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Orbit determination is a set of techniques for estimating the orbits of objects such as Moons, planets, and spacecraft. Determining the orbits of newly observed asteroids is a common usage of these techniques, both so the asteroid can be followed up with future observations, and also to verify that it has not been previously discovered. ''Observations'' are the raw data fed into orbit determination algorithms. Observations made by a ground-based observer typically consist of time-tagged azimuth, elevation, range, and/or range-rate values. Telescopes or radar apparatus are used, because naked-eye observations are inadequate for precise orbit determination. After orbit determination has taken place, mathematical propagation techniques can be used to predict the future positions of orbiting objects. As time goes by, the actual path of an orbiting object tends to diverge from the predicted path (this is especially true if the object is subject to difficult-to-predict perturbations such as atmospheric drag), and a new orbit determination using new observations serves to re-calibrate knowledge of the orbit. For US and partner countries, to the extent that optical, and radar resources allow, the Joint Space Operations Center gathers observations of all objects in Earth orbit. The observations are used in new orbit determination calculations that maintain the overall accuracy of the satellite catalog. Collision avoidance calculations may use this data to calculate the probability that one orbiting object will collide with another. A satellite's operator may decide to adjust the orbit, if the risk of collision in the present orbit is unacceptable. (It is not possible to adjust the orbit every time a very-low-probability situation is encountered; doing so would cause the satellite to quickly run out of propellant.) When the quantity or quality of observations improves, the accuracy of the orbit determination process also improves, and fewer "false alarms" are brought to the attention of satellite operators. Other countries, including Russia and China, have similar tracking assets. ==History== Orbit determination has a long history, beginning with the prehistoric discovery of the planets and subsequent attempts to predict their motions. Johannes Kepler used Tycho Brahe's careful observations of Mars to deduce the elliptical shape of its orbit and its orientation in space, deriving his three laws of planetary motion in the process. The beginning of modern understanding of orbit determination is considered to be Anders Johan Lexell's work on computing the orbit of the comet discovered in 1770 that later was named Lexell's Comet,〔Valsecchi, G. '236 Years Ago...' in ''Near Earth Objects, Our Celestial Neighbors: Opportunity and Risk : Proceedings of the 236th Symposium of the International Astronomical Union'', Cambridge University Press, 2006, xvii-xviii〕 in which Lexell computed the interaction of comet with Jupiter that first made the comet fly close to Earth and then would have expelled it from the Solar system. Another milestone in orbit determination was Carl Friedrich Gauss' assistance in the "recovery" of the dwarf planet Ceres in 1801. He introduced a method which, when given three observations (in the form of pairs of right ascension and declination), would result in the six orbital elements that completely describe an orbit. The theory of orbit determination has subsequently been developed to the point where today it is applied in GPS receivers as well as the tracking and cataloguing of newly observed minor planets. In 2019, a new US asset is expected to become operational. The Space Fence—currently being built—will utilize S-band radar and will track a larger number of small objects than previous space radars: "about 200,000 objects and make 1.5 million observations per day, about 10 times the number" made by existing or recently-retired US assets.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「orbit determination」の詳細全文を読む スポンサード リンク
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