Dead reckoning

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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 known speed, elapsed time, and course. While traditional methods of dead reckoning are no longer considered primary in most applications, modern inertial navigation systems, which also depend upon dead reckoning, are very widely used.

Animal navigation

In studies of animal navigation, dead reckoning is more commonly (though not exclusively) known as path integration, and animals use it to estimate their current location based on the movements they made since their last known location.Some research has been performed with the intention of discovering how birds can find their way back from distant places they have never visited before. Most researchers believe that homing ability is based on a "map and compass" model, with the compass feature allowing birds to orient and the map feature allowing birds to determine their location relative to a goal site (home loft).[5] While the compass mechanism appears to be use of the sun, the map mechanism has been highly debated.[6] Some researchers believe that the map mechanism relies on the ability of birds to detect the Earth's magnetic field. Much of this work has been conducted by Rosthina and Wolfgang Witschko and has shown that alteration of magnetic fields around the home loft result in disrupted homing ability. Recently, researchers have attempted to determine how pigeons can detect magnetic fields, and two different mechanisms have been proposed. A light-mediated mechanism that involves the eyes and is lateralized has been examined somewhat, but recent developments have implicated the trigeminal nerve in magnetoreception.[7][8] Research by Floriano Papi (Italy, early 1970s) and more recent work, largely by Hans Wallraff, suggests that instead pigeons orient themselves using the spatial distribution of atmospheric odors[6] (See the August 20, 2005 issue of Science News.). Near their home lofts, in areas they have previously visited, pigeons probably are guided by visual landmarks.

Various experiments suggest that different breeds of homing pigeons rely on different cues to different extents. Charles Walcott at Cornell was able to demonstrate that one strain of pigeons was confused by a magnetic anomaly in the Earth that had no effect on another strain of birds. Other experiments have shown that altering the perceived time of day with artificial lighting or using air conditioning to eliminate odors in the pigeons' home roost affected the pigeons' ability to return home.

Some research also indicates that homing pigeons navigate by following roads and other man-made features, making 90 degree turns and following habitual routes, much the same way that humans navigate.[9]

Marine navigation

In marine navigation a DR plot generally does not take in to account the effect of currents or wind. Aboard ship a DR plot is considered important in evaluating position information and planning the movement of the vessel.

Dead reckoning (DR) begins with a known position, or fix. This fix is then advanced, mathematically or directly on the chart, by means of recorded heading, speed, and time. Speed can be determined by many methods. Before modern instrumentation, it was determined aboard ship using a chip log. More modern methods include, pit log referencing engine rpm against 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 an equivalent airspeed based upon known atmospheric conditions and measured errors in the indicated airspeed system. A naval vessel uses a device called a Pit Sword. A Pit sword (rodmeter) utilizes two sensors on a metal rod to measure the electromagnetic variance caused by the ship moving through water. This change is then converted to ship's speed. Distance is determined by multiplying the speed and the time. This initial position can then be adjusted resulting in an estimated position (EP) by taking into account the current (known as set and drift in marine navigation). If there is no positional information available, a new DR plot may start from an estimated position. In this case subsequent DR positions will have taken into account estimated set and drift.

DR positions are calculated at predetermined intervals, and are maintained between fixes. The duration of the interval varies. Factors including one's speed made good and the nature of heading and other course changes, and the navigator's judgment determine when DR positions are calculated.

Before the development of the marine 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.

Air navigation

Traditionally, in air navigation, displacement or position caused by wind were taken into account, using a tool called a wind triangle. Generally speaking, DR positions were calculated at least once every 300 miles and when making combined turns totaling more than 30 degrees from the initial heading out of the last DR position.

Today, dead reckoning is rarely used in this traditional form for air navigation, but it survives in the form of inertial navigation systems, which are nearly universal on more advanced aircrafts. The INS is used in combination with other navigation aids, such as GPS, in order to provide reliable navigation capability under virtually any conditions, with or without external navigation references.

Etymology

There is disagreement about the derivation of the phrase. It is popularly thought to come from deduced reckoning and is sometimes given in modern sources as an alternatively spelled 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 the perception of 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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Last updated on Sunday March 09, 2008 at 23:48:58 PDT (GMT -0700)
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