In an Instrument Landing System (ILS), what is the function of the localizer?

• A. It transmits distance to the runway.
• B. It measures wind shear.
• C. It provides vertical guidance for descent along the glide path.
• D. It provides lateral guidance to the runway centerline.

Answer: (D)

In an ILS, the localizer handles the lateral guidance to the runway centerline. It transmits signals that your cockpit equipment converts into a course deviation indicator, showing whether you’re left or right of the extended runway centerline. The localizer antenna array at the runway threshold is aligned with that centerline, so keeping the CDI centered steers you precisely toward the runway path. It does not provide distance information—that’s the job of distance measuring equipment or markers. It also doesn’t measure wind shear; wind-shear detection is handled by separate systems. Vertical guidance, to descend along the approach path, comes from the glide slope component, which works alongside the localizer but serves a different purpose.