GNSS; LEO; Spire; CubeSats; Precise orbit determination; Raw observation approach
Abstract :
[en] CubeSats, a widely used class of nanosatellites, have made significant contributions to both geodetic and non-geodetic applications by offering low-cost, lightweight, and energy-efficient solutions. Spire Global Inc. has deployed over 120 3U CubeSats into various orbits, each equipped with a STRATOS GNSS receiver, a zenith-looking POD antenna, one or two RO antennas, and an attitude determination and control system. These features make Spire's LEMUR CubeSats ideal candidates for precise orbit determination (POD) and geodetic research. This study presents the first application of the raw observation approach for POD of commercial 3U CubeSats, focusing on 10 Spire GNSS-RO CubeSats with diverse orbital characteristics. The POD algorithm includes essential steps, such as the determination of a priori orbits and inertial quaternions for the Spire LEMUR CubeSats. Precise orbits are determined through both kinematic and reduced dynamic orbit solutions for the year 2020. In-flight calibrated antenna center variations (ACVs) are derived from the POD processing, revealing significant deviations from ground-based calibrations. Moreover, the calibrated ACVs vary among Flight Modules (FMs) with similar satellite buses and orbital characteristics, highlighting the necessity for independent in-flight calibration for each FM. To further enhance accuracy, azimuth- and elevation-dependent accuracy maps are generated from observation residuals and incorporated into the POD processing. The agreement between the reduced dynamic and kinematic orbits results in a 3D RMS difference of less than 5 cm. External validation further supports the reliability of the POD solutions. Comparisons with AIUB solutions for FM099 and FM103 in June 2020 show 3D RMS position agreements of 7.76 cm and 8.92 cm, and velocity agreements of 0.08 mm/s and 0.08 mm/s, respectively. Over the entire year, comparisons between the University of Luxembourg reduced dynamic orbits and Spire official L1B solutions achieve a 3D RMS of 22-29 cm in position and 0.25-1.04 mm/s in velocity. These results demonstrate the robustness of the raw observation approach for CubeSat POD and its potential to advance geodetic applications, such as the recovery of Earth's gravity field.
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