Instrument Landing System (ILS) — Page 147, Lesson 134

I want to walk you through the Instrument Landing System, the ILS, and I want to start by laying out the whole ground installation so you can see how the pieces fit together before we talk about frequencies. The ILS is built around two main radio transmitters. The first is the localizer, which gives you lateral guidance — left and right. The second is the glide path transmitter, the GP, which gives you vertical guidance — up and down. Now, the glide path transmitter operates in the UHF band, and it is frequency paired with the localizer. That pairing is a big deal, and we'll come back to it. Physically, the GP transmitter sits 300 metres in from the threshold, and about 200 metres from the runway edge, abeam the touchdown point. So picture it off to the side of the runway, roughly level with where your wheels are meant to touch. Then we have the marker beacons. These transmit at 75 MHz in the VHF band. There are three of them you need to know: the outer marker, the OM, the middle marker, the MM, and possibly an inner marker, the IM. Their job is to let the pilot cross-check the aircraft's height against range and timing to the runway threshold. So as you fly over each marker, you get an indication, and you can verify that your altitude matches what it should be at that distance from the runway. Now, a couple of variations on the standard setup. Some countries allow back course approaches. That means an aircraft can make a non-precision approach on the back beam of the localizer transmitter — that's the signal radiating behind the antenna, away from the runway. It's a non-precision approach because you don't get the glide path guidance on the back course. Some ILS installations also have a co-located, low-powered NDB — that's a non-directional beacon — called a locator, designated with the letter L, and it sits at the site of the outer marker beacon. And increasingly, Distance Measuring Equipment, DME, that is frequency paired with the ILS frequencies, is being provided to supplement or even replace the range information that the marker beacons give you. Now let's get into the frequencies, because this is where the precision really lives. The localizer operates in the VHF band, between 108 and 111.975 MHz, and it provides 40 channels. Examples would be 108.1, 108.15, 108.3, 108.35, 108.5, 108.55, all the way up to 111.95 MHz. Here's a critical detail: this part of the frequency band is shared with VOR. The frequencies allocated are odd decimals, and odd decimals plus 0.05 MHz. So that's how the two systems coexist without stepping on each other. The glide path operates in the UHF band, between 329.15 and 335 MHz, and it provides 40 complementary channels. Examples: 329.15, 329.3, 329.45, 329.6, up to 335 MHz. The markers, as I said, all transmit at 75 MHz. And there's no interference problem there, because the radiation pattern is a narrow, fan-shaped vertical beam. So the signal is tightly focused upward, not spraying sideways into other receivers. Now, the frequency pairing — this is the elegant part. The GP frequency is paired with the localizer, and selection of the frequency is automatic. The localizer and glide path transmissions are frequency paired in accordance with the list published at ICAO. For example, 108.1 MHz is paired with 334.7 MHz, and 111.95 MHz is paired with 330.95 MHz. Why does this matter? Four advantages, and I want you to hold onto all of them. One: one switch activates both receivers — that reduces the pilot's workload. Two: frequency selection is made easier and quicker, because there's only one to consider. Three: the potential for a wrong frequency selection is reduced. And four: only one identifier is needed. So when you tune the localizer, the glide path comes along automatically, and you only have to verify one Morse code identifier. Let me show you the layout of all this on the ground. That's the ILS ground installation. And here's the frequency pairing chart so you can see the localizer and glide path channels lined up. And this one shows you the marker beacon positions relative to the runway. So to tie it together: the localizer gives you left-right, the glide path gives you up-down, the markers and DME give you distance, and the whole thing is frequency paired so you select one frequency and get both guidance signals plus the identifier. That's the ILS ground system.