With the advent of GPS, published approaches can be created to get an air craft to almost any runway or point in space. That wasn't always the case, of course, and before GPS came along there were operational needs for approaches where no navaids existed. Enter the radar approach, basically the ultimate set of vectors. Like published procedures based on navaids or GPS, radar approaches come in two basic flavors: precision and non-precision. The non-precision version is known as a surveillance approach—or ASR approach, for "airport surveillance radar"—and provides lateral guidance only; the pilot is respon sible for managing an appropriate descent to the ATC-specified minimum altitude. The precision radar approach, meanwhile, is known variously as a "precision approach-radar," PAR for short, or as a GCA, "ground-controlled approach," at military facilities. Here's how they work.
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