Everything about Dead Reckoning totally explained
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. Studies on the
homing pigeon have shown the birds use this ability in returning to find their nest. 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.
Marine navigation
In marine navigation a dead reckoning plot generally doesn't take in to 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.
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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 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 by taking into account the current (known as
set and drift in marine navigation). If there's 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.
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.
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 isn't documented in the
Oxford English Dictionary or any other historical dictionary. Dead reckoning is navigation without stellar observation. With stellar observation, you're "live," working with the stars and the movement of the planet. With logs, compasses, clocks, but no sky, you're 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 isn't close enough to the predicted state. Other programs in the network use the same prediction algorithm to fill in the gaps between entity updates.
Further Information
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