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A book that contains astronomical or meteorological data arranged according to days, weeks, and months of a given year and may also include diverse information of a nonastronomical character. This article is restricted to astronomical and…

Any device which measures the height of an aircraft. The two chief types are the pressure altimeter, which measures the aircraft's distance above sea level, and the radar altimeter, which measures distance above the ground. Pressure Altimeter A…

An anchored or moored floating object, other than a lightship, intended as an aid to navigation. Buoys are the most numerous of all engineered aids to navigation, with some 20,000 in United States waters alone.A great variety of buoys are in use.…

A discipline that combines the means to know the location and heading of an aircraft with respect to some map or coordinate system, with guidance which means steering the aircraft along some desired path toward a destination. The construction of…

A service to promote the safe, orderly, and expeditious flow of air traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground; assisting aircraft in avoiding hazardous weather; assuring that…

A device that reduces the risk of midair collision by providing advisories to the flight crew. It is known in the United States as a Traffic Alert and Collision Avoidance System (TCAS), and operates independently of the ground-based air-traffic…

An instrument that indicates the bearing, or angle of the direction in which an aircraft is pointing in the horizontal plane. A compass may indicate magnetic heading or bearing, bearing referenced to a radio signal source, or bearing with respect to…

A coordinated group of instruments that provides the flight crew with information about the aircraft and its subsystems. Together with the controls, aircraft instruments form the human–machine interfaces that enable the flight crew to operate the…

A ground mapping system that uses analog radar equipment to provide surveillance of aircraft and other surface vehicles on an airport surface. It is used by air-traffic controllers in airport control towers to monitor and control the movement of…

A device that computes and displays speed of an aircraft relative to the air mass in which the aircraft is flying. Three common types are the indicated airspeed meter, true airspeed indicator, and Machmeter.The commonest type is the indicated…

The satellite navigation systems GPS (the U.S.–managed Global Positioning System) and GLONASS (the Russian global navigation satellite system) have been operational in some form for decades. Other global and regional navigational satellite systems…

A device that provides the pilot with symbols representative of the attitude of an aircraft relative to an artificial horizon. In an automatic or artificial horizon a vertical gyro moves a horizon bar relative to a fixed aircraft index. Motion of…

The means for guiding and controlling aircraft from an initial approach altitude to a point where safe contact is made with the landing surface. Such systems differ from low-approach systems in three major respects: (1) They furnish not only…

An automatic means for steering an aircraft, ship, or other vehicle. The terminology means to automatically (autonomously) control a mode of the vehicle, usually its trajectory. Examples include altitude, speed, heading, accelerations, body rates,…

Navigation with the aid of celestial bodies, primarily for determination of position when landmarks are not available. In celestial navigation, position is not determined relative to the objects observed, as in navigation by piloting, but in…

A form of navigation that determines position of a craft by advancing a previous position to a new one on the basis of assumed distance and direction moved. The name probably stems from the early practice of determining speed by throwing overboard a…

Equipment used for determining the direction from which a received signal is being radiated. Direction finders serve a variety of applications. Direction finders can be used for navigation purposes to determine the heading of a vehicle relative to a…

An internationally standardized navigation system which allows an aircraft to measure its distance from a selected ground-based beacon. Such beacons are used throughout the world by all airliners, most of the military aircraft of the West, and a…

A very high-frequency omnidirectional radio range (VOR) employing the Doppler principle. The bearing accuracy achieved by users when VOR beacons are installed in the vicinity of obstructions, or when aircraft using VOR signals fly over mountainous…

Systems that locate suitably equipped users by means of measurements made by electronic sensors. Typical measurements include distance or bearing from a vehicle to a known point, or direct determination of the present position of the vehicle in a…

An aircraft landing instrument that provides the pilot with a visual indication of vertical and horizontal angular deviations from a glide path or final approach segment (FAS) that indicates the off-center position of the aircraft from the FAS with…

A system carried on many aircraft to warn the pilot that the aircraft may be in danger of inadvertent contact with the ground. It is intended to reduce the occurrence of controlled-flight-into-terrain (CFIT) accidents, in which aircraft with no…

The algorithms, computers, and sensors used to produce steering commands in the control of mobile platforms. The types of vehicles include airplanes, rockets, missiles, ships, torpedoes, drones, and others, including robotic vehicles used in…

A north-seeking form of gyroscope used as a directional reference in navigation. The first practical gyrocompasses were developed by H. Anschütz (Germany) in 1908, E. A. Sperry (United States) in 1911, and S. G. Brown (England) in 1916. Modern…

A device that is used to define a fixed direction in space or to determine the change in angle or the angular rate of its carrying vehicle with respect to a reference frame. Gyroscopes (also called gyros) respond to vehicle angular rates, that is,…

