Phase residuals analysis in kinematic orbit determination of GRACE-FO

The GRACE-FO based scientific research will benefit from the improvement of the kinematic orbit quality. This study investigated the errors that could potentially impact the accuracy of orbit determination and applied error corrections to the observations, including phase center variation (PCV) and multipath error, with a specific focus on enhancing the performance of GRACE-FO kinematic precise orbit determination based on the zero-difference method. When the observation model for precise orbit determination is highly consistent with the actual situation, the phase residuals should only consist of observation noise. However, the phase residuals encompass both modeled errors and unmodeled errors, such as PCV and multipath errors. The study utilized the GPS observation data of the GRACE-FO satellites in January 2022 to derive the fixed ambiguity solutions based on the zero-difference ionosphere-free linear combination, thereby obtaining the phase residuals. Subsequently, the antenna PCV was estimated based on the residual method, resulting in a substantial reduction in carrier phase residuals through PCV correction. A comparison with the reference orbits revealed that the root mean square (RMS) of the fixed solutions with PCV correction improved by 2.8 to 4.3 mm in three directions. Furthermore, the wavelet decomposition was performed on the phase residuals to extract the multipath errors and reconstruct the carrier phase observations. The RMS of the kinematic orbits improved by 4.3 to 5.9 mm with the incorporation of both the PCV and multipath error corrections. The RMS of the satellite laser ranging (SLR) residuals of the two GRACE-FO (GRACE-C and GRACE-D) satellites were 1.43 cm and 1.48 cm, respectively. The results indicate that these methods have the potential to enhance the kinematic orbit accuracy of the GRACE-FO satellites, bringing it closer to the a posteriori scientific orbit accuracy.