Instrument landing system ils it is a ground based

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Instrument Landing System (ILS) It is a ground-based instrument approach system which provides precise guidance to an aircraft approaching a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during Instrument (c) UPES, Not for Reproduction/ Sale
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Notes ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ Aviation Safety & Security Management Meteorological Conditions (IMC), such as low clouds or reduced visibility due to fog, rain, or blowing snow. Principle of Operation An ILS consists of two independent sub-systems, one providing lateral guidance (Localizer), the other vertical guidance (Glideslope or GlidePath) to aircraft approaching a runway (Figure 2.1). Figure 2.1: ILS A localizer (LOC, or LLZ in Europe) antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. It provides lateral guidance to the landing aircraft, by the help of radio signals that assist the aircraft to come in line of the runway. Two signals are transmitted on a carrier frequency between 108.10 MHz and 111.975 MHz. One is modulated at 90 Hz, the other at 150 Hz (Known as Yellow and Blue rays) and these are transmitted from separate but co-located antennas. Each antenna transmits a fairly narrow beam, one slightly to the left of the runway centre line, the other to the right. The localizer receiver on the aircraft measures the Difference in the Depth of Modulation (DDM) of the 90 Hz and 150 Hz signals. For the localizer, the depth of modulation for each of the modulating frequencies is 20 per cent. The difference between the two signals varies depending on the position of the approaching aircraft from the centre line. (c) UPES, Not for Reproduction/ Sale
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UNIT 2: Radio Communication Notes ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ If there is a predominance of either 90Hz or 150Hz modulation, the aircraft is off the centre line. In the cockpit, the needle on the Horizontal Situation Indicator, or HSI (The Instrument part of the ILS), will show that the aircraft needs to fly left or right to correct the positional error to fly down the centre of the runway. If the DDM is zero the receiver aerial and therefore, the aircraft, is on the centre line of the localizer coinciding with the physical runway centre line. A glideslope or Glidepath (GP) provides vertical guidance to the landing aircraft by the help of radio signals that assist the aircraft to come at the right angle of descent for touch down on the runway. Glidepath antenna array is sited to one side of the runway touchdown zone. The GP signal is transmitted on a carrier frequency between 329.15 and 335 MHz using a technique similar to that of the localizer. The centre line of the glideslope signal is arranged to define a glideslope of approximately 3° above the horizon. Localizer and glideslope carrier frequencies are paired so that only one selection is required to tune both receivers.
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  • Fall '19
  • Instrument approach, Runway, Rajiv, Aviation Safety & Security Management

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