 During lunar orbit, the Earth, the Moon and the Sun also move, so attitude control is a complex three-vector control process.The Chang'e satellites need to adjust their attitude very carefully to maintain an optimal angle towards all three bodies.During the second phase of the program, in which the spacecraft were required to soft-land on the lunar surface, it was necessary to devise a system of automatic hazard avoidance in order that the landers would not attempt to touch down on unsuitable terrain.