SWARM - Multi-approach gravity field models from Swarm GPS data

Although the knowledge of the gravity of the Earth has improved considerably with the CHallengingMini-Satellite Payload (CHAMP) (Reigber et al. 1996, 2002), Gravity Recovery And Climate Experiment (GRACE) (Tapley et al. 1996, 2004) and Gravity field and steadystate Ocean Circulation Explorer (GOCE) (Balmino et al. 1999; Floberghagen et al. 2011) satellite missions, the geophysical community has identified the need for the continued monitoring of its time-variable component with the purpose of estimating the hydrological and glaciological yearly cycles and long-term trends. Currently, the GRACE satellites are the sole provider of this data and the GRACE Follow On (GRACE-FO) (Sheard et al. 2012; Larkin 2012; Flechtner et al. 2014) mission aims to take over this role after December 2017. The current proposal aims at providing the high-quality gravity field models from Swarm data that constitute an alternative source of gravimetric data, which could help alleviate the consequences of the potential gap between GRACE and GRACE-FO, as well as the short gaps in the existing GRACE data. Independently on the existence of gravimetric data from dedicated satellite missions, the gravity field models derived from Swarm GPS data constitute an independent source of information about Earth’s gravity field, e.g. for monitoring large mass transport processes (Teixeira Encarnação et al., 2017). Recently, the geodetic community has realised that the combination of the different gravity field solutions is superior to any individual model, as demonstrated for GRACE by Jean et al. (2015a,b), and for Swarmby Teixeira da Encarnação et al. (2016). In response to the call for Swarm gravity field models issue by the Data, Innovation and Science Cluster (DISC) consortium, we intend to exploit of this fact and deliver to highest quality gravity field models, resulting from the combination of 4 different gravity field estimation approaches. Additionally, we intend to evaluate the added value of Kinematic Baselines (KBs) in the quality of the combined gravity field model. For that purpose, 2 different KBs solutions will be studied within the context of the project. The choice of which KB solution (if any) is used for the production of the models of the various institutes is left to the respective responsible scientist. To answer the Statement ofWork (SoW), we will also determine the benefit of two different models of non-gravitational forces and the measured non-gravitational accelerations from Swarm-C to the quality of the gravity field models. The strong points of our proposal are i) the statistically-optimal combination of models derived with 4 different gravity estimation strategies, each producing state-of-the-art Swarm gravity field models, ii) the use of 3 independently-developed Precise Orbit Determination (POD) software packages to estimate the Kinematic Orbits (KOs), iii) the analysis of the improvement resulting fromusing accelerometer measurements from Swarm-Cor 2 different non-gravitational force models and iv) the study of the added value to the quality of the gravity field models when considering the KBs computed with 2 different strategies. A prerequisite for the proposed objectives is the involvement of numerous institutions. This has the unfortunate disadvantage of diluting the funding and limiting considerably the time devoted to project activities. In spite of this, we have constructed a WP arrangement that minimizes overhead costs and maximizes independent progress, in order to fully take advantage of the available resources.