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

Let's start with the glide path transmitter, because that's where the excerpt opens. The glide path, or GP, transmitter operates in the UHF band — that's Ultra High Frequency. And it's frequency paired with the localizer, which we'll get to in a moment. Physically, it's located 300 metres in from the threshold — that's the runway threshold, the start of the runway — and about 200 metres from the runway edge, abeam the touchdown point. So it's sitting off to the side of the runway, roughly level with where the wheels are meant to touch down. Now, marker beacons. These transmit at 75 MHz in the VHF band — Very High Frequency. There are three types: the outer marker, or OM; the middle marker, or MM; and possibly an inner marker, the IM. Their job is to let the pilot cross-check the aircraft's height against ranges and timing to the runway threshold. So they give you a position check along the approach path. Then there's the back course approach. Some countries allow these. It lets an aircraft make a non-precision approach on the back beam of the localizer transmitter — that's the signal radiating behind the localizer, away from the runway. It's called non-precision because there's no glide path guidance on the back course. Some ILS installations also have a co-located low-powered NDB — a Non-Directional Beacon — called a locator, abbreviated L. It sits at the site of the outer marker beacon. And finally, Distance Measuring Equipment, DME, that's frequency paired with the ILS frequencies, is now increasingly provided to supplement or replace the range information that marker beacons give you. Now let's talk frequencies, because this is where the precision comes in. The localizer operates in the VHF band between 108 and 111.975 MHz, providing 40 channels — for example 108.1, 108.15; 108.3, 108.35; 108.5, 108.55, up to 111.95 MHz. This part of the band is shared with VOR — that's VHF Omni-directional Range. The frequencies allocated are odd decimals and odd decimals plus 0.05 MHz. So you'll notice the pattern: 108.1, then 108.15 — the odd decimal and the odd decimal plus half a hundredth. The glide path operates in the UHF band between 329.15 and 335 MHz, providing 40 complementary channels — for example 329.15, 329.3, 329.45, 329.6, up to 335 MHz. All markers transmit at 75 MHz. There's no interference problem because the radiation pattern is a narrow fan-shaped vertical beam — so it's tightly focused upward, not spreading out to cause trouble. Now, frequency pairing. This is the key concept. The GP frequency is paired with the localizer, and selection of the frequency is automatic. The localizer and glide path transmissions are paired in accordance with a 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. One: one switch activates both receivers — that reduces the pilot's workload. Two: frequency selection is 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 — you only have to listen for one Morse code identifier to confirm both the localizer and glide path are working. So the whole system hangs together on this pairing: you tune one frequency, and both the localizer and glide path receivers are set automatically. That's the elegance of the ILS design.