DEAD RECKONING

the determination without the aid of celestial observations 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. Virtually every navigation system uses dead reckoning. Some marine navigators differentiate between the dead-reckoning position, for which they use the course steered and their estimated speed through the water, from 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 navigation is used now and then to determine a reliable position (called a fix), and a new dead reckoning is begun from that point. A number of devices used for the determination of dead reckoningsuch as a plotter (a protractor attached to a straightedge) and computing charts, now chiefly used by operators of smaller vehicleshave 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 increasingly sophisticated computers use Doppler or inertial sensing units, 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.