Final answer:
The SDF provides course guidance without glidepath information, while the localizer provides both course and glidepath guidance. The SDF has wider azimuth coverage than the localizer.
Step-by-step explanation:
The main differences between the SDF (Simplified Directional Facility) and the localizer of an ILS (Instrument Landing System) lie in their functions and capabilities.
The SDF provides course guidance to the pilot but does not provide glidepath information. It is typically used for non-precision approaches, where the final descent is done visually by the pilot. The SDF has a wider azimuth coverage compared to the localizer, which means it can provide guidance to aircraft flying at larger angles from the runway centerline.
On the other hand, the localizer provides both course and glidepath guidance to the pilot. It aligns the aircraft with the centerline of the runway and provides vertical guidance for the descent. This makes it suitable for precision approaches, where the pilot can rely on the instrument indications to accurately fly the aircraft to the runway touchdown point.