Two signals are transmitted on one of 40 ILS channels. Some runways have ILS only in one direction, this can however still be used for the opposite direction (with a lower precision) known as back beam or "Back Course" which is not associated with a glide path. In parts of Africa and Asia large airports may lack any kind of transmitting ILS system. An older aircraft without an ILS receiver cannot take advantage of any ILS facilities at any runway, and much more importantly, the most modern aircraft have no use of their ILS instruments at runways which lack ILS facilities. ![]() In aviation, a localizer is the lateral component of the instrument landing system (ILS) for the runway centerline when combined with the vertical glide path, not to be confused with a locator, although both are parts of aviation navigation systems.Ī localizer (like a glide path) requires both a transmitting airport runway system and receiving cockpit instruments. Many aircraft possess the ability to route signals into the autopilot, allowing the approach to be flown automatically by the autopilot.Horizontal guidance system Localizer as component of an ILS (KMEZ runway 27, Mena, Arkansas) Emission patterns of the localizer and glide path signalsĪn instrument landing system localizer, or simply localizer ( LOC, or LLZ prior to 2007 ), is a system of horizontal guidance in the instrument landing system, which is used to guide aircraft along the axis of the runway. Vertical guidance, shown on the instrument by the glideslope indicator, aids the pilot in reaching the runway at the proper touchdown point. This ensures the aircraft is following the ILS centreline (i.e., it provides lateral guidance). 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 328.6 and 335.4 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 slightly 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 in the carrier frequency range between 108.10 MHz and 111.95 MHz (with the 100 kHz first decimal 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). ![]() Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison.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. 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.
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