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==Dead reckoning== Dead reckoning (DR) is the process of estimating a global position of a navigating agent by advancing a known position using direction, speed, time and distance of travel. This method of navigation is used in ships, aircraft, automobiles, rail engines, construction site engines (e.g., in tunnels) and, more recently, mobile robots. Animals also use dead reckoning (more commonly, in biological circles, known as path integration: see below) in order to estimate their current location based on the movements they made since their last known location. In marine navigation, dead reckoning uses the craft's last known position (fix), then plots the craft's approximate position, at any time, using true course and distance run. Distance run is determined by time and speed (rate x time = distance). Speed is determined by log, engine rpm or other method. Typically a Dead Reckoning plot is kept graphical on the chart with DR positions plotted at the same time intervals as the fix frequency. Estimated Position If allowance for winds and tides is used to correct the DR, the position determined is called an estimated postion (EP). In modern navigation, this plotted position is compared to a fix, taken at the time for which the DR was plotted, to determine set and drift (the combined external forces which act upon a ship causing it to deviate from its intended course). The difference between actual position (fix) and DR position helps the navigator determine a course and speed that will allow for set and drift in maintaining the ordered course and speed of advance. In air navigation, the wind triangle serves a similar purpose. If other methods of determing positon are not available, the navigator incorporats his estimation of these forces (wind, current, helmsman error, etc.) in his DR plot the resulting position is called an estimated position. This may be the only method used if, for example, the sky was overcast and a celestial observation could not be made. Before the development of the 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.
Dead reckoning (path integration) by animals Charles Darwin and JJ Murphy first postulated an inertially based navigation system in animals, in 1873. Studies beginning in the middle of last century confirmed that animals could return directly to a starting point, such as a nest, in the absence of vision and having taken a circuitous outwards journey. This shows that they can use cues indicating distances and directions moved to calculate position, and hence a vector home. This process was called path integration to capture the concept of continuous integration of movement cues over the journey, and manipulation of inertial cues confirmed that at least one of these movement (or idiothetic) cues is information from the vestibular organs, which detect movement in the three dimensions. Other cues probably include proprioception (information from muscles and joints about limb position), motor efference (information from the motor system telling the rest of the brain what movements were commanded and executed) and optic flow (information from the visual system signaling how fast the visual world is moving past the eyes). Together, these sources of information can tell the animal which direction it is moving, at what speed and for how long. The question of how all this information is put together by the brain, to enable extraction of position and homing information, is one that is currently unanswered. Extensive studies in arthropods, most notably in the desert ant ant Cataglyphis fortis, reveal the existence of highly effective path integration mechanisms that depend on determination of directional heading (by polarizing cues such as the sun) and distance computations (by, essentially, “counting footsteps”). How these sources of information are integrated is not yet known. In mammals, three important discoveries promise to shed light on this issue. The first, in the early 1970s, is that neurons in the hippocampal formation, called place cells, respond to the position of the animal. The second, in the early 1990s, is that neurons in neighboring regions (including anterior thalamus and post-subiculum, called head direction cells, respond to the head direction of the animal. This enables a much more fine-grained study of path integration, since it is possible to manipulate movement information and see how place and head direction cells respond: a much simpler procedure than training an animal, which is very slow. The third finding was that neurons in the dorso-medial entorhinal cortex, which feeds information to the place cells in the hippocampus, fire in a metrically regular way across the whole surface of a given environment. The activity patterns of these grid cells looks very much like a hexagonally organized sheet of graph paper, and suggest a possible metric system that place cells can use to compute distances. Whether place and grid cells actually compute a path integration signal remains to be seen, but computational models exist suggesting this is plausible. Certainly, brain damage to these regions seems to impair the ability of animals to path integrate. References 1. Best PJ, White AM, Minai A (2003) Spatial processing in the brain: the activity of hippocampal place cells. Annu Rev Neurosci 24: 459-486. 2. Etienne AS, Jeffery KJ (2004) Path integration in mammals. Hippocampus 14: 180-192. 3. McNaughton BL, Battaglia FP, Jensen O, Moser EI, Moser MB (2006) Path integration and the neural basis of the 'cognitive map'. Nat Rev Neurosci 7: 663-678. 4. Taube JS (1998) Head direction cells and the neurophysiological basis for a sense of direction. Prog Neurobiol 55: 225-256. Etymology There is some controversy about the derivation of the phrase. It is popularly thought to come from deduced reckoning and is sometimes given in modern sources as 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 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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