Eight years of temporal gravity changes observed by the Swarm satellites

<p>The GPS data collected by the Swarm satellites form the basis for monthly global gravity field models that are complete and uninterrupted since late 2013, thus already covering a period of eight years. A nice aspect is that this time series covers the gap between the GRACE and GRACE-FO missions, as well as any other short gaps in their time series, with a spatial resolution of roughly 1500 km.</p><p>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 have teamed up to routinely provide these monthly models, with the support of the European Space Agency and the International&#160;Combination&#160;Servicefor&#160;Time-variable&#160;Gravity Fields (COST-G). The models are published every 3 months at ESA&#8217;s Swarm Data Access server (https://swarm-diss.eo.esa.int) as well at the International Centre for Global Earth Models (http://icgem.gfz-potsdam.de/series/02_COST-G/Swarm). Our gravity field models do not rely on any other source of gravimetric data nor any a priori information in for example the form of temporal and spatial correlations. The strength of our approach is that each institute exploits different gravity inversion strategies, thus producing independent solutions, which are combined at the solution level using weights derived with Variance Component Estimation.</p><p>Considering a reference parametric model derived from GRACE/GRACE-FO data, our models traditionally agree at the level of roughly 4 cm Equivalent Water Height (EWH). Since early 2020, developments in the processing of the kinematic orbits have improved this figure to 3 cm. A particularity of the Swarm gravity field models is that the deep ocean areas are ~30-50% noisier than land areas, with the underlying reason yet unknown. At the spatial resolution of Swarm, the time series of large water storage basins show a temporal correlation of 0.75 when compared with GRACE models, and their trends agree within 1 cm/year in terms of EWH.</p>