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

Let's start with the localizer, because it's the heart of the ILS lateral guidance. The localizer antenna sits at the far end of the runway, and it produces two overlapping lobes of radio energy that project back along the approach direction — that's the runway QDM, the magnetic bearing you fly toward the runway. Both lobes are transmitted on a single VHF ILS frequency, so your receiver can't tell them apart by frequency. Instead, each lobe is identified by its 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 as you fly, your receiver is comparing how much 150 Hz signal it's getting versus how much 90 Hz signal it's getting. Now here's the key: the depth of modulation, the DoM, increases as you move away from the centre line. That means the amplitude of the modulating signal grows the further you are from the runway centre line. So if your aircraft is approaching from the right of the centre line, you'll receive more of the 150 Hz signal than the 90 Hz. That difference in depth of modulation — the DDM — is what tells you your angular displacement from the centre line. The DDM energizes the vertical needle of the ILS indicator, and it commands Go Left. Conversely, if you're approaching from the left, you receive more of the 90 Hz signal than the 150 Hz, and that DDM energizes the vertical needle to command Go Right. When the DDM is zero, you have a perfect balance between the two modulations. That gives you zero needle deflection, which means you're exactly on the runway centre line. So the whole localizer principle is: the greater your angular displacement, the greater the DDM, and the further the needle deflects. Now, there's an important complication — the back course. Behind the localizer aerial there's a mirror image of the radiation pattern, so your aircraft equipment will receive ILS indications even when you're flying away from the approach end. Back Course ILS is used in some countries, but it is not permitted in the United Kingdom. So if you're operating in the UK, you must ignore any back course indications entirely. And there are three specific disadvantages to the back course that you need to know. First, the glide path indications are incorrect — if you used them, they would guide the aircraft to the wrong end of the runway. Second, the CDI needle — that's the course deviation indicator, the localizer needle — is sense reversed. So when you're flying toward the runway, the needle tells you the opposite of what you'd expect. 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 they overlap. So to summarise the localizer: two lobes, one frequency, 150 Hz on the right, 90 Hz on the left, DoM increasing away from centre line, DDM driving the vertical needle, and a zero DDM meaning you're on the centre line. And remember — in the UK, back course indications are to be ignored.