Frame of Reference
State vectors are defined with respect to some frame of reference, usually but not always an inertial reference frame. One of the more popular reference frames for the state vectors of bodies moving near the earth is the earth-centered equatorial system defined as follows:
The origin is the earth's center of mass;
The Z axis is coincident with the earth's rotational axis, positive northward;
The X/Y plane coincides with the earth's equatorial plane, with the +X axis pointing toward the vernal equinox and the Y axis completing a right-handed set.
This reference frame is not truly inertial because of the slow, 26,000 year precession of the earth's axis, so the reference frames defined by the earth's orientation at a standard astronomical epoch such as B1950 or J2000 are also commonly used.
Many other reference frames can be used to meet various application requirements including those centered on the Sun or on other planets or moons, the one defined by the barycenter and total angular momentum of the solar system, or even by a spacecraft's own orbital plane and angular momentum.
Read more about this topic: Orbital State Vectors
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