Simplified directional facility (SDF) is a localizer-based instrument non-precision approach to an airport, which provides final approach course similar to instrument landing system (ILS) and localizer type directional aid (LDA) approaches, although not as precise.
The SDF signal is fixed at either 6 or 12 degrees, as necessary to provide maximum flyability and optimum course quality. Unlike an ILS, an SDF does 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.
Current SDF approaches in the United States
The following SDF approaches are published in the United States (as of March 2017):
- KISW, SDF RWY 02, Alexander Field South Wood County Airport, Wisconsin Rapids, WI
- KMFI, SDF RWY 34, Marshfield Municipal Airport, Marshfield, WI
- KMOR, SDF RWY 05, Moore–Murrell Airport, Morristown, TN