Where is the localizer typically placed in relation to the runway?

The localizer is a critical component of an Instrument Landing System (ILS), providing lateral guidance to pilots as they approach the runway. Typically, the localizer is positioned at the threshold of the runway, which coincides with the beginning of the usable part of the runway for landing.

This positioning ensures that the localizer antenna emits signals that align with the runway centerline, guiding aircraft to properly align with it as they descend. The localizer needs to provide precise directional guidance so that the aircraft can maintain the intended approach path throughout the final stages of descent.

While considering the other options, it’s important to note that placing the localizer at the midpoint of the runway or at the departure end would not provide the necessary lateral guidance for landing. Additionally, while the consideration of the centerline may seem relevant since the localizer aligns with it, the antenna itself is not physically located at that point, but rather at the threshold, where the landing actually begins. Thus, the most accurate description of the localizer's placement is at the runway threshold.