS extends the runway environment out towards the landing aircraft and allows low-visibility operations. CAT II and III ILS approaches generally require complex high-intensity approach light systems, while medium-intensity systems are usually paired with CAT I ILS approaches. At many non-towered airports, the pilot controls the lighting system; for example, the pilot can key the microphone 7 times to turn on the lights, then 5 times to reduce them to medium intensity.
Once established on an approach, the pilot follows the ILS approach path indicated by the localizer and descend along the glide path to the decision height. This is the height at which the pilot must have adequate visual reference to the landing environment (i.e. approach or runway lighting) to decide whether to continue the descent to a landing; otherwise, the pilot must execute a missed approach procedure, then try the same approach again, try a different approach, or divert to another airport.
There are three categories of ILS equipment which support similarly named categories of approach/landing operation. Information below is based on ICAO, FAA, and JAA; certain states may have filed differences.
ICAO classifies ILS approaches as being in one of the following categories:
|Approach category||Decision height or alert height|
(minimum above runway threshold or touchdown zone)
|Runway visual range ("RVR")||Visibility minimum||Notes|
|I||200 feet (61 m)||550 m or 2400 ft (1200 ft is approved at some airports), increased to 800 m for single crew operations||800 m|
(1600 ft or 1200 ft in Canada)
|Either visibility not less than 800 m or 2400 ft or a Runway visual range (RVR) not less than 550 meters (1,800 ft) on runway with touchdown zone and centerline lighting.|
FAA Order 8400.13D allows for special authorization of CAT I ILS approaches to a decision height of 150 feet (46 m) with RVR >= 1,400 feet (430 m). The aircraft and crew must be approved for CAT II operations and a heads-up display in CAT II or III mode must be used to the decision height. CAT II/III missed approach criteria apply.
|II||100 feet (30 m)||300 m or 1000 ft||N/A||RVR 350 meters (1,150 ft) (ICAO and FAA) or 300 meters (980 ft) (JAA).|
|IIIa||50 ft < DH < 100 feet (30 m)||200 meters (660 ft)||N/A|
|IIIb||0 < DH < 50 feet (15 m)||75 meters (246 ft) (JAA) < RVR < 200 meters (660 ft)||N/A||RVR > 50 meters (160 ft) (ICAO and FAA). This is currently the best in operation.|
|IIIc||No DH||No RVR||N/A||As of 2012[update] this category is not yet in operation anywhere in the world as it requires guidance to taxi in zero visibility as well. Category IIIc is not mentioned in EU-OPS.|
Smaller aircraft generally are equipped to fly only a CAT I ILS. On larger aircraft, these approaches typically are controlled by the flight control system with the flight crew providing supervision. CAT I relies only on altimeter indications for decision height, whereas CAT II and CAT III approaches use radio altimeter (RA) to determine decision height.
An ILS must shut down upon internal detection of a fault condition. Higher categories require shorter response times; therefore, ILS equipment is required to shut down faster. For example, a CAT I localizer must shut down within 10 seconds of detecting a fault, but a CAT III localizer must shut down in less than 2 seconds.
In contrast to other operations, CAT III weather minima do not provide sufficient visual references to allow a manual landing to be made. CAT III minima depend on roll-out control and redundancy of the autopilot. because they give only enough time for the pilot to decide whether the aircraft will land in the touchdown zone (basically CAT IIIa) and to ensure safety during rollout (basically CAT IIIb). Therefore an automatic landing system is mandatory to perform Category III operations. Its reliability must be sufficient to control the aircraft to touchdown in CAT IIIa operations and through rollout to a safe taxi speed in CAT IIIb (and CAT IIIc when authorized). However, special approval has been granted to some operators for hand-flown CAT III approaches using "heads up display" (HUD) guidance which provides the pilot with an image viewed through the windshield with eyes focused at infinity, of necessary electronic guidance to land the airplane with no true outside visual references.
In the United States, many but not all airports with CAT III approaches have listings for CAT IIIa, IIIb, and IIIc on the instrument approach plate (U.S. Terminal Procedures). CAT IIIb RVR minimums are limited by the runway/taxiway lighting and support facilities, and are consistent with the airport Surface Movement Guidance Control System (SMGCS) plan. Operations below 600 ft RVR require taxiway centerline lights and taxiway red stop bar lights. If the CAT IIIb RVR minimums on a runway end are 600 feet (180 m), which is a common figure in the U.S., ILS approaches to that runway end with RVR below 600 feet (180 m) qualify as CAT IIIc and require special taxi procedures, lighting, and approval conditions to permit the landings. FAA Order 8400.13D limits CAT III to 300 ft RVR or better. Order 8400.13D (2009) allows special authorization CAT II approaches to runways without ALSF-2 approach lights and/or touchdown zone/centerline lights, which has expanded the number of potential CAT II runways.
