A 3D coordinate system, with its origin at the center of the Earth.
For a list of all members of this type, see IGeocentricCoordinateSystem Members.
The X axis points towards the prime meridian. The Y axis points East or West. The Z axis points North or South. By default the Z axis will point North, and the Y axis will point East (e.g. a right handed system), but you should check the axes for non-default values.