![]() 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 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). 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. The glide slope must transmit in a different frequency than the localizer, of the receiver wouldn't be able to distinguish it from the localizer signal, no? I guess the nav radio “knows”, when it's tuned to an ILS frequencies, to look for the associated glide slope signal, at whatever frequency it's transmitting on, probably at a standard offset from the one you tune.Īn 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. I suppose the localizer part of the ILS signal could be a VOR transmitter, but I always assumed the localizer provided more precise deviation info than a VOR station. ![]() It never made sense to me that my VOR needle should point to the ILS station, as though it was just another VOR signal. Ah! This has been the source of my confusion over this. I have noticed that FSX gives points with ILS, but this is not correct. And yes, ILS frequencies are in the 108 Mhz to 111 Mhz range and odd tenths selected. With an ILS frequncy selected no point to the station is provided. ![]() ILS signals are highly directional along the runway heading, providing altitude and heading references to the landing aircraft. The VOR signals are omnidirectional and provide relative bearing to the station. The VOR and ILS signals are detected by the same receiver.
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