Navigation

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Global Positioning Systems (GPS)

The Transit system of six polar-orbit satellites provides a worldwide positioning service to military and research ships, but the system will probably be abandoned in the 1990s. The United States Navstar GPS and the former Soviet Union's Glonass system of military satellites are also available for civilian use. The European Space Agency is planning a Navsat system of 18 spacecraft. The International Maritime Satellite Organization (Inmarsat) is also developing a worldwide system of navigational aids. GPS are becoming accepted as the most convenient form of navigation aid for many applications.

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Ground-Controlled Approach (GCA)

GCA is an instrument-approach system consisting of extremely high-precision microwave radar equipment that gives the position of an aircraft in range, azimuth, and elevation. It is primarily designed to bring the pilot through conditions of low visibility to make a normal landing by visual contact. Skilled operation of this system in the aircraft and on the ground permits emergency landing under conditions of nearly zero visibility. GCA uses two sets of radarscopes. One locates planes at a considerable distance, such as 15 to 25 km (about 9 to 15 mi). The controller using this set of scopes maintains communications with planes waiting to land, “stacks” them (that is, assigns each one to a separate altitude at which it can circle without danger of collision), and brings them in one at a time along a standard approach pattern until they are on the final leg of the approach. On the final approach leg the final-approach controller, using precision scopes, takes control. This controller also broadcasts verbal instructions, principally concerning altitude and lateral deviation from the desired glide path, and guides the pilot virtually to the end of the runway.

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Instrument Landing System (ILS)

This system is primarily designed for instrument approach, but in emergency can be used for landing. It consists essentially of two beams, similar to radio range beams, one horizontal and the other vertical. The horizontal beam (called the localizer) is identical with the visual-aural range (VAR) beam, an ordinary radio range with only two beams instead of four. The vertical beam (called the glide-path) is extremely narrow and is inclined to the ground at an angle of 2.5°. The pilot follows the two beams by means of two pointers, one horizontal and one vertical, on a single dial.

Both ILS and GCA receive valuable supplementary aid from a standardized high-intensity lighting system along the runway and approach so that the pilot can make visual contact with the ground even in extremely bad weather and identify the position of the aircraft in relation to the runway.

Proposals have been made to replace ILS and GCA by a single microwave landing system (MLS). However, there is now some doubt as to whether this system will be implemented or whether it will be overtaken by the use of GPS.

Most radio navigational systems that are currently in use are operated in conjunction with high-speed computers.

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Missile Navigation Systems

The development of rockets and guided missiles has accelerated the introduction of new and sophisticated electromechanical systems of navigation, including automatic celestial systems, Doppler navigation, and inertial navigation.

The automatic celestial system, known also as the star-tracking system, consists of an electronic device capable of computing a celestial solution and feeding it to a unit designed to track a celestial body or bodies automatically. The tracking unit feeds back the information to computers that then record the actual position of the vehicle.

Doppler navigation, named after the 19th-century Austrian physicist and mathematician Christian Johann Doppler, is concerned primarily with air navigation and involves the analysis of the shift in radio frequency resulting from reflection of radar waves by an approaching or receding object. See Doppler Effect.

Inertial navigation, which is based on inertial guidance, is a self-contained system, wholly independent of either visual or electronic information from outside the craft in which it is operating. This system consists of a certain type of accelerometer, stabilized by gyroscopes that register the magnitude of the acceleration of the craft in both a north-south and an east-west direction from a known starting point; the accelerations are converted by computers into a precise position for the craft. Originally developed for use in long-range ballistic missiles and nuclear-powered submarines, inertial systems now find much wider applications, such as guiding long-range transport aircraft.

See also Air Traffic Control.