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ECEF ("Earth-Centered, Earth-Fixed"), also known as ECR ("Earth Centered Rotational"), is a geographic coordinate system and Cartesian coordinate system, and is sometimes known as a "conventional terrestrial" system.〔Alfred Leick, 2004, ''GPS Satellite Surveying'', Wiley〕 It represents positions as an X, Y, and Z coordinate. The point (0,0,0) is defined as the center of mass of the Earth,〔http://www.gmat.unsw.edu.au/snap/gps/clynch_pdfs/coorddef.pdf〕 hence the name Earth-Centered. Its axes are aligned with the International Reference Pole (IRP) and International Reference Meridian (IRM) that are fixed with respect to the surface of the Earth,〔http://www.weblab.dlr.de/rbrt/pdf/TN_0001.pdf〕〔http://metadata.dod.mil/mdr/ns/GSIP/crs/WGS84C_3D〕 hence the name Earth-Fixed. This term can cause confusion since the Earth does not rotate about the z-axis (unlike an inertial system such as ECI), and is therefore alternatively called ECR. The z-axis is pointing towards the north but it does not coincide exactly with the instantaneous Earth rotational axis.〔 The slight "wobbling" of the rotational axis is known as polar motion.〔http://www.iers.org/nn_10398/IERS/EN/Science/EarthRotation/PolarMotion.html?__nnn=true〕 The x-axis intersects the sphere of the Earth at 0° latitude (Equator) and 0° longitude (Greenwich). This means that ECEF rotates with the earth and therefore, coordinates of a point fixed on the surface of the earth do not change. Conversion from a WGS84 Datum to ECEF can be used as an intermediate step in converting velocities to the North East Down coordinate system. ==See also== * Geodetic system * Earth Centered Inertial coordinate system * International Terrestrial Reference System (ITRS) * Orbital state vectors 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「ECEF」の詳細全文を読む スポンサード リンク
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