Temporal gravity observed by the ESA Swarm satellites during the past decade

Since the launch of the ESA Swarm satellites in 2014, GPS data has been used to monitor the monthly changes in Earth’s gravity field at a spatial resolution larger than 1,500 km (half-wavelength). We have chosen processing strategies that do not rely on assumptions of temporal and spatial correlations, producing models that are independent of GRACE and GRACE-FO data. We are a team composed of the Astronomical Institute of the University of Bern, the Astronomical Institute of the Czech Academy of Sciences, the Delft University of Technology, the Institute of Geodesy of the Graz University of Technology, and the School of Earth Sciences of the Ohio State University. We are supported by the European Space Agency and the Swarm Data, Innovation, and Science Cluster. Given the good health of the Swarm satellites, we will continue to produce high-accuracy hl-SST gravity field solutions in the foreseeable future.

We produce individual gravity field models following independent gravity field inversion strategies. The International Combination Service for Time-variable Gravity Fields (COST-G) combines these individual models using weights derived with Variance Component Estimation, with the main objective of keeping the combined model unbiased to any individual solution. The models are published quarterly, pending the successful processing of kinematic orbits in the increasingly challenging environment resulting from the increased solar activity. The models are accessible at ESA’s Swarm Data Access server (https://swarm-diss.eo.esa.int) as well as at the International Centre for Global Earth Models (https://icgem.gfz.de/sp/02_COST-G_/Swarm).

These data monitor geophysical processes in parallel to the ll-SST technique, offering the opportunity to validate the ll-SST models, act as an alternative in the event of gaps in their data records or to bridge the potential gap between the GRACE-FO and GRACE-C/MAGIC mission period.

We show that the signal variability over the oceans of our models is a reliable measure of their accuracy by comparing it with the differences over land between Swarm and the GRACE and GRACE-FO solutions. Despite GRACE/GRACE-FO’s higher spatial resolution, we demonstrate that our Swarm models are able to resolve large-scale hydrological signals. Finally, we show that our effort has mitigated the challenges of estimating gravity field models derived from GPS data during the occurrence of high solar activities, which degraded the GPS signals.