dead reckoning
American Heritage Dictionary:
Top
dead reckoning
n.
- A method of estimating the position of an aircraft or a ship without astronomical observations, as by applying to a previously determined position the course and distance traveled since.
- Predictive calculation based on inference; guesswork.
[Possibly alteration of ded., abbr. of deduced, from deduce, to trace from the beginning. See deduce.]
McGraw-Hill Science & Technology Encyclopedia:
Top
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.
Oxford Dictionary of the US Military:
Top
dead reckoning
[ܒded ܒrekniŋ]
See the Introduction, Abbreviations and Pronunciation for further details.
Science Q&A:
Top
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.
| Previous question: | What is a Texas tower? |
| Next question: | What is the name of the carved wooden figure of a woman on a sailing ship? |
McGraw-Hill Boating Encyclopedia:
Top
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
 |
McGraw-Hill Dictionary of Aviation:
Top
dead reckoning
In navigation, determination of position by advancing a previous known position using courses and distances. This navigational technique employs direct computation, using the triangle of velocities, from which the ground speed can be calculated and the course to steer can be determined. It is also a method of finding one’s position by means of a compass and calculations based on speed, time elapsed, effects of wind, and direction from a known position.
Random House Word Menu:
Top
categories related to 'dead reckoning'
- Seamanship and Port - dead reckoning: calculation of position from course and distance sailed
Bradford's Crossword Solver's Dictionary:
Top
dead reckoning
See crossword solutions for the clue Dead reckoning.
Wikipedia on Answers.com:
Top
Dead reckoning
For other uses, see Dead reckoning (disambiguation).
 |
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2010) |
In navigation, dead reckoning (also ded (for deduced) reckoning or DR) is the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course.
Dead reckoning, using best estimates of speed and direction, is subject to cumulative errors. Advances in navigational aids which give accurate information on position, in particular satellite navigation using the Global Positioning System, has made simple dead reckoning by humans obsolete for most purposes; however, inertial navigation systems, which provide very accurate directional information, use dead reckoning and are very widely applied.
By analogy with their navigational use, the words dead reckoning are also used to mean the process of estimating the value of any variable quantity by using an earlier value and adding whatever changes have occurred in the meantime. Often, this usage implies that the changes are not known accurately. The earlier value and the changes may be measured or calculated quantities.
There is speculation on the etymological origin of the term, but no reliable information.
Errors
Dead reckoning may give the best available information on position, but is subject to significant errors due to many factors as both speed and direction must be accurately known at all instants for position to be determined accurately. For example, if displacement is measured by the number of rotations of a wheel, any discrepancy between the actual and assumed diameter, due perhaps to the degree of inflation and wear, will be a source of error. As each estimate of position is relative to the previous one, errors are cumulative.
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][2]
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.[3]
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 to collect the data needed for dead reckoning.
Before the advent of modern aids dead reckoning was widely used in air navigation, taking into account displacement of position caused by wind as far as possible, often using a tool called a wind triangle. As a rule dead reckoning (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.
Simple dead reckoning fell out of use for air navigation, but is used by inertial navigation systems (INSes), 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, without the need for external navigation references.
However, simple dead reckoning is still widely used[citation needed] by civil aircraft not equipped with GPS or radio navigation aids. The pilot periodically establishes a location fix from visual sighting of landmarks with reference to a map, allowing errors in dead reckoned course to be corrected. Pilots of advanced aircraft are trained in dead reckoning, which remains usable in case of failure of advanced navigational systems.
Dead reckoning is today implemented in some high-end automotive navigation systems in order to overcome the limitations of GPS/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 know the wheel diameter and record wheel rotations 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 to integrate the always-available sensor data with the accurate but occasionally unavailable position information from the satellite data into a combined position fix.
Dead reckoning is utilized in some lower-end, non mission-critical, or tightly constrained by time or weight, robotic applications. It is usually used to reduce the need for sensing technology, such as ultrasonic sensors, GPS, or placement of some linear and rotary encoders, in an autonomous robot, thus greatly reducing cost and complexity at the expense of performance and repeatability. The proper utilization of dead reckoning in this sense would be to supply a known percentage of electrical power or hydraulic pressure to the robot's drive motors over a given amount of time from a general starting point. Dead reckoning is not totally accurate, which can lead to errors in distance estimates ranging from a few millimeters (in CNC machining) to kilometers (in UAV's), based upon the duration of the run, the speed of the robot, the length of the run, and several other factors.
