Dead reckoning is the process of estimating a global position of a vehicle by advancing a known position using course, speed, time and distance to be traveled. In other words figuring out where you momentarily are or where you will be at a certain time if you hold the speed, time and course you plan to travel.

Dead reckoning (DR) is a method of navigation used in ships, aircraft, trucks, cars, rail engines, construction sites engines e.g. in tunnels and, more recently, mobile robots. Essentially it is used to estimate an object's position based on the distance it traveled in its current direction from its previous position.

A navigator using this method uses the craft's last known position (fix), then plots the craft's expected position for a given fix interval (elapsed time from one fix to the next) according to the compass course it is steering, the speed it is making, and allowance for winds and tides. In modern navigation, this plotted position is compared to a fix, taken at the time for which the DR was plotted, to determine set and drift (the combined external forces which act upon a ship causing it to deviate from its intended course). The difference between actual position (fix) and DR position helps the navigator determine a course and speed that will allow for set and drift in maintaining the ordered course and speed of advance.

Before modern navigational methods were available, the navigator incorporated his estimation of these forces (wind, current, helmsman error, etc.) in his DR plot. The DR plot was often the only method used if, for example, the sky was overcast and a celestial observation could not be made.

Before the development of the chronometer, dead reckoning was the primary method of determining longitude available to mariners such as Christopher Columbus and John Cabot on their trans-Atlantic voyages.

## Etymology

There is some controversy about the derivation of the phrase. It is popularly thought to come from deduced reckoning and is sometimes given in modern sources as ded reckoning. However, according to the Oxford English Dictionary, the phrase dead reckoning dates from Elizabethan times (1605-1615).

The popular etymology from deduced is not documented in the Oxford English Dictionary or any other historical dictionary. Dead reckoning is navigation without stellar observation. With stellar observation, you are "live", working with the stars and the movement of the planet. With logs, compasses, clocks, but no sky, you are working "dead".

## Computer games and simulations

Dead reckoning is also a method used in networked computer games and simulations to reduce lag caused by network latency and bandwidth issues. Programs do this by predicting the future state of an entity based on its current state (such as predicting the path of a fighter jet based on its velocity and position). Then the program only sends updated information about the entity's current state if it is not close enough to the predicted state. Other programs in the network use the same prediction algorithm to fill in the gaps between entity updates.

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