Dead reckoning

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For other uses, see Dead reckoning (disambiguation).
The navigator plots his 9am position, indicated by the triangle, and, using his course and speed, estimates his position at 9:30am and 10am.

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 or estimated speeds over 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.

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[edit] 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. Animals such as ants, rodents, and geese have also been shown to continuously keep track of their locations relative to a starting point and return to it, an important skill to have for creatures that forage for food and then return to a fixed home.[1]

[edit] Marine navigation

In marine navigation a "dead" reckoning plot generally does not take into account the effect of currents or wind. Aboard ship a dead reckoning plot is considered important in evaluating position information and planning the movement of the vessel.[2]

Dead reckoning begins with a known position, or fix, which 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 speed (e.g. in 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 (rodmeter), which uses 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 by taking into account the current (known as set and drift in marine navigation). If there is no positional information available, a new dead reckoning plot may start from an estimated position. In this case subsequent dead reckoning positions will have taken into account estimated set and drift.

Dead reckoning 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 dead reckoning 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.

[edit] 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, dead reckoning 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 aircraft. 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.

[edit] Ground Transportation

Prior to the advent of GPS Technology some public transportation systems used dead reckoning to track their fleets. Transponders placed along the service routes would act as the fixed points. Transponders would be installed at major intersections and hydro poles along the routes enabling the management to monitor operations from a central location and make service adjustments based on the data. A major downside to this technology for the purpose of Ground Transportation is that if a bus goes off route for any reason (diversion for an accident, roadwork, etc.) the Vehicle would simply appear to be held at its last known position or drop off the tracking map all together. The Toronto Transit Commission still uses this technology for its streetcar and bus fleets, however has indicated that they soon plan on implementing GPS on all their surface vehicles.[3]

[edit] 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."

In an extended discussion on "dead vs "ded", the Straight Dope Science Advisory Board summarises it as follows:

The term dates from the seventeenth century, so we have to look to the sea for the origin of the term, not the air. One theory is that dead reckoning was first called "deduced reckoning," which someone then abbreviated (in a ship's log) as "ded. reckoning." Later someone reading it thought "ded" didn't make much sense, so he wrongly (according to this theory) thought it must be a misspelling for "dead." The other theory is that it was "dead reckoning" from the beginning, but since this sort of navigation doesn't seem to have much to do with death, someone assumed the derivation from "deduced," which must have made more sense to him. In either case, folk etymology is at work, but it isn't immediately obvious which is the real etymology and which is the folk etymology.

There is an extended bibliography and encyclopedic references accompanying the above article.


[edit] See also

[edit] References

  1. ^ Gallistel. The Organization of Learning. 1990.
  2. ^ http://www.irbs.com/bowditch/pdf/chapt07.pdf
  3. ^ http://findarticles.com/p/articles/mi_m0BPW/is_6_15/ai_n6175512

[edit] External links

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