In each case, a suitably equipped aircraft and appropriately qualified crew are required. For example, CAT IIIb requires a fail-operational system, along with a crew who are qualified and current, while CAT I does not. A head-up display (HUD) which allows the pilot to perform aircraft maneuvers rather than an automatic system is considered as fail-operational. A head-up display allows the flight crew to fly the aircraft using the guidance cues from the ILS sensors such that if a safe landing is in doubt, the crew can respond in an appropriate and timely manner. HUD is becoming increasingly popular with "feeder" airlines and most manufacturers of regional jets are now offering HUDs as either standard or optional equipment. A HUD can provide capability to take off in low visibility.
Some commercial aircraft are equipped with automatic landing systems that allow the aircraft to land without transitioning from instruments to visual conditions for a normal landing. Such autoland operations require specialized equipment, procedures and training, and involve the aircraft, airport, and the crew. Autoland is the only way some major airports such as Paris-Charles de Gaulle Airport remain operational every day of the year. Some modern aircraft are equipped with enhanced vision systems based on infrared sensors, that provide a day-like visual environment and allow operations in conditions and at airports that would otherwise not be suitable for a landing. Commercial aircraft also frequently use such equipment for takeoffs when takeoff minima are not met.
For both automatic and HUD landing systems, the equipment requires special approval for its design and also for each individual installation. The design takes into consideration additional safety requirements for operating an aircraft close to the ground and the ability of the flight crew to react to a system anomaly. The equipment also has additional maintenance requirements to ensure that it is capable of supporting reduced visibility operations.
At a controlled airport, air traffic control will direct aircraft to the localizer course via assigned headings, making sure aircraft do not get too close to each other (maintain separation), but also avoiding delay as much as possible. Several aircraft can be on the ILS at the same time, several miles apart. An aircraft that has turned onto the inbound heading and is within two and a half degrees of the localizer course (half scale deflection or less shown by the course deviation indicator) is said to be established on the approach. Typically, an aircraft is established by at least 2 nautical miles or 3 km prior to the final approach fix (glideslope intercept at the specified altitude).
Aircraft deviation from the optimal path is indicated to the flight crew by means of a display dial (a carryover from when an analog meter movement indicated deviation from the course line via voltages sent from the ILS receiver).
The output from the ILS receiver goes to the display system (head-down display and head-up display if installed) and may go to a Flight Control Computer. An aircraft landing procedure can be either coupled where the autopilot or Flight Control Computer directly flies the aircraft and the flight crew monitor the operation, or uncoupled where the flight crew flies the aircraft manually to keep the localizer and glideslope indicators centered.
Tests of the ILS system began in 1929 in the United States. The Civil Aeronautics Administration (CAA) authorized installation of the system in 1941 at six locations. The first landing of a scheduled U.S. passenger airliner using ILS was on January 26, 1938, when a Pennsylvania Central Airlines Boeing 247-D flew from Washington, D.C., to Pittsburgh, Pennsylvania, and landed in a snowstorm using only the Instrument Landing System. The first fully automatic landing using ILS occurred in March 1964 at Bedford Airport in UK.
The advent of the Global Positioning System (GPS) provides an alternative source of approach guidance for aircraft. In the US, the Wide Area Augmentation System (WAAS) has been available in many regions to provide precision guidance to Category I standards since 2007. The equivalent European Geostationary Navigation Overlay Service (EGNOS) was certified for use in safety of life applications in March 2011.
Local Area Augmentation System (LAAS) is under development to provide for Category III minimums or lower. The FAA Ground-Based Augmentation System (GBAS) office is currently working with the industry in anticipation of the certification of the first GBAS ground stations in Memphis, TN; Sydney, Australia; Bremen, Germany; Spain and Newark, NJ. All four countries have installed GBAS systems and are involved in technical and operational evaluation activities.
The Honeywell and FAA team obtained System Design Approval of the world's first Non-Federal U.S. approval for LAAS Category I at Newark Liberty International Airport, operations on Sept. 2009 and Operational Approval on Sept. 28, 2012.
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