IPN Progress Report 42-159 November 15, 2004 New Empirically Derived Solar Radiation Pressure Model for Global Positioning System Satellites Y. Bar-Sever 1 and D. Kuang 1 We describe the development and testing of a set of new and improved solar radiation pressure models for Global Positioning System (GPS) satellites that is based on four and one-half years of precise GPS orbital data. These empirical mod- els show improved performance in both GPS orbit fit and prediction relative to the state-of-the-art models. Orbit-fit rms is improved 80 percent for Block IIR satel- lites and 24 percent for Block IIA satellites. Orbit-prediction accuracy improved 58 percent for Block IIR satellites and 32 percent for Block IIA satellites. These new models are designated GSPM.04. It is shown that, after the implementation of these new models, Block IIA and Block IIR satellites perform about the same in orbit fit and in orbit prediction. I. Introduction Orbiting at an altitude of about 20,000 km, with no drag and with limited sensitivity to the details of the Earth’s gravitational pull, the Global Positioning System (GPS) satellites seem in little need of a complex dynamical model. Yet the relatively poor geometry of the observation system (providing mostly radial information), combined with the demand of some applications for extremely high accuracy, creates the need for a very careful modeling of the forces acting on a GPS satellite. The solar radiation pressure force is the largest perturbation acting on GPS satellites after the grav- itational attraction from the Earth, Sun, and Moon, and it is the largest error source in the modeling of GPS orbital dynamics. Various efforts have been made since the inception of the GPS to develop high-fidelity GPS solar radiation pressure models for precise orbit determination. These efforts can be classified under two basic approaches. The ground-model approach is based on pre-launch models and measurements of the spacecraft optical and thermal properties [3,4,6]. The empirical approach uses the observed orbital motion of the spacecraft to infer the solar radiation forces (and other forces) acting on the spacecraft [2,5]. 2 This study is a follow up investigation to the work reported by Bar-Sever, 3 using 1 Tracking Systems and Applications Section. 2 Y. E. Bar-Sever, New and Improved Solar Radiation Pressure Models for GPS Satellites Based on Flight Data, JPL Report (internal document), Jet Propulsion Laboratory, Pasadena, California, 1997. 3 Ibid. The research described in this publication was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. 1