ple, during the night and when landing and taking off in low cloud ceilings. When flying IFR, pilots rely on navigation instruments in conjunction with instructions from air traffic control agencies issuing clearances for routes

taking off in low cloud ceilings. When flying IFR, pilots rely on navigation instruments in conjunction with instructions from air traffic control agencies issuing clearances for routes and altitudes. In congested areas, air traffic controllers essentially “steer” aircraft using radar vectoring. To ensure an orderly flow of traffic, most phases of flight must follow a track, or flight path, that coincides with ground-based navigation facilities. This requirement has always been a major constraint on IFR operations. The ability to navigate with a high degree of accuracy, independent of ground-based facilities, is considered the most beneficial aspect of GPS with regard to aviation. Precision and Nonprecision Approaches. Aeronautical navigation involves four phases of flight: en route, terminal, approach, and departure. En route navigation encompasses point-to-point navigation, usually between cities. Departure navigation is the portion of flight from take-off to the en route phase. And terminal navigation refers to the transition between the en route phase and the approach, or landing phase. For approaches, there are two basic procedures: precision and nonprecision. A navigation system permitting a precision approach provides both lateral (horizontal) and vertical guidance to a decision altitude/height (DA/H). If the required visual references, such as approach lights or the runway environment, are not in view at the DA/H, a pilot must execute a missed approach — that is, a specified, controlled routing away from the runway. A nonprecision approach (NPA) provides lateral guidance only and uses a minimum descent altitude (MDA). MDA is defined as an altitude below which an aircraft must not descend until visual reference has been established (see Figure 1). Typically, MDAs range from 300 to 500 feet, but the actual value depends on the presence of obstacles such as hills, buildings, or towers in the airport’s vicinity. The International Civil Aviation Organization is in the process of introducing a new