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🔗 Mathematics 🔗 Statistics 🔗 Systems 🔗 Robotics 🔗 Systems/Control theory

In statistics and control theory, Kalman filtering, also known as linear quadratic estimation (LQE), is an algorithm that uses a series of measurements observed over time, containing statistical noise and other inaccuracies, and produces estimates of unknown variables that tend to be more accurate than those based on a single measurement alone, by estimating a joint probability distribution over the variables for each timeframe. The filter is named after Rudolf E. Kálmán, one of the primary developers of its theory.

The Kalman filter has numerous applications in technology. A common application is for guidance, navigation, and control of vehicles, particularly aircraft, spacecraft and dynamically positioned ships. Furthermore, the Kalman filter is a widely applied concept in time series analysis used in fields such as signal processing and econometrics. Kalman filters also are one of the main topics in the field of robotic motion planning and control and can be used in trajectory optimization. The Kalman filter also works for modeling the central nervous system's control of movement. Due to the time delay between issuing motor commands and receiving sensory feedback, use of the Kalman filter supports a realistic model for making estimates of the current state of the motor system and issuing updated commands.

The algorithm works in a two-step process. In the prediction step, the Kalman filter produces estimates of the current state variables, along with their uncertainties. Once the outcome of the next measurement (necessarily corrupted with some amount of error, including random noise) is observed, these estimates are updated using a weighted average, with more weight being given to estimates with higher certainty. The algorithm is recursive. It can run in real time, using only the present input measurements and the previously calculated state and its uncertainty matrix; no additional past information is required.

Optimality of the Kalman filter assumes that the errors are Gaussian. In the words of Rudolf E. Kálmán: "In summary, the following assumptions are made about random processes: Physical random phenomena may be thought of as due to primary random sources exciting dynamic systems. The primary sources are assumed to be independent gaussian random processes with zero mean; the dynamic systems will be linear." Though regardless of Gaussianity, if the process and measurement covariances are known, the Kalman filter is the best possible linear estimator in the minimum mean-square-error sense.

Extensions and generalizations to the method have also been developed, such as the extended Kalman filter and the unscented Kalman filter which work on nonlinear systems. The underlying model is a hidden Markov model where the state space of the latent variables is continuous and all latent and observed variables have Gaussian distributions. Also, Kalman filter has been successfully used in multi-sensor fusion, and distributed sensor networks to develop distributed or consensus Kalman filter.

🔗 Philosophy 🔗 Philosophy/Ethics

The hedonic treadmill, also known as hedonic adaptation, is the observed tendency of humans to quickly return to a relatively stable level of happiness despite major positive or negative events or life changes. According to this theory, as a person makes more money, expectations and desires rise in tandem, which results in no permanent gain in happiness. Brickman and Donald T. Campbell coined the term in their essay "Hedonic Relativism and Planning the Good Society" (1971). The concept dates back centuries, to such writers as St. Augustine, cited in Robert Burton's 1621 Anatomy of Melancholy: "A true saying it is, Desire hath no rest, is infinite in itself, endless, and as one calls it, a perpetual rack, or horse-mill."

The hedonic (or happiness) set point has gained interest throughout the field of positive psychology where it has been developed and revised further. Given that hedonic adaptation generally demonstrates that a person's long-term happiness is not significantly affected by otherwise impacting events, positive psychology has concerned itself with the discovery of things that can lead to lasting changes in happiness levels.

🔗 Sociology

Computational sociology is a branch of sociology that uses computationally intensive methods to analyze and model social phenomena. Using computer simulations, artificial intelligence, complex statistical methods, and analytic approaches like social network analysis, computational sociology develops and tests theories of complex social processes through bottom-up modeling of social interactions.

It involves the understanding of social agents, the interaction among these agents, and the effect of these interactions on the social aggregate. Although the subject matter and methodologies in social science differ from those in natural science or computer science, several of the approaches used in contemporary social simulation originated from fields such as physics and artificial intelligence. Some of the approaches that originated in this field have been imported into the natural sciences, such as measures of network centrality from the fields of social network analysis and network science.

In relevant literature, computational sociology is often related to the study of social complexity. Social complexity concepts such as complex systems, non-linear interconnection among macro and micro process, and emergence, have entered the vocabulary of computational sociology. A practical and well-known example is the construction of a computational model in the form of an "artificial society", by which researchers can analyze the structure of a social system.

🔗 Geographical coordinates 🔗 Microformats

Geohash is a public domain geocode system invented in 2008 by Gustavo Niemeyer and (similar work in 1966) G.M. Morton, which encodes a geographic location into a short string of letters and digits. It is a hierarchical spatial data structure which subdivides space into buckets of grid shape, which is one of the many applications of what is known as a Z-order curve, and generally space-filling curves.

Geohashes offer properties like arbitrary precision and the possibility of gradually removing characters from the end of the code to reduce its size (and gradually lose precision). As a consequence of the gradual precision degradation, nearby places will often (but not always) present similar prefixes. While in rare cases nearby places may have very short shared prefixes, the longer their shared prefix is, the closer two places are guaranteed to be.

🔗 Computing 🔗 Computing/Software

Norton Commander (NC) is a discontinued prototypical orthodox file manager (OFM), written by John Socha and released by Peter Norton Computing (later acquired in 1990 by the Symantec corporation). NC provides a text-based user interface for managing files on top of MS-DOS. It was officially produced between 1986 and 1998. The last MS-DOS version of Norton Commander, 5.51, was released on July 1, 1998.

A related product, Norton Desktop, a graphical shell for MS-DOS and Windows, succeeded Norton Commander. It came in two variants, Norton Desktop for DOS and Norton Desktop for Windows.

