Instrument landing system

In aviation, the instrument landing system (ILS) is a 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 approach until it is over the ground, within a mile of the runway. At that point the runway should be visible to the pilot; if it is not, they perform a missed approach. Bringing the aircraft this close to the runway dramatically improves the weather conditions in which a safe landing can be made. Later versions of the system, or "categories", have further reduced the minimum altitudes.
ILS uses two directional radio signals, the "localizer" (108 to 112 MHz frequency), which provides horizontal guidance, and the "glideslope" (329.15 to 335 MHz frequency) for vertical. The relationship between the aircraft's position and these signals is displayed on an aircraft instrument, often additional pointers in the attitude indicator. The pilot attempts to maneuver the aircraft to keep these indicators centered while they approach the runway to the decision height. Optional "markers" provide distance information as the approach proceeds, including the "middle marker" placed close to the position of the decision height. ILS may also include high-intensity lighting at the end of the runways.
A number of radio-based landing systems were developed between the 1920s and 1940s, notably the Lorenz beam which saw relatively wide use in Europe prior to the war. The US-developed SCS-51 system was more accurate while also adding vertical guidance. Many sets were installed at airbases in the United Kingdom during World War II, which led to it being selected as the international standard after the formation of the International Civil Aviation Organization (ICAO) in 1947. Several competing landing systems have been developed, including the radar-based ground-controlled approach (GCA) and the more recent microwave landing system (MLS), but few of these systems have been deployed. ILS remains a widespread standard to this day.
The introduction of precision approaches using low-cost GPS systems is leading to the replacement of ILS. Providing the required accuracy with GPS normally requires only a low-power omnidirectional augmentation signal to be broadcast from the airport, which is dramatically less expensive than the multiple, large and powerful transmitters required for a full ILS implementation. By 2015, the number of US airports supporting ILS-like LPV approaches exceeded the number of ILS systems, and this is expected to lead to the eventual removal of ILS at most airports.
Principle of operation.
An "instrument landing system" operates as a ground-based instrument approach system that provides precision lateral and vertical 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.
Beam systems.
Previous blind landing radio aids typically took the form of "beam" systems of various types. These normally consisted of a radio transmitter that was connected to a motorized switch to produce a pattern of Morse code dots and dashes. The switch also controlled which of two directional antennae the signal was sent to. The resulting signal sent into the air consists of dots sent to one side of the runway and dashes to the other. The beams were wide enough so they overlapped in the center.
To use the system an aircraft only needed a conventional radio receiver. As they approached the airport they would tune in the signal and listen to it in their headphones. They would hear dots or dashes if they were to the side of the runway, or if they were properly aligned, the two mixed together to produce a steady tone, the "equisignal". The accuracy of this measurement was highly dependant on the skill of the operator, listening to the signal on earphones in a noisy aircraft whilst often communicating with the tower at the same time.
Accuracy of the system was normally on the order of 3 degrees. While this was useful for bringing the aircraft onto the direction of the runway, it was not accurate enough to safely bring the aircraft to visual range in bad weather; an aircraft normally descends at a rate of about 3 to 5 degrees, and if they were 3 degrees below that they would crash. Beams were used only for lateral guidance, and the system was not enough on its own to perform landings in heavy rain or fog. Nevertheless, the final decision to land was made at only from the airport.
ILS concept.
The ILS system, developed just prior to the start of the war, used a more complex system of signals and an antenna array to achieve higher accuracy. This requires significantly more complexity in the ground station and transmitters, with the advantage that the aircraft instruments are simplified.
Key to its operation is a concept known as the amplitude modulation index, a measure of how strongly the amplitude modulation is applied to the underlying carrier frequency. In the earlier beam systems, the signal was turned on and off entirely, corresponding to a modulation index of 100%. In ILS, a more complex system of signals and antennas varies the modulation across the width of the beam pattern, and this modulation can be accurately measured using electronic means even in the presence of interference. This provides angular resolution of less than a degree, and allows the construction of a precision approach.
The system relies on the creation of sidebands, secondary frequencies that are created when two different signals are mixed. For instance, if one takes a radio frequency signal at 100 MHz and mixes that with an audible tone at 2500 Hz, four signals will be produced, at the original 2500 as well as 99997500, 100000000 and 100002500. The original modulating signal is too low frequency to travel far from an antenna, but the other three signals are all radio frequency and can be effectively broadcast. The center frequency is known as the carrier and the signals on either side are the sidebands.
ILS starts by mixing two modulating signals to the carrier, one at 90 Hz and another at 150. This creates a signal with five radio frequencies in total, the carrier and four sidebands. This combined signal, known as the CSB for "carrier and sidebands", is sent out evenly from an antenna array. The CSB is also sent into a circuit that suppresses the original carrier, leaving only the four sideband signals. This signal, known as SBO for "sidebands only", is also sent to the antenna array.
For lateral guidance, known as the "localizer", the antenna is normally placed off the far end of the runway and consists of multiple antennas in an array normally about the same width of the runway. Each individual antenna has a phase shifter that is applied only to the SBO such that the signal on the left side of the runway is retarded 90 degrees and the advanced 90 on the right. Additionally, the 150 Hz signal is inverted on one side of the pattern, another 180 degree shift. Due to the way the signals mix in space, the SBO signals destructively interfere and eliminate each other along the centerline, leaving just the CSB. On either side, the SBO will not completely cancel out.
