GEI

This system has its Z axis parallel to the Earth's rotation axis (positive to the North) and its X axis towards the First Point of Aries (the direction in space defined by the intersection between the Earth's equatorial plane and the plane of its orbit around the Sun (the plane of the ecliptic). This system is (to first order) fixed with respect to the distant stars. It is convenient for specifying the orbits (and hence location) of Earth-orbiting spacecraft as one can specify a Keplerian orbit in this frame.

However note that the GEI system is subject to second order change with time owing to the various slow motions of the Earth's rotation axis with respect to the fixed stars. Thus for GEI coordinates one must specify the date (normally termed the epoch) to which the coordinate system applies. For space physics work one should use the epoch-of-date GEI system, i.e. the system applying at the same time as the data were taken. (Thus the rotation axis in GEI is identical with the GEO rotation axis.)