🔗 Mathematics

The shoelace formula or shoelace algorithm (also known as Gauss's area formula and the surveyor's formula) is a mathematical algorithm to determine the area of a simple polygon whose vertices are described by their Cartesian coordinates in the plane. The user cross-multiplies corresponding coordinates to find the area encompassing the polygon, and subtracts it from the surrounding polygon to find the area of the polygon within. It is called the shoelace formula because of the constant cross-multiplying for the coordinates making up the polygon, like tying shoelaces. It is also sometimes called the shoelace method. It has applications in surveying and forestry, among other areas.

The formula was described by Meister (1724–1788) in 1769 and by Gauss in 1795. It can be verified by dividing the polygon into triangles, and can be considered to be a special case of Green's theorem.

The area formula is derived by taking each edge AB, and calculating the area of triangle ABO with a vertex at the origin O, by taking the cross-product (which gives the area of a parallelogram) and dividing by 2. As one wraps around the polygon, these triangles with positive and negative area will overlap, and the areas between the origin and the polygon will be cancelled out and sum to 0, while only the area inside the reference triangle remains. This is why the formula is called the surveyor's formula, since the "surveyor" is at the origin; if going counterclockwise, positive area is added when going from left to right and negative area is added when going from right to left, from the perspective of the origin.

The area formula can also be applied to self-overlapping polygons since the meaning of area is still clear even though self-overlapping polygons are not generally simple. Furthermore, a self-overlapping polygon can have multiple "interpretations" but the Shoelace formula can be used to show that the polygon's area is the same regardless of the interpretation.

🔗 Internet 🔗 University of Cambridge

The Trojan Room coffee pot was a coffee machine located in the Computer Laboratory of the University of Cambridge, England. Created in 1991 by Quentin Stafford-Fraser and Paul Jardetzky, it was migrated from their laboratory network to the web in 1993 becoming the world's first webcam.

To save people working in the building the disappointment of finding the coffee machine empty after making the trip to the room, a camera was set up providing a live picture of the coffee pot to all desktop computers on the office network. After the camera was connected to the Internet a few years later, the coffee pot gained international renown as a feature of the fledgling World Wide Web, until being retired in 2001.

🔗 Time 🔗 Writing

Kairos (Ancient Greek: καιρός) is an Ancient Greek word meaning the right, critical, or opportune moment. The ancient Greeks had two words for time: chronos (χρόνος) and kairos. The former refers to chronological or sequential time, while the latter signifies a proper or opportune time for action. While chronos is quantitative, kairos has a qualitative, permanent nature. Kairos also means weather in Modern Greek. The plural, καιροί (kairoi (Ancient and Modern Greek)) means the times. Kairos is a term, idea, and practice that has been applied in several fields including classical rhetoric, modern rhetoric, digital media, Christian theology, and science.

🔗 Companies 🔗 Computing 🔗 United Kingdom

Andrews & Arnold Ltd (also known as AAISP) is an Internet service provider based in Bracknell in the United Kingdom founded in 1997 and launched in 1998, primarily serving businesses and "technical" home users.

In 2009 the company was judged the best niche provider in the Thinkbroadband Customer Service Awards, based on customer ratings and again in 2010.

The company's owner, Adrian Kennard (RevK), stated in a blog post that as of October 2010 the company is "xkcd/806" compliant, referring to xkcd comic number 806. This means that technical support callers who say the code word "shibboleet" will be transferred to a technical support representative who knows at least two programming languages, and presumably can offer more useful advice than a standard tech support script.

Andrews & Arnold provides IPv6 to all customers, for no additional charge.

Andrews & Arnold provides optional bonded multiple-link internet access. This allows multiple links to be used together to vastly increase speed and reliability. Special routers distribute individual IP packets between the available links in such a way that even one single download or upload operation will benefit fully from multiple speed, and it is not necessary to have several users, several running programs or computers to gain the speed benefit. Links can be of different types, each needs only to be a pipe that can carry IP packets. Multiple links can either be used together all the time, or some can be brought up as a back up if other links fail, so-called 'failover', or a combination of the two approaches can be set up.

Andrews & Arnold are strong advocates of not censoring Internet connections. Adrian Kennard has several blog posts discussing why Internet censorship as discussed in the UK is not workable, providing background for AAISP's decision.

🔗 Spaceflight 🔗 Physics 🔗 Physics/relativity

Gravity Probe B (GP-B) was a satellite-based experiment to test two unverified predictions of general relativity: the geodetic effect and frame-dragging. This was to be accomplished by measuring, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth-orbiting satellite at 650 km (400 mi) altitude, crossing directly over the poles.

The satellite was launched on 20 April 2004 on a Delta II rocket. The spaceflight phase lasted until 2005; Its aim was to measure spacetime curvature near Earth, and thereby the stress–energy tensor (which is related to the distribution and the motion of matter in space) in and near Earth. This provided a test of general relativity, gravitomagnetism and related models. The principal investigator was Francis Everitt.

Initial results confirmed the expected geodetic effect to an accuracy of about 1%. The expected frame-dragging effect was similar in magnitude to the current noise level (the noise being dominated by initially unmodeled effects due to nonuniform coatings on the gyroscopes). Work continued to model and account for these sources of error, thus permitting extraction of the frame-dragging signal. By August 2008, the frame-dragging effect had been confirmed to within 15% of the expected result, and the December 2008 NASA report indicated that the geodetic effect was confirmed to be better than 0.5%.

In an article published in the journal Physical Review Letters in 2011, the authors reported analysis of the data from all four gyroscopes results in a geodetic drift rate of −6601.8±18.3 mas/yr and a frame-dragging drift rate of −37.2±7.2 mas/yr, in good agreement with the general relativity predictions of −6606.1±0.28% mas/yr and −39.2±0.19% mas/yr, respectively.