The impact of gravity field modelling on GNSS orbits and on station coordinates

Newtonien mechanics is only hold in inertial systems. In order to avoid the computation of centrifugal and Coriolis forces, the integration of satellite orbits are computed in inertial frame. The origin of the used reference system should be at the mass center of the Earth (CM), otherwise the system becomes non-inertial. However, CM can not be easily realized. In general, the origin of the used reference frames, such as the international terrestrial reference frame (ITRF), is realized with the center of the station network (CN). Thus the satellite orbits computed with ground stations are referred not to CM but to CN. The location of CM and that of CN may not be the same. Therefore, the satellite orbits are in many cases integrated in a non-inertial system. The differences between the orbits integrated in a CM system and a CN system are investigated in this study. With observations collected at the International GNSS Service (IGS) stations, the motion of the CM relative to CN is computed for the year 2011. The daily solution shows that the geocenter moves up to 3 cm along z-direction. The station coordinate differences between the solution of estimating gravity field coefficients and that of not estimating gravity field coefficients are analyzed. It shows that the correlation between gravity field coefficients and the IGS station coordinates is very small, with the condition of minimum constraint applied.