Instrument Landing System (ILS) is the international slandered for approach landing guidance . ILS was adopted by International Civil Aviation Organization (ICAO) in 1947 and will be in service until least 2005. The Instrument Landing System (ILS) provides a means for safe landing of aircraft at airports underconditions of low ceiling and limited visibility. The use of the system materially reduces interruptions of service at airports resulting from bad weather by allowing operations to continue at lower weather minimums. The ILS also increases the traffic handling capacity of the airport under all weather conditions.
The functions of an ILS is to provide the PILOT or AUTOPILOT of a landing aircraft with the
guidance to and along the surface of the runway. This guidance must be of very high integrity to ensure that each landing has a very high probability of success.
Components of ILS
The basically philosophy of ILS is that ground installations, located in the vicinity of the runway,
transmit coded signals in such a manner that pilot is given information indicating position of the
aircraft with respect to correct approach path.
To provide correct approach path information to the pilot, three different signals are required to be transmitted. The first signal gives the information to the pilot indicating the aircraft’s position relative to the centerline of the runway. The second signal gives the information indicating the aircraft’s position to the required angle of descent, where as the third signal provides distance information from some specified point.
Theses three parameters which are essential for a safe landing are Azimuth Approach Guidance and Range from the touch down point .These are provided to the pilot by three components of the ILS namely Localizer, Glide Path and Marker Beacons respectively. At some airports the Marker Beacons are replaced by a Distance Measuring Equipment (DME).
1 Localizer Unit
The localizer unit consists of an equipment building, the transmitter equipment, a platform, the
antennas, and field detectors. The antennas will be located about 1,000 feet from the stop end of the runway and the building about 300 feet to the side. The detectors are mounted on posts a short distance from the antennas.
2. Glide Path Unit
The Glide Path unit is made up of a building, the transmitter equipment, the radiating antennas and monitor antennas mounted on towers. The antennas and the building are located about 500 feet to one side of the runway centre line at a distance of approximately 1,000 feet from the approach end of the runway.
3. Marker Units
Three Marker Units are provided. Each Marker unit consists of a building, transmitter and directionalantenna array. The system will be located near the runway centre line, extended. The transmitters are 75 MHz, low power units with keyed tone modulation. The units are controlled via lines from thetower. The outer marker will be located between 4 and 7 miles in front of the approach end of the runway, so the pattern crosses the glide angle at the intercept altitude. The modulation will be 400 Hz keyed at 2 dashes per second.
The middle marker will be located about 3500 feet from the approach end of the runway, so the pattern interests the glide angle at 200 feet. The modulation will be a 1300 Hz toon keyed by continuous dot, dash pattern. Some ILS runways have an inner marker located about 1,000 feet from the approach end of therunway, so the pattern intersects the glide angle at 100 feet. The transmitter is modulated by a tone of3000 Hz keyed by continuous dots.
4. Distance Measuring Equipment (DME)
Where the provision of Marker Beacons is impracticable, a DME can installed co-located with the Glide Path facility.
5. Locater Beacons
The ILS should be supplemented by sources of guidance information which will provide effected
guidance to the desired course, Locator Beacons, which are essentially low power NDBs, installed at Outer Marker and middle Marker locations will serve this purpose.
6. Aircraft ILS Component
The Azimuth and Elevation guidance provided by the Localizer and Glide Path respectively to the pilot continuously by an on-board meter called the Cross Deviation Indicator (CDI).Range information isprovided continuously in the form of digital readout if DME is used with ILS. However range information is not visual indications of specific distances are provide by means of audio coded signals and lighting of appropriate coloured in the cockpit.
ILS Signal Format
Introduction
ILS employs amplitude modulation of a radio frequency carrier to provide the guidance information. The modulating signals used in ILS are pure sine waves of 90 Hz and 150 Hz frequency. This handout deals with the characteristics feathers of singles radiated by Localizer and Glide Path.
Principles of operation
An ILS consists of two independent sub-systems, one providing lateral guidance, the other vertical 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. Localizer and glideslope signals are displayed on a cockpit instrument, called a Course deviation indicator (CDI), as vertical and horizontal needles (or an electronic display simulating needles). The pilot controls the aircraft so that the needles remain centred on the display, the aircraft then follows the ILS centreline. The signals are also fed into autopilot systems to allow approaches to be flown on autopilot.
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