Instrument Landing System
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The Instrument Landing System (ILS) is an instrument approach system which provides precise guidance to an aircraft approaching a runway and in the case of one type of Category III approach, it also provides guidance along the runway surface.
Principle of Operation
An ILS consists of two independent sub-systems, one providing lateral (localizer) guidance, the other vertical (glideslope) guidance to aircraft approaching a runway.
A localizer (LOC) antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. Two signals are transmitted on a carrier frequency between 108 MHz and 111.975 MHz. One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located aerials. Each aerial transmits a fairly narrow beam, one slightly to the left of the runway centreline, the other to the right. The localizer receiver on the aircraft measures the difference in the depth of modulation of the 90 Hz and 150 Hz signals. When this difference is zero the receiver aerial is on the centreline of the localizer which normally coincides with the runway centreline.
A glideslope (GS) antenna array is sited to one side of the runway touchdown zone. The GS signal is transmitted on a carrier frequency between 328.6 MHz and 335.4 MHz using a technique similar to that of the localizer, the centreline of the glideslope signal being arranged to define a glideslope at approximately 3° above the horizontal.
Localizer and glideslope carrier frequencies are paired so that only one selection is required to tune both receivers.
These signals are displayed on an instrument in the cockpit. The pilot controls the aircraft so that the indications on the instrument remain centered on the display. This ensures the aircraft is following the ILS centreline. Some aircraft possess the ability to route signals into the autopilot, which allows the approach to be flown automatically by the autopilot.
Localizer
In addition to the previously mentioned navigational signals, the localizer provides for ILS facility identification by periodically transmitting a 1020 Hz morse code identification signal. For example, the ILS for runway 04R at John F. Kennedy International Airport transmits IJFK to identify itself to users whereas runway 04L is known as IHIQ. This lets users know the facility is operating normally and that they are tuned to the correct ILS. The glideslope transmits no identification signal and relies on the localizer for identification.
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. This signal is reverse sensing so a pilot may have to fly opposite the needle indication (depending on the equipment installed in the aircraft). Highly directional antennas do not provide a sufficient signal to support a backcourse. In the United States, backcourse approaches are commonly associated with Category I systems at smaller airports that do not have an ILS on both ends of the primary runway.
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 modulating tone of the beacon is audible to the pilot. The height at which these signals will be received in an aircraft on the correct glideslope is promulgated.
Outer Marker
The outer marker should be located 7.2 km (3.9 NM) from the threshold except that, where this distance is not practicable, the outer marker may be located between 6.5 and 11.1 km (3.5 and 6 NM) from the threshold. The modulation is two dashes per second of a 400 Hz tone, the indicator is blue. 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, an 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.Middle Marker
The middle marker should be located so as to indicate, in low visibility conditions, that visual contact with the runway is imminent, Ideally at a distance of 1050m from the threshold. It is modulated with a 1300 Hz tone as alternate dots and dashes.Inner Marker
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. The modulation is 3000 Hz dots at 6 per second..DME
Distance Measuring Equipment (DME) is replacing markers in many installations. This provides more accurate and continuous monitoring of correct progress on the ILS to the pilot, and does not require an installation outside the airport boundary. The DME is frequency paired with the ILS so that it is automatically selected when the ILS is tuned.Other Means of Determining Distance
DME, GPS, Radar, and Cross Radials (through use of VOR receivers), a form of triangulation may also be used to determine distance.Monitoring
It is essential that any failure of the ILS to provide safe guidance is detected very rapidly by the pilot. Monitors are provided that continuously 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.Approach Lighting
Most installations include medium or high intensity approach light systems. The approach light system (abbreviated ALS) assists the pilot in transitioning from instrument to visual flight, and to align the aircraft visually with the runway centreline. At many non-towered airports, the intensity of the lighting system can be adjusted by the pilot.ILS categories
There are three categories of ILS which support similarly named categories of operation.- Category I - A precision instrument approach and landing with a decision height not lower than 60 m (200 ft) and with either a visibility not less than 800 m or a runway visual range not less than 550 m.
- Category II - Category II operation: A precision instrument approach and landing with a decision height lower than 60 m (200 ft) but not lower than 30 m (100 ft), and a runway visual range not less than 350 m.
- Category III is further subdivided
- *Category III A - A precision instrument approach and landing with: a) a decision height lower than 30 m (100 ft), or no decision height; and b) a runway visual range not less than 200 m.
- *Category III B - A precision instrument approach and landing with: a) a decision height lower than 15 m (50 ft), or no decision height; and b) a runway visual range less than 200 m but not less than 50 m.
- *Category III C - A precision instrument approach and landing with no decision height and no runway visual range limitations.
(Reference ICAO Annex 10 AERONAUTICAL TELECOMMUNICATIONS Volume 1 RADIO NAVIGATION AIDS 2.1.1)
An ILS is required to shut down upon internal detection of a fault condition as mentioned in the monitoring section. With the increasing categories, ILS equipment is required to shutdown faster since higher categories require shorter response times. For example, a Cat I localizer must shutdown within 10 seconds of detecting a fault, but a Cat III localizer must shutdown in less than 2 seconds.
Limitations and alternatives
Due to the complexity of ILS localizer and glideslope systems, there are some limitations. Localizer systems are sensitive to obstructions in the signal broadcast area like large buildings or hangars. Glideslope systems are also limited by the terrain in front of the glideslope 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, ILS only supports straight in approaches. Installation of ILS can also be costly due to the complexity of the antenna system and siting criteria.
In the 1970s there was a major US & European effort to establish the Microwave Landing System, which are not similarly limited and which allow curved approaches. However, a combination of slow development, airline reluctance to invest in MLS, and the rise of GPS has resulted in its failure to be widely adopted. The Transponder Landing System (TLS) is another alternative to an ILS that can be used where a conventional ILS will not work or is not cost-effective.
Future
The advent of the Global Positioning System (GPS) provides an alternative source of guidance for aircraft. Wide Area Augmentation System signals (WAAS) is enough to provide guidance to Category I standards beginning 2007. Other methods of augmentation are in development, including Local Area Augmentation System, or Local Area Augmentation System (LAAS), which will provide for Category III mins or better.See also
- Instrument flight rules (IFR)
- VHF Omni-directional Range (VOR)
- Distance Measuring Equipment (DME)
- Non-Directional Beacon (NDB)
- Global Positioning System (GPS)
- Transponder Landing System (TLS)
- Local Area Augmentation System (LAAS)
- Wide Area Augmentation System (WAAS)
Notes
}References
- ICAO Annex 10 Volume 1, Radio Navigation Aids, Fifth Edition — July 1996
External links
- [2001 Federal Radionavigation Plan (FRP)]—FRS publication has detailed description of ILS and other navigational systems
- [Thales ATM ILS page]—Manufacturer of ILS equipment; includes pictures of antenna systems
- [Antenna Products—ILS antenna page]—Manufacturer of ILS antennas (V-ring and traveling-wave antennas)
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