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
Famous quotes containing the words frame of, frame and/or reference:
“I can imagine no more comfortable frame of mind for the conduct of life than a humorous resignation.”
—W. Somerset Maugham (1874–1966)
“The warped, distorted frame we have put around every Negro child from birth is around every white child also. Each is on a different side of the frame but each is pinioned there. And ... what cruelly shapes and cripples the personality of one is as cruelly shaping and crippling the personality of the other.”
—Lillian Smith (1897–1966)
“I think, for the rest of my life, I shall refrain from looking up things. It is the most ravenous time-snatcher I know. You pull one book from the shelf, which carries a hint or a reference that sends you posthaste to another book, and that to successive others. It is incredible, the number of books you hopefully open and disappointedly close, only to take down another with the same result.”
—Carolyn Wells (1862–1942)