摘要：Although the knowledge of the gravity of the Earth has improved considerablywith CHAMP, GRACE, and GOCE (see appendices for a list of abbreviations) satellite missions, the geophysical community hasidentified the need for the continued monitoring of the time-variablecomponent with the purpose of estimating the hydrological and glaciologicalyearly cycles and long-term trends. Currently, the GRACE-FO satellites arethe sole dedicated provider of these data, while previously the GRACEmission fulfilled that role for 15 years. There is a data gap spanning fromJuly 2017 to May 2018 between the end of the GRACE mission and start the ofGRACE-FO, while the Swarm satellites have collected gravimetric data withtheir GPS receivers since December 2013. We present high-quality gravity field models (GFMs) from Swarm data thatconstitute an alternative and independent source of gravimetric data, whichcould help alleviate the consequences of the 10-month gap between GRACE andGRACE-FO, as well as the short gaps in the existing GRACE and GRACE-FOmonthly time series. The geodetic community has realized that the combination of differentgravity field solutions is superior to any individual model and set up theCombination Service of Time-variable Gravity Fields (COST-G) under theumbrella of the International Gravity Field Service (IGFS), part of theInternational Association of Geodesy (IAG). We exploit this fact and deliver the highest-quality monthly GFMs, resulting from the combination of fourdifferent gravity field estimation approaches. All solutions areunconstrained and estimated independently from month to month. We tested the added value of including kinematic baselines (KBs) in ourestimation of GFMs and conclude that there is no significant improvement.The non-gravitational accelerations measured by the accelerometer on boardSwarm C were also included in our processing to determine if this wouldimprove the quality of the GFMs, but we observed that is only the case when theamplitude of the non-gravitational accelerations is higher than during thecurrent quiet period in solar activity. Using GRACE data for comparison, we demonstrate that the geophysical signalin the Swarm GFMs is largely restricted to spherical harmonic degrees below12. A 750km smoothing radius is suitable to retrieve the temporal variationsin Earth's gravity field over land areas since mid-2015 with roughly 4cmequivalent water height (EWH) agreement with respect to GRACE. Over oceanareas, we illustrate that a more intense smoothing with 3000km radius isnecessary to resolve large-scale gravity variations, which agree with GRACEroughly at the level of 1cm EWH, while at these spatial scales the GRACE observes variations with amplitudesbetween 0.3 and 1cm EWH. The agreement with GRACE and GRACE-FO over nineselected large basins under analysis is 0.91cm, 0.76cmyr−1, and 0.79 in terms oftemporal mean, trend, and correlation coefficient, respectively. The Swarm monthly models are distributed on a quarterly basis at ESA's EarthSwarm Data Access (at https://swarm-diss.eo.esa.int/, last access: 5 June 2020, followLevel2longterm and then EGF) and at the International Centre for Global Earth Models(http://icgem.gfz-potsdam.de/series/02_COST-G/Swarm, last access: 5 June 2020), aswell as identified with the DOI https://doi.org/10.5880/ICGEM.2019.006 (Encarnacao et al.,2019).
