GNSS Satellite Force Modeling: Unveiling the Origins of the Galileo Y-bias

The Y-bias as present on most global navigation satellite system (GNSS) spacecraft plays an important role in precise orbit determination and prediction. Accurate knowledge about the Y-bias and its temporal variability is particularly relevant for the Galileo system in order to fulfil its once-in-a-lifetime station-keeping maneuver requirements. Despite the widely recognized importance, however, no consensus has been reached on the physical mechanism that is responsible for the Y-bias. In this presentation, we shed light on the origins of the Galileo Y-bias using temperature and attitude data series from spacecraft telemetry to analytically determine Y-bias time histories for different Galileo satellites. We start by calculating the thermal radiation pressure forces generated by the two surface radiators at the main body's +Y and -Y sides of satellite GSAT0204 over a period of five years, from the activation of the spacecraft's search and rescue payload in early 2016 to the deactivation of its navigation payload in December 2017 and beyond. The net force from both radiators yields the Y-bias as it evolves over time, with some striking discontinuities due to abrupt changes in the amount of dissipated heat after the payload units have been turned on or off. Comparison against empirical Y-bias estimates from satellite laser ranging long arc analyses proves the correctness of our Y-bias model. In addition, we report on yearly variations in the Y-bias acceleration of GSAT0101 between -0.10 nm/s² and +0.05 nm/s², leading to a secular increase in the satellite orbit's semi-major axis since January 2016. Yaw error measurements from the spacecraft's fine sun sensor (FSS) spanning 2016-2019 provide compelling evidence that these Y-bias variations originate from an attitude-related mispointing of the satellite's solar panels by a few tenths of a degree. Least square fitting of the FSS measurements led to the development of a refined yaw model for GSAT0101. As a result of this new model, estimates of the Y-bias parameter are significantly reduced in magnitude and less dependent upon the position of the sun relative to the orbit plane. Overall, our analyses provide the first hard evidence that the Galileo Y-bias is primarily of thermal origin and, contrary to popular belief, that solar panel orientation errors only play a secondary role. The implications for precise orbit determination will be discussed. In addition, our results confirm the long-standing hypothesis that Y-bias and solar panel orientation error are linearly related.