Simplified directional facility
Simplified directional facility was a localizer-based instrument non-precision approach to an airport, which provided final approach course similar to instrument landing system and localizer type directional aid approaches, although not as precise.
The SDF signal was fixed at either 6 or 12 degrees, as necessary to provide maximum flyability and optimum course quality. Unlike an ILS, an SDF did not provide vertical guidance in the form of a glideslope. The SDF course may or may not be aligned with the runway because its antenna may be offset from the runway centerline. Usable off-course indications are limited to 35 degrees either side of the course centerline.