Gravity field determination from Spire CubeSat data with non-gravitational force modelling

In recent years, the use of non-scientific low Earth orbiting (LEO) satellites for gravity field determination has been increasingly explored. In principle, the same methods as for non-dedicated scientific missions are applied by analysing the satellites’ orbital perturbations to gain information about the Earth’s gravity field. 

With currently over 100 CubeSats in orbit, the Spire commercial constellation provides a huge amount of GNSS tracking data in the same timeframe as scientific missions.  This can be exploited to increase the spatio-temporal resolution of estimated gravity field solutions. Although the data quality is limited, previous analyses using data from a 2020 ESA Announcement of Opportunity project have shown that a combined processing of Spire CubeSats can achieve monthly gravity field solutions of similar quality as non-dedicated scientific missions. In our work, we make use of both the Spire data from 2020 and a new dataset from 2023 provided by EUMETSAT. 

At the Astronomical Institute of the University of Bern, non-gravitational forces acting on the satellite have so far not been explicitly modelled when determining the gravity field, but instead absorbed by pseudo-stochastic parameters such as piecewise constant accelerations. As these forces are prominent for CubeSats due to their low altitude and their large area-to-mass ratio, this study explores how the estimation of the orbits and gravity field solutions improves when explicitly modelling these forces. To achieve this, we determine kinematic orbit positions from Spire CubeSat GNSS phase and code data and introduce them as pseudo-observations in an orbit and gravity field recovery step, where dedicated non-gravitational force modelling is applied using macro-model information provided by Spire. The combination of the different Spire satellites is performed at the normal equation level using a least-squares approach.