An instrument approach system that provides both lateral and vertical guidance to landing aircraft.
The term precision approach refers to those types of instrument approach procedures (iaps) which provide vertical, as well as lateral, guidance to incoming aircraft; they stand in contrast to the many types of non-precision-approach, which provide only lateral guidance.
There are several types of precision approaches:
- Most common, and oldest, is the ils (Instrument Landing System), which uses two sets of radio beams (one for vertical and one for lateral guidance) to guide aircraft to the runway. It can guide aircraft all the way down to touchdown and rollout, but requires the use of a straight, constant-angle descent and the installation of a considerable amount of expensive equipment at each ILS-equipped runway (often in inconvenient locations).
- Next came the par (Precision Approach Radar) approach, where an aircraft is painted simultaneously by two radar beams (one measuring its height relative to the glidepath, the other its lateral offset relative to the approach course), and its vertical and horizontal position are depicted to an air-traffic-controller, who then gives the flightcrew instructions for staying on course and on glidepath. PAR is extremely labor-intensive due to the need for each landing aircraft to be talked down to the runway by a dedicated controller, and is, nowadays, used almost exclusively on aircraft-carriers.
- MLS (microwave-landing-system) approaches were developed in the 1970s and 1980s to replace the ILS. MLS has a number of advantages over ILS; it uses antennae that are much smaller than those used for an ILS and not subject to the latter's siting limitations, a single MLS installation can serve an entire airport (whereas an ILS-equipped airport needs one ILS per runway), and an MLS provides guidance over a wide fan-shaped area, allowing aircraft to approach along a wide variety of courses (which need not be straight courses, either - curved approaches are perfectly kosher with MLS) at a wide variety of approach angles (whereas an ILS stuffs all its landing aircraft down one single straight line in the sky in single file). However, by the time MLS was ready for widespread deployment, satellite navigation (better known as GPS, for "Global Positioning System"; gnss is a more recent, rarely-used synonym for GPS) had overtaken it, and only a very few MLS approaches were ever brought online.
- rnav (aRea NAVigation, although this doesn't even begin to do justice to everything RNAV is capable of doing) approaches use GPS signals, plus an additional signal that tells the receiver how much to add or subtract to correct for the error in the GPS signal at that time and place, to guide the aircraft to the runway. These go MLS one better; whereas MLS still requires some equipment installation on the ground (although much less than ILS), RNAV requires, in some cases, nothing at all (in terms of new construction on the ground), and, even for those RNAV approaches that do require equipment on the ground, they still require less than even MLS. Also, RNAV technology can be used for navigation during other phases of flight as well, instead of only providing guidance during approach and landing. There are several subtypes of RNAV approaches; some provide only lateral guidance (and are, thus, nonprecision approaches), but some are true precision approaches, providing both lateral and vertical guidance.