Dead reckoning (DR) is the process of estimating one's current position based upon a previously determined position, or fix, and advancing that position based upon measured velocity, time, heading, as well as the effect of currents or wind. While this method of navigation is no longer considered primary in ships, aircraft, automobiles, rail engines, and construction site engines (tunnels), it is still often used as a backup in case of failure of the electronic navigation systems. In the animal kingdom, dead reckoning is known as path integration, and animals use it to estimate their current location based on the movements they made since their last known location. Naturally, animals do not employ the same tools, such as math, in their dead reckoning as do humans, so the term "dead reckoning" as it applies to animals is mute. The word fix has several possible meanings: a fix is the result of position fixing in navigation a fix is a dose of a drug taken by an addict to fix is to prepare or to repair something to fix is to fasten one object to another using adhesive or... Table of geography, hydrography, and navigation, from the 1728 Cyclopaedia. ... Charles Darwin and J.J. Murphy first postulated an inertially-based navigation system in animals in 1873. ...

In marine and aerial navigation, dead reckoning (DR) begins with a known position, or fix. This fix is then advanced, mathematically, by means of recorded heading, velocity, and time. Velocity can be determined by many methods. Before modern instrumentation, it was determined aboard ship by dropping a floating object tied to a rope of known length into the water, and timing how long it took until the length of rope because taught. More modern methods include referencing engine rpm again a table of total displacement (for ships) or referencing one's indicated airspeed fed by the pressure from a pitot tube. This measurement is converted to calibrated and equivalent airspeeds based upon known atmospheric conditions and measured errors in the indicated airspeeed system. Modern ships use a similar pressure method to measure their velocity with respect to the water over which they travel. Distance is determined by multiplying the velocity and the time. This initial position is then adjusted to the DR position by taking into account the current (known as set and drift in marine navigation) or the winds (for aerial navigation). In air navigation, this tool is referred to as a wind triangle. The word fix has several possible meanings: a fix is the result of position fixing in navigation a fix is a dose of a drug taken by an addict to fix is to prepare or to repair something to fix is to fasten one object to another using adhesive or... In navigation set and drift are characteristics of the current in which a ship is sailing. ... The principles of air navigation are the same for all aircraft, big or small. ... The wind triangle graphically represents the relationships among velocity vectors used for air navigation. ...

DR positions are calculated at predetermined intervals, and are maintained between fixes. There duration of the interval is arbitrary. Factors including one's velocity made good and the nature of heading and other course changes, and the navigator's judgement determine when DR positions are calculated. Generally speaking, DR positions are calculated at least once every 300 miles and when making combined turns totalling more than 30 degrees from the initial heading out of the last DR position.

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. A chronometer is a timekeeper precise enough to be used as a portable time standard, usually in order to determine longitude by means of celestial navigation. ... Longitude, sometimes denoted by the Greek letter λ (lambda),[1][2] describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... Christopher Columbus (Genoa?, Italy, 1451? – Valladolid, Spain, May 20, 1506) was a navigator and maritime explorer credited as the discoverer of the Americas. ... Giovanni Caboto (c. ...

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 Oxford English Dictionary print set The Oxford English Dictionary (OED) is a dictionary published by the Oxford University Press (OUP), and is generally regarded as the most comprehensive and scholarly dictionary of the English language. ...

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". A fake etymology, is an invented explanation (etymology) for the origin of a word. ...

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. A computer game is a game composed of a computer-controlled virtual universe that players interact with in order to achieve a defined goal or set of goals. ... A computer simulation or a computer model is a computer program that attempts to simulate an abstract model of a particular system. ... Lag often refers to delays experienced in computing communications, however it may also apply to written or other forms of communication. ... Latency is the time a message takes to traverse a system. ... This article or section does not cite its references or sources. ...

External links

• US Coast Guard Auxiliary: "Dead reckoning"
• Bowditch Online: "Dead reckoning"
• Straight Dope: Is "dead reckoning" short for "deduced reckoning"?
• Jesse Aronson: "Dead Reckoning: Latency Hiding for Networked Games" (requires free registration to Gamasutra.com)