In article ,
says...

Im new to IFR and was wondering what an SDF approach is.


Look it up in any book, and you'll find probably a full paragraph, indicating
the amount of printing ink the publishers wish to squander on this approach -
the SDF (Simplified Directional Facility, IIRC) is like a LOC approach, except
instead of being aligned with a friendly runway it's aligned with a big old
tree or communications antenna several degrees off the extended centerline.
Lateral precision is reduced, compared with a LOC, and MDA is correspondingly
higher.

There are very few of them left - SME (Somerset KY) was one of them, until a
KingAir took out a communications tower a few years back.

I think you can pretty much bank on them being phased out entirely pretty
soon, though I wouldn't want to put a time scale on that.

G Faris