Proper Frame of Reference

The proper frame of reference of a body is the reference frame associated with the body—that is, the reference frame in which the body is at rest. Since a body may move with acceleration, including rotation, the proper frame of reference is, in the general case, noninertial. For example, the proper frame of reference of an unstabilized artificial earth satellite is represented by an imaginary coordinate system that is rigidly attached to the satellite and flies and tumbles with it and by a clock that is fixed to the satellite and registers time.