The main source of perturbation for a satellite in GSO is the combined gravitational attractions of the Sun and Moon, which causes the orbital inclination to increase by nearly one degree per year. This is countered by a north-south station-keeping maneuver about once every two weeks so as to keep the satellite within 0.05° of the equatorial plane. The average annual velocity change (delta v) needed is about 50 m/s, which represents 95% of the total station-keeping propellant budget. Additionally, the bulge of Earth causes a longitudinal drift, which is compensated by east-west station-keeping maneuvers about once a week, with an annual delta v of less than 2 m/s, to keep the satellite within 0.05° of its assigned longitude. Finally, solar radiation pressure caused by the transfer of momentum from the Sun's light and infrared radiation both flattens the orbit and disturbs the orientation of the satellite. The orbit is compensated by an eccentricity control maneuver that can sometimes be combined with east-west stationkeeping, whereas satellite's orientation is maintained by momentum wheels supplemented by magnetic torquers and thrusters. Ion propulsion systems, notably XIPS, are being used increasingly for station-keeping.