To get the Hidden ZWX-F04/ORBIT Back Weapon Part, choose the "Zio City Suppression" mission requested by Earth Government. At the begining of the mission, fly over/through the tower directly in front of you. You will see some green buildings and the Orbit Cannon lying on the ground. (coordinates: X -000260, Y 000015, Z -000243)(note: due to the fact that there are several green "mazelike" buildings and you'll probably be hammered from all directions, I suggest you use these coordinates to your advantage.