Can the Spire GNSS-RO CubeSats Constellation Observe Earth's Gravity Field?

Dual-frequency GNSS data from onboard Precise Orbit Determination (POD) antennas has emerged as a promising tool for investigating the time-variable components of Earth's gravity field. Over the past two decades, POD antennas onboard Low Earth Orbit (LEO) satellites have significantly advanced the study of the long-wavelength components of Earth's gravity field. CubeSats, a commonly used class of nanosatellites, now support a wide range of geodetic and non-geodetic applications while maintaining minimal costs, mass, and power requirements. In this context, the Spire CubeSats constellation, comprising over 100 CubeSats, offers potential contributions to the study of Earth’s long-wavelength gravity field components. This study uses precise orbit solutions of Spire GNSS-RO CubeSats, determined via the raw observation approach, to derive Earth's gravity field solutions using the short-arc approach for 2020. While this technique has been previously applied to scientific LEO satellite missions, this marks its first application to commercial CubeSats. Monthly gravity field solutions are calculated for each Flight Module (FM) using consistent parametrization. On average, geoid height differences between the individual gravity field solutions and the GOCO06s model are within ±2 cm. The individual Spire gravity solutions from each FM are further combined using the Variance Component Estimation (VCE) approach at the normal equation (NEQ) level. The quality of both individual and combined Spire GNSS-RO-based monthly gravity field solutions is compared with the ITSG-GRACE gravity field model.