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ILS is a radio aid to the final approach and is used only within a short distance from the airport.

Scheduled service would be impossible without a way to land in poor weather.

Tests of the first ILS began in 1929

The first scheduled passenger airliner to land using ILS was in 1938.

A Pennsylvania-Central Airlines Boeing 247-D from Washington to Pittsburgh.

In 1949, ICAO adapted an ILS standard developed by the US Army as a standard system for all of its member countries.

Until the mid-1950s, only visual landing procedures were possible

1958-First IFR landing system developed

1966-First ILS system developed and tested at AIRPORT in USA

1968-First ILS applications installed at major airports

1974-ILS systems mandated by FAA for at least two major runways at all Regional, and International Airports.

A Precision Approach is an approved descent procedure using a navigation facility aligned with a runway where glide slope information is given.

When all components of the ILS system are available, including the approved approach procedure, the pilot may execute a precision approach.

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The distance measuring equipment (DME) system gives the pilots distance to a DME ground station.

The pilot can tune one DME station with the navigation control panel.The DME-distance shows on the navigation displays unit

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ILS (instrument landing system)

Primarily consists of three instruments:Marker beacons are used to measure how far aircraft until landingGlideslope or GlidePath for vertical guidanceLocalizer for lateral guidance

All of these systems transmitt signals at different frequencies that are translated by electronics to determine the position of the aircraft

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Localizer operates in VHF band between 108 to 111.975 MHz.

Glide Path operates in UHF band between 329.15 and 335 MHz

Marker Beacons operated in VHF band which is 75 MHz.

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Glide Path operates in UHF band between 329.15 and 335 MHz

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Localizer is sensitive to obstructions like buildings and terrain.

Glide Path errors can occur if terrain is sloping or is uneven in front of the antenna.

Since antennas point in a single direction, only straight approaches are available.

Can be costly.Both the localizer and and glideslope emit two tones one at 90 hertz and the other at 150.

Category I

An instrument runway served by Instrument Landing Systems (ILS) and/or Microwave Landing Systems (MLS) for lateral/vertical guidance and visual aids intended for operating

Decision Height (DH) more than 60m

Visibility more than 880 m

Runway Visual Range (RVR) more than 550 m

Most common of P runways

Category II

Same as Cat I except Cat II has DH more than 30m but less than 60m and RVR more than 350 m.

Category III

Same as Cat I except Cat III has DH less than 30m and RVR less than 350 m.

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There are three categories of ILS the operation.

Category I - A precision instrument approach and landing with a decision height not lower than 60 m (200 ft) above touchdown zone elevation and with either a visibility not less than 800 m or a runway visual range not less than 550 m. An aircraft equipped with an Enhanced Flight Vision System may, under certain circumstances, continue an approach to CAT II minimums.

Category II - Category II operation: A precision instrument approach and landing with a decision height lower than 60 m (200 ft) above touchdown zone elevation 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) above touchdown zone elevation, 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) above touchdown zone elevation, 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. A Category III C system is capable of using an aircraft's autopilot to land the aircraft and can also provide guidance along the runway surface.

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Primarily consists of three instruments:Marker beacons are used to measure how far aircraft until landingGlideslope or GlidePath for vertical guidanceLocalizer for lateral guidance

All of these systems transmitt signals at different frequencies that are translated by electronics to determine the position of the aircraft

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Aircraft flying into Heathrow Airport in fog or poor visibility.

On a clear day, about 44 planes an hour land at Heathrow.

However, if the visibility drops and aircraft have to use the ILS system to land, only 24 aircraft could land per hour.

This is because the radio transmitter at the end of the runway needs good line of sight to the approaching aircraft, but because it is at the far end of the runway, planes have to land and taxi clear before a full signal is restored.

However, MLS allows an extra six aircraft an hour to land, meaning that while fog will still cause disruption, its effects will be less prominent.

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There was heavy rain at Guam so visibility was significantly reduced and the crew was attempting an instrument landing. Air traffic control in Guam advised the crew that the glideslope Instrument Landing System (ILS) in runway 6L was out of service. Air traffic control cleared Flight 801 to land in runway 6L at around 1:40 am. The crew noticed that the plane was descending very steeply, and noted several times that the airport "is not in sight". At 1:42 am, the aircraft crashed into Nimitz Hill, about 3 nautical miles (5 km) short of the runway, at an altitude of 660 feet (201 m).

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