navigation

A French mariner using an astrolabe to fix the position of a star, from a vellum manuscript of Jacques Devaulx (1583; Bibliothèque Nationale, Paris, France). The mariner's astrolabe was introduced in the mid-15th century, but did not see general use until the beginning of the 16th century. It was supplanted by the sextant in the 18th century.

An 18th-century sextant (West Australia Museum, Australia). The sextant is used to determine the observer's position on the Earth's surface by measuring angles between the horizon and celestial bodies. The sextant has been a basic navigational tool for more than two centuries.

The science and technology of finding the position, course, and distance travelled by a ship, plane, or other craft. Traditional methods include the magnetic compass and sextant. Today the gyrocompass is usually used, together with highly sophisticated electronic methods, employing beacons of radio signals, such as Decca, Loran, and Omega. Satellite navigation uses satellites that broadcast time and position signals.

The US global positioning system (GPS) was introduced in 1992. When complete, it will feature 24 Navstar satellites that will enable users (including eventually motorists and walkers) to triangulate their position (from any three satellites) to within 15 m/50 ft.

In 1992, 85 nations agreed to take part in trials of a new navigation system which makes use of surplus military space technology left over from the Cold War. The new system, known as FANS or Future Navigation System, will make use of the 24 Russian Glonass satellites and the 24 US GPS satellites. Small computers will gradually be fitted to civil aircraft to process the signals from the satellite, allowing aircraft to navigate with pinpoint accuracy anywhere in the world. The signals from at least three satellites will guide the craft to within a few metres of accuracy. FANS will be used in conjunction with four Inmarsat satellites to provide worldwide communications between pilots and air-traffic controllers.