Global geodetic parameter estimation using the combination of GPS, Galileo and LEO satellites

The global geodetic parameters are essential for the realization of an International Terrestrial Reference Frame (ITRF), which has broad geophysical and geodetic applications. With well-distributed global tracking stations and continuous observations, GNSS is suited to providing high quality geodetic parameters, including geocenter coordinates (GCCs) and Earth rotation parameters (ERPs). However, the spurious draconitic signals have been observed in GNSS-based geodetic products. Fortunately, the fast development of multi-GNSS and LEO satellite techniques in recent years provides new opportunity to improve the determination of geodetic parameters by combining observations from constellations with different orbital characteristics. In this study, we perform the integrated precise orbit determination (IPOD) under the ITRF2020 with dual-system GPS/Galileo observations from the Sentinel-6A satellite and 61 ground stations for geodetic parameter estimation. The effects of the LEO solar radiation pressure (SRP) model are analyzed and the contribution of the multi-GNSS combination and the LEO is investigated.

Three Sentinel-6A macro models, from the manufacturer, the German Aerospace Center (DLR) and the University of Colorado and Jet Propulsion Laboratory (UoC/JPL), are used to perform GPS-based IPOD solutions to investigate the impact of LEO SRP model on geodetic parameter estimation. The relationship between the estimated constant SRP scale and sun elevation is first analyzed. The estimated SRP scale based on the UoC/JPL macro model is found to have a weaker correlation with sun elevation compared the other two macro models. By comparing the geodetic parameters obtained using different macro models, we find that the GCC-Z estimates are more sensitive to the LEO priori SRP model than the GCC-X/Y and ERPs, and the best solution with the smallest amplitude in 3 cpy frequency is achieved by UoC/JPL macro model.

Based on the UoC/JPL macro model, we further investigate the performance of the geodetic parameter estimation in IPOD with dual-system GPS and Galileo observations. The introduction of the LEO improves the ERPs estimates, with a reduction in the length of day (LoD) formal error of about 60%, and a reduction in the drift rate of the GNSS-derived dUT1 away from the IERS 20 C04 reference value of over 30%. The introduction of the LEO also improves the GCC estimates with a reduction in GCC-Z formal error and STD value by about 20% and 10% respectively. Compared to single system solution, the combination of GPS, Galileo and the LEO obtains the most stable GCC estimates with the smallest STD values, except for the GCC-Z for the period above 30 days, mainly due to the failure to eliminate the Galileo-induced 3 cpy spurious signal. However, the introduction of Sentinel-6A also induces the spurious annual signal due to its SRP model deficiency. Though this deficiency can be partly compensated by changing the parameterization of SRP scale factor from constant to piece-wise constant, the influence of residual errors of LEO SRP modelling is still visible in the Z component of GCC. These results indicate that refining the a priori macro model of LEO satellite is necessary to obtain accurate geodetic parameters in IPOD with multi-GNSS.