Ils (Instrument Landing System)

The ILS is designed to provide an approach path for exact alignment and descent of an aircraft on final approach to a runway.

The ground equipment consists of two highly directional transmitting systems along with three (or fewer) marker beacons. The directional transmitters are known as the localizer and glide path transmitters.

The system may be divided functionally into three parts: guidance information – localizer, glidepath Range information – marker beacons, Visual information – approach lights, touchdown and centerline lights, runway lights.

1. The localizer transmitter, operating on one of the twenty ILS channels emits signals which provide the pilot with course guidance to the runway centerline.

2. The UHF (ultra high frequency) glidepath transmitter, operating on one of the twenty ILS channels radiates the signals principally in the direction of the final approach.

3. Ordinarily, there are two marker beacons associated with an ILS; the outer marker and middle marker. However, some locations may employ a third beacon – the inner marker.

The outer marker normally indicates a position at which an aircraft at the appropriate altitude on the localizer course will intercept the ILS glide path.

The middle marker indicates a position at which an aircraft is approximately 3500 feet from the landing threshold. This will also be the position at which an aircraft on the glidepath will be at an altitude of approximately 200 feet above the elevation of the touchdown zone.

The inner marker, where installed, will indicate a point at which an aircraft is at a designated decision height on the glidepath between the middle marker and landing threshold.

Radar

The principles of radar are not new: in fact, some early experiments were made back in 1880s. In 1904 a German engineer had invented, as he explained, a “radio-echo collision prevention device”

The word “radar” was originally derived from the descriptive phrase “Radio Detection and Ranging”.

The application of radar in the air traffic control system consists of two basic designs. The initial type of radar, called primary radar, began to be used for advanced air traffic control. When the word “radar” is used alone it usually includes both primary and secondary radar.

There are three additional forms associated with primary and secondary radar:

Radar Echo – the visual indication on display of a radar signal transmitted from an object.

Radar Response – the visual indication on display of a radar signal transmitted from an object in reply to an interrogation.

Radar Blip – the collective term meaning either echo or response.

Primary Radar

In primary radar a beam of individual pulses of energy is transmitted from the ground equipment. These pulses hit the aircraft from 16 to 34 times each scan. An aircraft in the path of this radar beam will reflect back some of the pulses which are picked up by a receiver. This reflected energy produces a bright “echo” or “target” on a cathode ray tube.