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For those who aren't up on the requirements for instrument approaches or low visibility systems, we have started this tutorial. It attempts to introduce the regulatory and practical aspects of achieving precision approaches, low visibility landings, and terminal area operational safety. This is a huge undertaking so we must ask for both your patience as it goes together and your ongoing criticism of its correctness. The word precision has a special meaning when used to describe an approach procedure: the procedure provides both lateral and vertical guidance continually to the pilot. A non-precision approach only provides continuos lateral guidance with the pilot having to determine the altitude as a function of his position in the approach. Examples of non-precision approaches are those conducted using ADF, VOR, VOR/DME, LOC, or SDF ground signals. Today's precision approaches are almost universally based on the Instrument Landing System which has served worldwide since the 1950's (Figure 1). In the last few years the Microwave Landing System has began to supplement ILS, especially in Europe. Enhanced GPS systems using Wide Area Augmentation Systems or Local Area Augmentation System are being proposed sources for future precision approach data. The APALS Navigator is the latest entry into the market. Precision approaches are grouped into three major categories, described as Cat I, Cat II, and Cat III. There are three sub-categories for Cat III: Cat IIIa, Cat IIIb, and Cat IIIc. |
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