Editing Orbital state vectors (section)

==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 [[Earth]] is the [[Earth-centered inertial]] (ECI) system defined as follows:<ref name=om/>{{rp|p=23}}
*The [[origin (geometry)|origin]] is Earth's [[center of mass]];
*The Z axis is coincident with Earth's rotational axis, positive northward;
*The X/Y plane coincides with Earth's equatorial plane, with the +X axis pointing toward the [[Equinox (celestial coordinates)|vernal equinox]] and the Y axis completing a right-handed set.

The ECI reference frame is not truly inertial because of the slow, 26,000 year [[axial precession (astronomy)|precession of Earth's axis]], so the reference frames defined by Earth's orientation at a standard [[epoch (astronomy)|astronomical epoch]] such as B1950 or J2000 are also commonly used.<ref>{{cite book | doi=10.1007/978-3-662-50367-6_2 | chapter=Coordinate and Time Systems | year=2016 | last1=Xu | first1=Guochang | last2=Xu | first2=Yan | title=GPS | pages=17–36 | isbn=978-3-662-50365-2 }}</ref>{{rp|p=24}}

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]] (in particular the [[International Celestial Reference System|ICRF]]), or even a spacecraft's own orbital plane and angular momentum.