Examination and Enhancement of solar radiation pressure model for BDS-3 satellites

With the successful launch of the last Geostationary Earth Orbit (GEO) satellite in June 2020, China has completed the construction of the third generation BeiDou navigation satellites system (BDS-3). BDS-3 global services have been initiated in July 2020 with the constellation of 3 GEO, 3 Inclined Geosynchronous Orbit (IGSO) and 24 Medium Earth Orbit (MEO) satellites. In order to further improve the performance of BDS-3 services, the quality of BDS-3 precise orbit product needs further enhancements.

       The solar radiation pressure (SRP) is the main non-conservative orbit perturbation for GNSS satellites and is the key to improve BDS-3 precise orbit determination. In this study, we focus on the SRP models for BDS-3 satellites. Firstly, the widely used Extended CODE Orbit Model with five parameters (ECOM-5) is assessed. With one-year observations of 2020 from both iGMAS and MGEX networks, the five parameters of ECOM model (D0, Y0, B0, Bc and Bs) are estimated for each BDS-3 satellite. The D0 estimates show an obvious dependency on the elevation angle of the Sun above the satellite orbital plane (denoted as β). In addition, large variations can be noticed in eclipse seasons, which indicate the dramatic changes of SRP. The Y0 estimates vary from -0.6 nm/s² to 0.6 nm/s² for MEO, -1.0 to 1.0 nm/s² for IGSO and -1.0 to 1.5 nm/s² for GEO satellites. The B0 estimates of several satellites exhibit a clear dependency on the β angle. The largest variation of B0 appears at C45 and C46, changing from 1.0 nm/s² at 15 deg to 8.3 nm/s² at 64 deg, which implies that the solar panels of these two satellites may have an obvious rotation lag. To compensate the deficiencies of BDS-3 SRP modeling, we introduce several additional parameters into ECOM-5 model (e.g. introducing higher harmonic terms). The POD performances can be improved by about 10% and 40% for BDS-3 MEO/IGSO and GEO satellites, respectively.

       Except for the empirical model, we also study the semi-empirical SRP model such as the a priori box-wing model. Since the geometrical and optical properties from BDS-3 metadata are general and rough, we apply more detailed geometrical and optical coefficients for BDS-3 satellites. The POD performance can be improved by about 10% compared to empirical SRP models. Furthermore, considering Earth radiation pressure will have an impact of about 1.3 cm in radial component for MEO satellites.