dead reckoning

navigation

Written and fact-checked by The Editors of Encyclopaedia Britannica

Related Topics:: navigation; dead-reckoning position; plotter; drift; estimated position

dead reckoning, determination without the aid of celestial navigation of the position of a ship or aircraft from the record of the courses sailed or flown, the distance made (which can be estimated from velocity), the known starting point, and the known or estimated drift.

Some marine navigators differentiate between the dead-reckoning position, for which they use the course steered and their estimated speed through the water, and the estimated position, which is the dead-reckoning position corrected for effects of current, wind, and other factors. Because the uncertainty of dead reckoning increases over time and maybe over distance, celestial observations are taken intermittently to determine a more reliable position (called a fix), from which a new dead reckoning is begun. Dead reckoning is also embedded in Kalman filtering techniques, which mathematically combine a sequence of navigation solutions to obtain the best estimate of the navigator’s current position, velocity, attitude angles, and so forth.

A number of devices used for the determination of dead reckoning—such as a plotter (a protractor attached to a straightedge) and computing charts, now chiefly used by operators of smaller vehicles—have been replaced in most larger aircraft and military vessels by one or more dead-reckoning computers, which input direction and speed (wind velocity can be manually inserted). Some of these computers include an inertial guidance system or have a unit that measures Doppler effects, and some can be programmed to pick up signals from electronic or optical sensing units. The use of more than one such device tends to increase reliability.

celestial navigation

Written and fact-checked by The Editors of Encyclopaedia Britannica

Article History

Key People:: Martin Behaim

Related Topics:: octant; cross-staff; ground position; quadrant; backstaff

See all related content

celestial navigation, use of the observed positions of celestial bodies to determine a navigator’s position. At any moment some celestial body is at the zenith of any particular location on the Earth’s surface. This location is called the ground position (GP). GP can thus be stated in terms of celestial coordinates, with the declination of the celestial object equal to latitude and the Greenwich hour angle equal to longitude. Almanacs such as those published by the Nautical Almanac Office of the U.S. Naval Observatory provide these coordinates for the Sun, Moon, and planets (or navigator’s stars); the tabulations are given in terms of Greenwich Civil Time. From this information a line of position can be plotted. In principle, the line could be drawn on a very large sphere, but, in practice, a Mercator chart, or plotting sheet, is used. The navigator then uses a sextant or bubble octant to measure the altitude of the celestial object and records this altitude using Greenwich Civil Time. The navigator estimates his position, this being the dead-reckoning position. The altitude and the bearing that the celestial object would have at this position are calculated or taken from tables. The dead-reckoning position is marked on the plotting sheet and a line drawn in the direction of the celestial object’s calculated bearing. From this information and from the difference between the observed and computed altitudes of the celestial object, known as the intercept, the position of the navigator can be calculated.