Abstract: An autonomous real-time orbit determination for satellite flying around the Earth by the extended Kalman filtering is proposed in this paper. The observables are as follows, Satellite-Sun direction vector measured by Sun sensor, Satellite-Earth direction vector and Satellite-Moon direction vector measured by ultraviolet sensor, and the geocentric distance measured by radar altitude. Moreover, the satellite attitude to the Earth is determined. The simulation results show that orbit determination precision is better than 200m. By simulation experiments of autonomous orbit determination, the effects of sample period, orbit inclination, orbit eccentricity and orbit height on orbit determination precision are analyzed and compared. The changing law is submitted, which can improve satellite autonomous orbit determination precision.