| Dictionary: dead reckoning |
[Possibly alteration of ded., abbr. of deduced, from deduce, to trace from the beginning. See deduce.]
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| Sci-Tech Encyclopedia: Dead reckoning |
A form of navigation that determines position of a craft by advancing a previous position to a new one on the basis of assumed distance and direction moved. The parameters of dead reckoning are direction of motion and distance traveled. The intended direction of travel, the course, may differ from the direction steered because of the anticipated offsetting effect of wind (called leeway) or current, or both. When it is desired to distinguish between the two directions, mariners call the second the course steered, or heading, while aviators refer to it as the heading. A compass is used to indicate direction. Distance is usually determined indirectly by measurement of speed and time, but it may be measured directly.
In addition to several magnetic compasses, nearly all naval vessels and ocean liners are equipped with one or more north-seeking gyrocompasses. Gyrocompasses have replaced magnetic compasses as the primary source of directional information on many modern vessels. See also Gyrocompass; Magnetic compass.
Aboard ship, distance or speed is measured by means of a log or by an engine revolution counter. The pitometer log uses a pitot-static tube. The Forbes log uses a small rotor in a tube projecting below the bottom of the vessel. An electromagnetic log has a sensing element which produces a voltage directly proportional to speed through the water.
In aircraft, speed through the air is measured by means of an airspeed indicator or Mach meter. The latter provides an indication of speed in units of the speed of sound, which varies with density of the atmosphere. For measurement of air speed a pitot-static tube is generally used with a suitable registering device.
To determine position by dead reckoning, air and land navigators, and some marine navigators, use the best estimate of direction and distance traveled over the surface. Many marine navigators, however, prefer to use course steered and estimated speed through the water, without allowance for leeway, for their dead reckoning; they consider positions determined by allowance of estimated effects of wind and current as estimated positions.
The uncertainty of a dead-reckoning position, however determined, increases with time and, if there is an error in direction measurement, it also increases with distance traveled. From time to time an independent determination of position is made by means of external references. When a reliable position, called a fix, is so obtained, a new dead reckoning is started from this point.
In many large naval vessels and commercial ships the dead reckoning is performed automatically by a device that receives inputs of direction from a gyrocompass and speed from a log and continuously computes dead-reckoning position, which is displayed on dials or traced on a chart or plotting sheet.
The Doppler effect, a frequency shift that is proportional to the speed of relative motion between transmitter and receiver or reflector of radiant energy, either acoustic or electromagnetic, is used in a system to accomplish dead reckoning, automatically. In the ship version, called a Doppler sonar navigator, ultrasonic energy is transmitted obliquely downward (typically 30° from the vertical) and the frequency of the return echo is noted. By using four beams separated 90° laterally, the system provides an indication of vessel speed in both the fore-and-aft and athwartship directions, so that total speed and direction of motion can be determined if the device is properly oriented. When reflections are from the sea bottom (bottom return mode), true ground speed (speed relative to the solid Earth) is measured. When reflections are from suspended particulate matter in the water (volume reverberation mode), the speed is relative to the water. In either mode the speed is integrated to determine distance from a starting point. Doppler sonar navigation has proved particularly useful in survey and geophysical exploration vessels. See also Sonar.
Similar systems called Doppler navigators, which use electromagnetic energy, have been used in aircraft, but these systems have largely been replaced by inertial navigators. See also Doppler effect; Doppler radar.
Most aircraft that are used for long overwater flights, and some others, as well as some ships, notably submarines, are equipped with one or more inertial navigators. This device, when properly aligned, provides a continuous indication of speed, position, and heading by means of appropriate inertial sensors. Gyroscopes are used to sense angular motions of the craft and maintain accelerometers in the correct orientation to sense linear accelerations, or changes in speed. Single integration of the accelerations provides a measure of speed, and double integration produces a measure of distance. An inertial navigator is free from effects of wind and current, but like all dead-reckoning systems its output degrades with elapsed time and distance traveled, and must be reset periodically. Its use is particularly attractive in aircraft because of their high speed and hence relatively short time in transit. See also Inertial guidance system.
