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The perifocal coordinate (PQW) system is a frame of reference for an orbit. The frame is centered at the focus of the orbit, i.e. the celestial body about which the orbit is centered. The unit vectors lie in the plane of the orbit. has a true anomaly () of 90 degrees past the periapsis. The third unit vector And, since Where h is the specific relative angular momentum. The position and velocity vectors can be determined for any location of the orbit. The position vector, r, can be expressed as: : Where is the true anomaly and the radius ''r'' may be calculated from the orbit equation. The velocity vector, v, is found by taking the time derivative of the position vector: : where is the gravitational parameter of the focus, ''h'' is the specific relative angular momentum of the orbital body, ''e'' is the eccentricity of the orbit, and is the true anomaly. is the radial component of the velocity vector (pointing inward toward the focus) and is the tangential component of the velocity vector. By substituting the equations for and into the velocity vector equation and simplifying, the final form of the velocity vector equation is obtained as:〔Curtis, H. D. (2005). ''Orbital Mechanics for Engineering Students.'' Burlington, MA: Elsevier Buttersorth-Heinemann. pp 76–77〕 : ==Transformation from equatorial coordinate system== The perifocal coordinate system can also be defined using the orbital parameters inclination (''i''), right ascension () and the argument of perigee (). The following equations transform an orbit from the equatorial coordinate system to the perifocal coordinate system.〔Snow, K. (1999). ''Orbits in Space.''〕 : where : and , and are the unit vectors of the equatorial coordinate system. 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「Perifocal coordinate system」の詳細全文を読む スポンサード リンク
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