# Next-Generation Gravity Missions: Sino-European Numerical Simulation Comparison Exercise
## 5 Conclusions and Outlook  
Temporal gravity retrieval results of a future Bender-type double pair mission concept performed by five processing centers of a Sino-European NGGM study team based on jointly defined mission scenarios have been inter-compared and assessed. In spite of some remaining differences, the solutions of the contributing teams TU Munich, Institute of Geodesy and Geophysics, Tongji University, Wuhan University, and Huazhong University of Science and Technology show quite similar performances. In general, these results demonstrate that one of the main goals of this inter-comparison exercise, the consistency of simulation results among the contributing groups, could be achieved to a large extent. Remaining differences might be explained by different processing strategies and partially different parameterizations, such as the co-parameterization of accelerometer biases and drifts, or empirical parameters to compensate for long-wavelength errors in the observation time series. Some remaining differences also exist in the error level of the co-estimated daily solutions.  
As already mentioned above, the main study goal was not to identify a \"winner\" among the contributing groups, but rather to achieve consistency of numerical simulation results of NGGM concepts to the best possible extent, even though five independent software packages, which are based partly on different gravity retrieval methods, have been applied. This positive result is an important pre-requisite and basis for future joint activities towards the realization of NGGMs.  
Based on this successful inter-comparison exercise of numerical simulators and in view of the fact that in the future several satellite pairs might be in orbit in parallel, further joint studies on optimized constellations of triple- and multi-pairs will be performed, and their impact on the main fields of applications, such as hydrology, glaciology, and solid Earth physics, will be quantified. The results of these studies will be presented in a follow-up paper.  
Conceptualization, R.P, H.-C.Y, W.F., Z.L., Y.S., M.Z., H.X.; Methodology, R.P., Q.C., M.H., A.P., X.X., B.Z., H.Z., X.Z.; Software, Q.C., M.H., A.P., X.X., B.Z., H.Z., Z.L., Y.Z.; Validation of results, A.P., R.P.; Writing--original draft preparation, R.P.; Writing--review and editing, R.P., Q.C., W.F., R.H., M.H., Z.L., A.P., Y.S., C.W., X.X., B.Z., H.Z., B.L., Y.Z., X.Z., H.Y., M.Z.; visualization, R.P.  
Figure 10: Degree RMS of co-parameterized daily gravity solutions up to d/o 15, averaged over the whole recovery period.  
**Funding:** This research was performed in the framework of the study \"Assessment of satellite constellations for monitoring the variations in Earth gravity field (ADDCON)\", ESA-ESTEC, Contract AO/1-7317/12/NL/AF funded by the European Space Agency. This study was also funded by the National Natural Science Foundation of China (projects no. 41704012, 41931074 and 41731069), and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant nos. XDB23030100 and XDA15017700).  
**Acknowledgments:** We further acknowledge the contribution of the ADDCON project partners [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] for a successful completion of the ESA project, as well as [PERSON] and [PERSON] for their valuable support during this project.  
**Conflicts of Interest:** The authors declare no conflict of interest.