Estimating surface optical properties and thermal thrust for Galileo satellite body and solar panels

Abstract

Precise orbit determination of GNSS (Global Navigation Satellite System) satellites requires accurate models of perturbing forces acting on the spacecraft, i.e., solar radiation pressure (SRP) and thermal radiation forces. With the officially published satellite metadata, the analytical box-wing model is usually used to describe most of the SRP accelerations and the rest is assumed to be compensated by estimating ECOM/ECOM2 (Empirical CODE Orbit Model) parameters. However, we find that the precision of Galileo satellite orbits shows notable degradation inside eclipse seasons for 3-day-arc solutions and 24-h predictions. For instance, the RMS (root-mean-square) of orbit misclosures increases by about a factor of two in the eclipse season when using the box-wing model as the a priori and the 5-parameter ECOM model on top. The reason is proven to be mostly due to ignoring imbalanced thermal radiation forces (i.e., radiator emission and thermal radiation of solar panels) as satellite thermal properties are unknown. These imbalanced thermal effects cannot be fully absorbed by the ECOM/ECOM2 parameters inside eclipse seasons because the earth’s shadowing of a satellite in orbit causes periodic changes of the thermal environment. To cope with this problem, we first estimate satellite optical and thermal parameters as part of orbit determination based on Galileo tracking data covering 1 year. Then, we add physical thermal radiation models for radiators and solar panels as part of the a priori model and evaluate the performance of different ECOM models in Galileo satellite orbit determination. As shown by orbit misclosures, 24-h orbit predictions and SLR (Satellite Laser Ranging) residuals, the 7-parameter ECOM2 model performs better than the 5-parameter ECOM and the 9-parameter ECOM2 model for Galileo satellites. When using the 7-parameter ECOM2 model on top, the impact of the radiator emission and the thermal radiation of solar panels on Galileo satellite orbits is about 1 and 2 cm, respectively, inside eclipse seasons for 3-day-arc solutions.