Light aircraft, flown by pilotage, typically have a simple set of navigational instruments, including an airspeed indicator (see pitot static system), an aneroid altimeter, and a magnetic compass. For supersonic and hypersonic aircraft the airspeed indicator is altered to show the airspeed as a Mach number, which is the ratio of the speed of an aircraft to the speed of sound. Advanced aircraft also use electronic systems to give the pilot highly accurate positional information for use during landing. The Instrument Landing System enables an airplane to navigate through clouds or darkness to an airport's runway; the Microwave Landing System, installed in U.S. airports beginning in 1988, is capable of landing the plane automatically, although the pilot always has the option of overriding manually.
Other navigational aids include the radio altimeter, a radar device that indicates the distance of the plane from the ground; the ground-speed indicator, which operates by measuring the Doppler shift in a radio wave reflected from the ground; and, in commercial airliners, the flight management computer, which can display altitude, speed, course, wind conditions, and route information, as well as monitor the airplane's progress through the airway. Other similar systems use inertial devices such as free-swinging pendulums and gyroscopes as references in determining position. These automated and semiautomated procedures free the pilot from many of the activities previously necessary for navigation and thus allow the pilot to concentrate on actually flying the aircraft. Another device which is useful in this way is the automatic pilot, which interprets data on direction, speed, attitude, and altitude to maintain an aircraft in straight, level flight on a given course at a given speed.
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