SDF is an instrument approach facility that shares some characteristics with a localizer but has a wider course width.

• A. A radar beacon used in enroute navigation
• B. A ground-based radar alignment tool
• C. Simplified Directional Facility
• D. A type of VOR/DME combination

Answer: (C)

SDF stands for Simplified Directional Facility, a ground-based navigation aid that provides lateral guidance to a runway just like a localizer, but with a wider course width. That wider width means the instrument guidance is less precise than a standard localizer, making the approach easier to implement and use, while still guiding you along a defined path to the runway. Importantly, SDF typically does not provide vertical guidance, so you’re following lateral guidance only, similar to other non-precision approaches.

So the best choice describes a facility that uses a localizer-type signal and offers lateral guidance with a broader course than a standard localizer. The other options refer to different navigation tools that don’t fit this concept.