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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Computation of radiowave attenuation in the atmosphere ： ウィキペディア英語版 Computation of radiowave attenuation in the atmosphere One of the causes of attenuation of radio propagation is the absorption by the atmosphere. There are many well known facts on the phenomenon and qualitative treatments in textbooks.〔''Antennas and radiowave propagation''. R. E. Collin. Mc Graw Hill, 1985〕 A document published by the International Telecommunication Union (ITU) 〔''ITU recommendation ITU-R'' pp. 676–8, 2009〕 provides some basis for a quantitative assessment of the attenuation. That document describes a simplified model along with semi-empirical formulas based on data fitting. It also recommended an algorithm to compute the attenuation of radiowave propagation in the atmosphere. NASA also published a study on a related subject.〔''http://trs-new.jpl.nasa.gov/dspace/handle/2014/41145''. NASA progress report〕 Free software from CNES based on ITU-R recommendations is available (for download ) and is available to the public. ==The model and the ITU recommendation== The document ITU-R P.676-8 of the ITU-R section considers the atmosphere as being divided into spherical homogeneous layers; each layer has a constant refraction index. By the use of trigonometry, a couple of formulas and an algorithm were derived. Through the use of an invariant, the same results can be directly derived: File:Optinv.png An incident ray at A under the angle Φ hits the layer B at the angle ''θ''. From basic Euclidean geometry: : $|CK|\; =\; R\backslash sin\; u\; =\; r\backslash sin\backslash theta.\; \backslash ,$ By Snell's law (or René Descartes' law from the French point of view!) : : $n\_1\; \backslash sin\backslash Phi\; =\; n\_2\backslash sin\; u,\; \backslash ,$ so that : $n\_1\; R\backslash sin\backslash Phi\; =\; n\_2\; r\; \backslash sin\backslash theta\; =\; INV\; \backslash ,$ Notes: * One proof 〔 starts from the Fermat's principle. As a result one gets the proof of the Snell’s law along with this invariance. This invariant is valid in a more general situation; the spherical radius is then replaced by the Radius of curvature at points along the ray. It is also used in equation (4) of the 2005 NASA's report〔 in an application of satellite tracking. * The assumption of the refraction index varying with the latitude is not strictly compatible with the notion of layers. However the variation of the index is very small, this point is usually ignored in practice. The ITU recommended algorithm consists of launching a ray from a radio source, then at each step, a layer is chosen and a new incidence angle is then computed. The process is iterated until the altitude of the target is reached. At each step, the covered distance ''dL'' is multiplied by a specific attenuation coefficient ''g'' expressed in dB/km. All the increments ''g'' ''dL'' are added to provide the total attenuation. Note that the algorithm does not guaranty that the target is actually reached. For this, a much harder boundary value problem would have to be solved. 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「Computation of radiowave attenuation in the atmosphere」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.