2.2.1. Earth-Surface Reference Frame *OExEyEz<sup>E</sup>*

The Earth-surface reference frame was defined to obtain a transformational relationship between the aircraft body and Earth and used to determine the attitude and heading of the aircraft. The selected takeoff point is the origin *OE*, the axis *z<sup>E</sup>* is vertical to the horizontal plane and points to the Earth's core, and the axis *x<sup>E</sup>* is located in the horizontal plane and points to the direction of the nose when the aircraft takes off. The axis *y<sup>E</sup>* is also located in the horizontal plane and is perpendicular to the axis *xE*, whose direction is determined by the right-hand rule.