A process of navigation by which a destination is approached by keeping some navigation parameter constant. In its early uses, the most commonly chosen parameter was the relative bearing from the vehicle to destination as determined from a signal…

A navigation system that produces hyperbolic lines or surfaces of position by measuring the difference in times of reception (or phase difference) between radio signals from two or more synchronized transmitters. Such systems require the use of a…

A self-contained system that can automatically determine the position, velocity, and attitude of a moving vehicle for the purpose of directing its future course. Based on prior knowledge of time, gravitational field, initial position, initial…

A collection of discrete radio navigation aids used by pilots of all types of aircraft for approach guidance to a specific airport runway, especially during times of limited visibility. Typically, three components constitute the instrument landing…

A distinctive structure, built on or near a shore, which exhibits a light of distinctive characteristics to serve as an aid to navigation (see illustration). Lesser lights may be displayed from fixed structures called beacons or from floating buoys…

A navigation system from which hyperbolic lines of position are determined by measuring the difference in times of arrival of pulses from widely spaced, synchronized transmitting stations. Since radio waves travel at the speed of light, a line of…

A compass depending for its directive force upon the attraction of the Earth's magnetism on a magnet free to turn in any horizontal direction. A compass is an instrument used for determining horizontal direction. Principle of operation The magnetic…

The process of directing a watercraft to a destination in a safe and expeditious manner. From a known present position, a course is determined that avoids dangers, and on this course estimates are made of time schedules. The task is to make…

An all-weather aircraft landing-guidance system that operates at microwave frequencies and provides deviations from the landing runway centerline using a time-referenced scanning beam (TRSB) technique. The MLS was standardized in 1988 and approved…

The process of directing the movement of a craft from one place to another. Navigation involves position, direction, distance, time, and speed.The process of keeping track of a craft's location by measuring and applying progress from a previous…

The form of navigation in which position is determined relative to external reference points, usually fixed points on the Earth. It is the oldest form of navigation. The following remarks are formulated in the context of marine navigation. However,…

The complex of navigational techniques modified from those used in other areas to suit the distinctive regional character of polar areas. Although polar navigation has become routine to a rising number of navigators operating in and through such…

A tracking system that provides a ground control approach (GCA) air-traffic controller with a precise display of an aircraft's position relative to a runway final-approach course. To ensure absolute safety, precise information is displayed on a plan…

A popular term for an automatic radio direction finder used for navigation purposes on ships and aircraft. It is not strictly a compass, because it indicates direction with respect to the radio station to which it is tuned, rather than to the north…

An aircraft instrument that provides an indication of the vertical change of the aircraft position within the air mass. It is more commonly known as the vertical-velocity or vertical-speed indicator. Contained within a sealed case, it is connected…

A navigation system in which one or more signals are emitted from a facility (or colocated facilities) to produce simultaneous indication of bearing and distance. Since a bearing is a radial line of position and a distance is a circular line of…

Radionavigation systems using artificial satellites as sources of radio signals and position references. Development of satellite-based systems for positioning and navigation began almost immediately after the launch of Sputnik I by the Soviet…

Any apparatus which swings, because of gravity, with a natural period of 84.4 min, that is, with the same period as a hypothetical simple pendulum whose length is the Earth's radius. In 1923 Max Schuler showed that such an apparatus has the unique…

A navigation instrument used for measuring angles, primarily altitudes of celestial bodies. Originally, the sextant had an arc of 60°, or one-sixth of a circle, from which the instrument derived its name. Because of the double-reflecting principle…

The selection of the most favorable route for a ship's transit in the open ocean. Safety, economics, and environmental protection play a key role in determining this selection. In the late 1890s, the practice of following predetermined routes for…

The determination of the position and velocity of a space probe with respect to a target body or a known reference body such as the Earth (navigation) and, based upon this determination, the application of propulsive maneuvers to alter the…

A device that determines the critical angle of attack for a given aircraft, at which point the lift stops increasing and the aircraft will no longer sustain itself in steady-state condition (level flight or climb/descent). The indicator usually…

A device that automatically measures the angular separation of stellar observations with respect to a reference platform. It is also referred to as an astrotracker.By utilizing the star tracker in conjunction with a precise time reference…

Any radar equipment used by civil or military authorities for locating aircraft, ships, or ground vehicles; most commonly, in civil air-traffic management, a radar used by controllers to indicate the position of aircraft aloft or on the airport…

A member of the rho-theta family of air navigation systems which define an aircraft's position by its distance and bearing to a single beacon. Such systems inherently answer the navigator's questions of “in what direction” and “how far,” without…