A receiver in front of the array will receive both of these signals mixed together. The SBO at their particular location will cause the original CSB modulation to be modified, either amplifying or reducing the sidebands. The receiver extracts the original 90 and 150 Hz signals and compares their "relative" strengths to determine location. For instance, if the 150 Hz signal is stronger, they are somewhere to the right of the runway and the difference in signal strength indicates how far. The two modulating signals can be extracted from simple electronics and mixed to produce a single electrical voltage. This is used, with analog instruments, to deflect a needle on a voltmeter that is used to represent the position of the aircraft relative to the centerline of the signal.
Although the encoding scheme is complex, and requires a considerable amount of ground equipment, the resulting signal is both far more accurate than the older beam-based systems and is far more resistant to common forms of interference. For instance, static in the signal will affect both sub-signals equally, so it will have no effect on the result. Similarly, changes in overall signal strength as the aircraft approaches the runway, or changes due to fading, will have little effect on the resulting measurement because they would normally affect both channels equally. The system is subject to multipath distortion effects due to the multiple frequencies, but because those effects are dependant on the terrain, they are generally fixed in location and can be accounted for through adjustments in the antenna or phase shifters.
Additionally, because it is the encoding of the signal within the beam that contains the angle information, not the strength of the beam, the signal does not have to be tightly focussed in space. In the older beam systems, the accuracy of the equisignal area was a function of the pattern of the two directional signals, which demanded that they be relatively narrow. The ILS pattern can be much wider. ILS systems are normally required to be usable within 10 degrees on either side of the runway centerline at , and 35 degrees on either side at . This allows for a wide variety of approach paths.
The "glideslope" works in the same general fashion as the localizer and uses the same encoding, but is normally broadcast to produce a centerline at an angle of 3 degrees above the horizon from a point beside the runway instead of the end. The only difference between the signals is that the localizer is broadcast using lower carrier frequencies, using 40 selected channels between 108.10 MHz and 111.95 MHz, whereas the glideslope has a corresponding set of 40 channels between 328.6 and 335.4 MHz. The higher frequencies generally result in the glideslope broadcast antennas being smaller. The channel pairs are not linear; localizer channel 1 is at 108.10 and paired with glideslope at 334.70, whereas channel two is 108.15 and 334.55. There are gaps and jumps through both bands.
Most illustrations of the ILS concept often show the system operating more similar to beam systems with the 90 Hz signal on one side and the 150 on the other. These illustrations are inaccurate,
Using ILS.
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.
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.
Localizer.
A localizer (LOC, or LLZ until ICAO standardisation) is an antenna array normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. 
The localizer will allow the aircraft to turn and match the aircraft with the runway. After that, the pilots will activate approach phase (APP). 
Glide slope (G/S).
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 of approximately 3° above horizontal (ground level) to remain above obstructions and reach the runway at the proper touchdown point (i.e. it provides vertical guidance).
Limitations.
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, such as 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.
Identification.
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.
Monitoring.
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.
Localizer back course.
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.
Marker beacons.
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.
DME substitution.
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, or an IFR approved GPS system (an RNAV system meeting TSO-C129/ -C145/-C146), to begin the approach.
Approach lighting.
Some installations include medium- or high-intensity approach light systems (abbreviated ALS). 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 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 ALS, and 3/8 mile visibility visual range is possible if the runway has high-intensity edge lights, touchdown zone and centerline lights, and an ALS that is at least long (see Table 3-3-1 "Minimum visibility values" in FAA Order 8260.3C). 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 seven times to turn on the lights on the high intensity, five times to medium intensity or three times for low intensity.
Decision altitude/height.
Once established on an approach, the pilot follows the ILS approach path indicated by the localizer and descends 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 (e.g. 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.
ILS categories.
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 more quickly. 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.
Special CAT II and CAT III operations.
In contrast to other operations, CAT III weather minima do not provide sufficient visual references to allow a manual landing to be made. CAT IIIb 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 a head-up display (HUD) guidance that 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, airports with CAT III approaches have listings for CAT IIIa and IIIb or just CAT III 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 , which is a common figure in the U.S., ILS approaches to that runway end with RVR below 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 HUD that allows the pilot to perform aircraft maneuvers rather than an automatic system is considered as fail-operational. A HUD 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 Charles de Gaulle Airport remain operational every day of the year. Some modern aircraft are equipped with enhanced flight 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.
Of course nearly all of this pilot training and qualification work is done in simulators with various degrees of fidelity.
Use.
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 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.
History.
Tests of the ILS system began in 1929 in the United States. A basic system, fully operative, was introduced in 1932 at Berlin-Tempelhof Central Airport (Germany) named LFF or "Lorenz beam" due its inventor, the C. Lorenz AG company. The Civil Aeronautics Board (CAB) of the United States 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 247D 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.
Future.
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 September 2009 and Operational Approval on September 28, 2012.
In Norway, a D-GPS based landing system, called SCAT-I, is in operation on some short runway airports.