Directional dead reckoning
Main article: South-pointing chariot
The south-pointing chariot was an ancient Chinese device consisting of a two-wheeled horse-drawn vehicle which carried a pointer that was intended always to aim to the south, no matter how the chariot turned. The chariot predated the navigational use of the magnetic compass, and could not detect the direction that was south. Instead it used a kind of directional dead reckoning: at the start of a journey, the pointer was aimed southward by hand, using local knowledge or astronomical observations e.g. of the Pole Star. Then, as it travelled, a mechanism possibly containing differential gears used the different rotational speeds of the two wheels to turn the pointer relative to the body of the chariot by the angle of turns made (subject to available mechanical accuracy), keeping the pointer aiming in its original direction, to the south. Errors, as always with dead reckoning, would accumulate as distance travelled increased.
Differential steer drive dead reckoning
Here are the dead reckoning equations for the coordinates (x and y), and heading (
) for a differential drive robot with encoders on both drives:
where 
are the encoder ticks recorded on drive one, 
are the encoder ticks recorded on drive two, 
is the radius of each drive wheel, 
is the separation between the wheels, and 
is the number of encoder ticks recorded in a full, in-place rotation.
Dead reckoning for networked games
Networked games and simulation tools routinely use dead reckoning to predict where an actor should be right now, using its last known kinematic state (position, velocity, acceleration, orientation, and angular velocity).[4] This is primarily needed because it is impractical to send network updates at the rate that most games run, 60 Hz. The basic solution starts by projecting into the future using linear physics[5]:
This formula is used to move the object until a new update is received over the network. At that point, the problem is that there are now two kinematic states: the currently estimated position and the just received, actual position. Resolving these two states in a believable way can be quite complex. One approach is to create a curve (ex cubic Bézier splines, Catmull-Rom splines, and Hermite curves)[6] between the two states while still projecting into the future. Another technique is to use projective velocity blending, which is the blending of two projections (last known and current) where the current projection uses a blending between the last known and current velocity over a set time.[4]
Dead reckoning in literature
In Walden, Henry David Thoreau suggests the following approach to life:
"In the midst of this chopping sea of civilized life, such are the clouds and storms and quicksands and thousand-and-one items to be allowed for, that a man has to live, if he would not founder and go to the bottom and not make his port at all, by dead reckoning, and he must be a great calculator indeed who succeeds."
In "Moby Dick, or, The Whale" Herman Melville states on page 507: "...and in these same perilous seas, gropes he not his way by mere dead reckoning of the error-abounding log?"
See also
References
- ^ Gallistel. The Organization of Learning. 1990.
- ^ Dead reckoning (path integration) requires the hippocampal formation: evidence from spontaneous exploration and spatial learning tasks in light (allothetic) and dark (idiothetic) tests, IQ Whishaw, DJ Hines, DG Wallace, Behavioural Brain Research 127 (2001) 49 – 69
- ^ http://www.irbs.com/bowditch/pdf/chapt07.pdf
- ^ a b Murphy, Curtiss. Believable Dead Reckoning for Networked Games. Published in Game Engine Gems 2, Lengyel, Eric. AK Peters, 2011, p 308-326.
- ^ Van Verth, James. Essential Mathematics for Games And Interactive Applications. Second Edition. Morgan Kaufmann, 1971, p. 580.
- ^ Lengyel, Eric. Mathematics for 3D Game Programming And Computer Graphics. Second Edition. Charles River Media, 2004.
External links
- Bowditch Online: "Dead reckoning"
- Straight Dope: Is "dead reckoning" short for "deduced reckoning"?
- Jesse Aronson: "Dead Reckoning: Latency Hiding for Networked Games"
- A paper about pedestrian dead reckoning: "Omni-directional Pedestrian Navigation for First Responders" by Stéphane Beauregard (2006)
- WFR, a Dead Reckoning Robot
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Post a question - any question - to the WikiAnswers community:
Copyrights:
American Heritage 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
McGraw-Hill Science & Technology Encyclopedia
McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.
Read more
Oxford Dictionary of the US Military
The Oxford Essential Dictionary of the U.S. Military. Copyright © 2001, 2002 by Oxford University Press, Inc. All rights reserved.
Read more
Science Q&A
The Handy Science Answer Book. 2003 ©Visible Ink Press (handyanswers.com). All rights reserved.
Read more
McGraw-Hill 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
McGraw-Hill Dictionary of Aviation
An Illustrated Dictionary of Aviation.. Copyright © 2005 by McGraw-Hill Companies, Inc. All rights reserved.
Read more
Random House Word Menu
© 2010 Write Brothers Inc. Word Menu is a registered trademark of the Estate of Stephen Glazier.
Write Brothers Inc.
All rights reserved.
Read more
Bradford's Crossword Solver's Dictionary
Collins Bradford's Crossword Solver's Dictionary © Anne Bradford, 1986, 1993, 1997, 2000, 2003, 2005, 2008
HarperCollins Publishers
All rights reserved.
Read more
Mentioned in
DR
(abbreviation)
dead-reckoning equipment
(navigation)
meridian sailing
(navigation)
dead-reckoning track
(navigation)
position indicator
(engineering)
» More