Knowledge of the real-time present position of a craft is generally considered essential to safe navigation. The Global Positioning System, using NAVSTAR artificial earth satellites, or an integrated system, using a computer and appropriate filter to synthesize outputs of several independent positioning systems, may provide an essentially continuous fix, which would then eliminate the need for dead reckoning. See also Celestial navigation; Electronic navigation systems; Piloting; Satellite navigation systems.
| US Military Dictionary: dead reckoning |
See the Introduction, Abbreviations and Pronunciation for further details.
| Science Q&A: What is dead reckoning? |
Dead reckoning is the determination of a craft's current latitude and longitude by advancing its previous position to the new one on the basis of assumed distance and direction traveled. The influences of current and wind as well as compass errors are taken into account in this calculation, all done without the aid of any celestial or physical observation. This is a real test of a navigator's skill.
Nuclear-powered submarines, which must retain secrecy of movement and cannot ascend to the surface, use the SINS system (Ship's Inertial Navigation System) developed by the United States Navy. It is a fully self-contained system, which requires no receiving or transmitting apparatus and thus involves no detectable signals. It consists of accelerometers, gyroscopes, and a computer. Together they produce inertial navigation, which is a sophisticated form of dead reckoning.
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| Boating Encyclopedia: Dead Reckoning |
Educated guesswork fuels this basic navigation method
Dead reckoning (DR) is a way of keeping track of a boat’s position without any help from external features. It consists of nothing more than charting the course steered and the distance run, and yet it’s the very basis of safe pilotage.Most authorities believe the term to have originated as deduced reckoning, the abbreviation for which was ded. reckoning. That, in turn may have been corrupted to dead reckoning.But another possibility, mentioned in The Complete Boating Encyclopedia, is that the phrase may have come from the nautical use of dead to mean directly; for example, in dead ahead. In that case, dead reckoning could mean direct reckoning, which fairly well sums it up.How does it work? From your last known position, plot (draw) on the chart the course(s) traveled and the estimated distance(s) covered. Let’s say, for example, that from a charted navigation buoy, you steam along a course of
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| Wikipedia: Dead reckoning |
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 means of navigation, modern inertial navigation systems, which also depend upon dead reckoning, are very widely used.
A disadvantage of dead reckoning is that since new positions are calculated solely from previous positions, the errors of the process are cumulative, so the error in the position fix grows with time.
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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]
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. Tools such as the Traverse board were developed to enable even illiterate crew members collect the data needed for dead reckoning.
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.
Dead reckoning is today implemented in some high-end automotive navigation systems in order to overcome the limitations of GNSS technology alone. Satellite microwave signals are unavailable in parking garages and tunnels, and often severely degraded in urban canyons and near trees due to blocked lines of sight to the satellites or multipath propagation. In a dead-reckoning navigation system, the car is equipped with sensors that record the wheel rotation and steering direction. These sensors are often already present in cars for other purposes (anti-lock braking system, electronic stability control) and can be read by the navigation system from the controller-area network bus. The navigation system then uses a Kalman filter in order to integrate the reliability and short-term accuracy of the sensor data with the long-term accuracy of the satellite data into a position fix that combines the best of both worlds.
There is disagreement about the derivation of the phrase. It is sometimes thought (particularly among sailors and pilots in the USA[3]) 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)[4], and no trace of it can be found in books published before the 20th Century.[3]
The popular etymology from deduced is not documented in the Oxford English Dictionary or any other historical dictionary. It is not clear what 'dead' might mean in the context of dead reckoning, and several alternative theories have been promulgated.[3]
Here are the dead reckoning formulas for the coordinates (x and y), and heading (θ) for a differential drive robot with encoders on both drives:
where T1 are the encoder ticks recorded on drive one, T2 are the encoder ticks recorded on drive two, RW is the radius of each drive wheel, and TR is the number of encoder ticks recorded in a full, in-place rotation.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| US Military Dictionary. The Oxford Essential Dictionary of the U.S. Military. Copyright © 2001, 2002 by Oxford University Press, Inc. All rights reserved. Read more |
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| Science Q&A. The Handy Science Answer Book. 2003 ©Visible Ink Press. All rights reserved. Read more |
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| Boating Encyclopedia. The Practical Encyclopedia of Boating. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Dead reckoning". Read more |
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