The localizer signal is usable in which directions?

The localizer signal is designed to provide precise horizontal guidance to aircraft as they approach a runway for landing. It projects a radio signal along the extended centerline of the runway, allowing pilots to determine their position in relation to the centerline. While lateral guidance is the primary purpose of the localizer, it does also provide vertical guidance when combined with other systems such as the glideslope.

The correct choice highlights the fact that the localizer provides both vertical and lateral guidance, although its primary function is lateral guidance to ensure that the aircraft is aligned with the runway centerline. This ability to guide an aircraft in both dimensions is essential for safe landings, particularly in low visibility conditions.

The other choices do not accurately reflect the capabilities of the localizer signal. For instance, the assertion that it operates only horizontally overlooks its vertical guidance features when used in conjunction with a glideslope, while suggesting it can be utilized in a forward and backward manner does not align with the nature of a localizer's directional use in the landing process. Thus, the emphasis on its dual capabilities of vertical and lateral guidance makes the correct choice the most accurate representation of the localizer's function.