the parameters by which the shape, dimensions and position of the orbit of a body in space can be defined in relation to a specified frame of reference
NOTE 1 – In order to determine the position of a body in space at any instant it is necessary to know, in addition to its orbital elements, the position of its centre of mass in its orbit at one given instant.
NOTE 2 – The frame of reference used is usually a direct rectangular co-ordinate system OXYZ, in which the origin is at the centre of mass of the primary body and the third axis OZ is perpendicular to the principal reference plane.
NOTE 3 – For an artificial earth satellite, the principal reference plane is the Earth's equatorial plane and the third axis OZ has a south-to-north orientation.
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