Precise Orbit Determination of the Spire GNSS-RO Constellation

This study introduces the raw observation approach for precise orbit determination (POD) of commercial 3U CubeSats equipped with a STRATOS receiver that collect dual-frequency GNSS observations. Different Spire CubeSats, each with different orbital characteristics and inclinations, are analyzed by processing reduced dynamic and kinematic orbits from 2020. The study examines the impact of orbital parameters on the derived code and phase residuals, as well as the calibrated antenna center variations (ACVs). The POD process starts with reduced dynamic orbit processing to determine the ACVs and accuracy maps, followed by the determination of kinematic orbits. The University of Luxembourg (UL) POD solutions are compared with those from Spire. The median root mean square (RMS) difference between UL reduced dynamic orbits and Spire solutions is between 24 and 27 cm in position and 1.1 mm/s in velocity. The agreement between reduced dynamic and kinematic orbits for all the flight modules (FMs) is within 5 cm 3D RMS. The highest receiver clock instability of FM103 resulted in higher daily RMS values of orbit differences between reduced dynamic and kinematic orbits compared to other FMs. The results indicate significant differences in the estimated ACVs among FMs with identical orbital characteristics and satellite properties, highlighting the necessity for in-flight calibration of each Spire FM individually. Additionally, epoch-wise receiver clock corrections are estimated for each FM in both reduced dynamic and kinematic orbit processing, revealing comparatively large drifts across all satellites.