Dead reckoning

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.