This is done by choosing frequencies in a manner which provides certain minimum cochannel and adjacent channel desired to undesired signal ratios at critical points of the service volume. The frequency assignment process is meant to insure interference-free service within the service volume. A VOR+GS would have relatively inaccurate lateral guidance, especially at long distances, making the accurate signal of the GS useless in improving the minimum altitude of the approach.THE SELECTION OF ILS LOCALIZER ANTENNA PATTERNS FOR USE IN THE FREQUENCY ASSIGNMENT PROCESS However, as pointed out above, the danger of misplacing the aircraft horizontally is greater than vertically. NDB+GS would also work, but with a lower accuracy.Tuning directly to the GS channel is possible with the existing hardware, but the cockpit controls and displays are not designed to do that at the moment. VOR+GS works, provided that the aircraft have two nav radios.If you have RNAV, might as well go completely RNAV anyway.GS+altimeter already gives you a DME of sort. So we must pair GS with some other navigation aid, if LOC is unavailable. the hangar or the planes on a nearby taxiway.įor the same reason, when flying an ILS approach, you should capture the localizer first, then the glideslope never the other way around. If you follow the GS all the way down, you would crash into something, e.g. A GS-only approach provides no information whether you are on the extended runway centerline.At the end you either see the runway well enough to switch to visual, or execute a missed approach. You'd step down your altitude as your DME reading decreases. A LOC+DME approach would be similar to a VOR+DME approach.A LOC-only approach simply won't work, you need at least one more piece of information, e.g.A LOC+GS approach can get you onto the runway.Let's consider the ILS on its own, with no supplement navigation aids like RNAV / GPS. Hence, there isn't really anything to be gained. What good would a lower minima do you, if, by the time you reached, say, 200ft, you find out that you are not actually aligned with the runway, because the lateral guidance is so inaccurate? So even with accurate vertical guidance, you would need time to align with the runway once you get below the ceiling, which means our theoretical NDB+GP approach would need minima similar to that of a pure NDB approach. Because of this, it would probably not be possible to define lower minima compared to a NDB/DME approach. If you were to combine an NDB with a GP instead, you would still have accurate vertical guidance, but not very accurate lateral guidance. The reason an ILS approach (LOC+GP) generally has lower minima is because a LOC and GP combined is very accurate. This is relatively inaccurate, so an NDB approach typically has a relatively high minimum descend altitude (the altitude at which the pilot must visually see the runway in order to continue). But if a locator/NDB or RNAV approach is available, the pilot could just use that approach for the vertical guidance as well.įor an NDB approach, there is no actual vertical guidance, but the pilot will descend manually, typically while cross checking distance to threshold using a DME. Where the pilot uses the glideslope for vertical guidance but relies on ground- or satellite-based navigation fixes for lateral guidance.Īs you correctly point out, if using only a glideslope, the pilot would have to rely on other navigation facilities for the horizontal guidance, for example a locator/NDB or RNAV. What allows the use for an approach of a localiser without a glideslope, but not a glideslope without a localiser?īecause there would be no benefit to doing so. While localiser- sans-glideslope approaches are quite common, there are, to the best of my knowledge (feel free to correct me if I'm mistaken here), no examples of the reverse: a glideslope- sans-localiser approach, where the pilot uses the glideslope for vertical guidance but relies on ground- or satellite-based navigation fixes for lateral guidance. For a localiser-only approach, the pilot uses a specified series of navigational fixes to mark off each segment of the approach each of these segments has a specified minimum safe altitude (which is shown on the approach plate), and the pilot uses this information, and their onboard baroaltimeter, to maintain terrain clearance during the approach. The two are typically used together, for an ILS approach, but the localiser can also be used on its own, for a localiser-only, or simply localiser ( LOC for short), approach (for instance, if the glideslope is inoperative or nonexistent, the aircraft lacks a functioning glideslope receiver, or the pilot is shooting a back-course approach). An ILS consists, at its most basic, of two components: a localiser (to provide lateral guidance) and a glideslope (to provide vertical guidance).
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