the orbital motion of GPS satellites is not only a result of the earth's gravitational attraction, there are several other forces that act on the satellite. the primary disturbing force are the non-spherical nature of the earth's gravity, the attractions of the sun and the moon, and solar radiation pressure. the best model of these forces is the actual motion of the satellites themselves and the government facilities distributed around the wold, known collectively as the Control Segment or the Operational Control System (OCS), track them for the reason, among other.