The pilot controls the aircraft so that the indications on the instrument (i.e., the course deviation indicator) remain centered on the display. This instrument is generally called the omni-bearing indicator or nav indicator. These signals are displayed on an indicator in the instrument panel. The beam is 1.4° deep 0.7° below the glideslope centerline and 0.7° above the glideslope centerline. The centerline of the glide slope signal is arranged to define a glide slope of approximately 3° above horizontal (ground level). The GP signal is transmitted on a carrier frequency between 329.15 and 335 MHz using a technique similar to that of the localizer. If the DDM is zero, the aircraft is on the centerline of the localizer coinciding with the physical runway centerline.Ī glide slope (GS) or glide path (GP) antenna array is sited to one side of the runway touchdown zone. In the cockpit, the needle on the horizontal situation indicator (HSI, the instrument part of the ILS), or course deviation indicator (CDI), will show that the aircraft needs to fly left or right to correct the error to fly down the center of the runway. If there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centerline. The difference between the two signals varies depending on the position of the approaching aircraft from the centerline. For the localizer, the depth of modulation for each of the modulating frequencies is 20 percent. The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right. One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located antennas. Two signals are transmitted on one out of 40 ILS channels between the carrier frequency range 108.10 MHz and 111.95 MHz (with the 100 kHz digit always odd, so 108.10, 108.15, 108.30, and so on are LOC frequencies but 108.20, 108.25, 108.40, and so on are not). Note that the glide slope beams are partly formed by the reflection of the glideslope aerial in the ground plane.Ī localizer (LOC, or LLZ until ICAO designated LOC as the official acronym) antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. The emission patterns of the localizer and glideslope signals. Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison. Principle of operation An ILS consists of two independent sub-systems, one providing lateral guidance (localizer), the other vertical guidance (glide slope or glide path) to aircraft approaching a runway. Radio-navigation aids must keep a certain degree of accuracy (set by international standards of CAST/ICAO) to assure this is the case, flight inspection organizations periodically check critical parameters with properly equipped aircraft to calibrate and certify ILS precision. Instrument approach procedure charts (or approach plates) are published for each ILS approach, providing pilots with the needed information to fly an ILS approach during instrument flight rules (IFR) operations, including the radio frequencies used by the ILS components or navaids and the minimum visibility requirements prescribed for the specific approach. An instrument landing system ( ILS) is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions (IMC), such as low ceilings or reduced visibility due to fog, rain, or blowing snow.
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