Instrument Landing System (ILS) — Page 157, Lesson 144

Right, let’s get into the Instrument Landing System — the ILS. This is the system that gives you precise vertical and horizontal guidance to the runway in poor visibility, and we’re starting with the principle of operation, specifically the localizer. The localizer is the part of the ILS that gives you lateral guidance — it tells you whether you’re left or right of the runway centre line. The localizer antenna produces two overlapping lobes along the runway approach direction, which is the QDM — that’s the magnetic bearing you fly to reach the runway. Both lobes are transmitted on a single VHF ILS frequency, so your receiver has to be able to tell which lobe it’s in. It does that by modulation. The right-hand lobe — the blue sector — is modulated at 150 Hz. The left-hand lobe — the yellow sector — is modulated at 90 Hz. So the same carrier frequency carries two different audio tones, one per side. Your ILS receiver measures which tone is stronger, and that tells it which side of the centre line you’re on. Now, here’s the key idea: the depth of modulation — the DoM — increases as you move away from the centre line. In other words, the amplitude of the modulating signal gets bigger the further you are from the centre line. So if you’re approaching the runway centre line from the right, you receive more of the 150 Hz signal than the 90 Hz. That difference in depth of modulation — the DDM — is what relates to your angular displacement from the centre line. The DDM energises the vertical needle of the ILS indicator, and it tells you to go left. Conversely, if you’re approaching from the left, you receive more of the 90 Hz signal than the 150 Hz. The DDM then energises the vertical needle to tell you to go right. And when the DDM is zero — when the two modulations are balanced — you get zero needle deflection, and that means you’re exactly on the runway centre line. So the whole lateral guidance boils down to comparing the depth of modulation of those two tones. Now, there’s a complication you need to know about: the back course. Behind the localizer aerial there’s a mirror image of the radiation pattern, so your aircraft equipment will still receive ILS indications when you’re on the opposite side of the aerial, flying away from the approach direction. Back Course ILS is used in some countries, but it is not permitted in the United Kingdom. So in the UK, you ignore any back course indications entirely. Why is it so problematic? There are three specific disadvantages. First, the glide path indications are incorrect — if you used them, they would guide you to the wrong end of the runway. Second, the CDI needle — that’s the course deviation indicator, the localizer needle — is sense reversed when flying to the runway. So the needle would point the wrong way. And third, there are no range-check markers on the back course, so you lose that distance-checking capability. Let me show you the radiation pattern so you can see those two lobes and how the modulation is split. So to pull it together: the localizer gives you left–right guidance by comparing the depth of modulation of a 150 Hz tone on one side and a 90 Hz tone on the other. The difference in depth of modulation drives the vertical needle, and zero DDM means you’re on the centre line. And in the UK, you disregard any back course indications because they’re not permitted and the guidance is unreliable.