We propose to assemble an international team for gravity field modelling in the context of the GRACE and GRACE-FO missions. These missions are dedicated to determine gravity field variations, hence mass transfers on the Earth, at a daily to monthly periodicity. In addition, the numerical simulation for possible synergistic observing of gravity satellites from Europe and China will be investigated for retrieving mass changes with higher spatio-temporal resolutions and accuracy in the near future.

The aim of the team members will be to join their effort for gravity field modelling especially the added contribution from China to generate established combined time-variable models to be included in the Combination Service for Time-variable Gravity Field Solutions (COST-G) and to establish so improved reference models as it has been demonstrated by the EGSIEM (European Gravity Service for Improved Emergency Management) project of the European Community. In addition, the simulation of constellation design for Chinese gravity satellites will be performed to pave the way to multi-pair gravity missions for improving our understanding of mass redistribution among different spheres of the Earth, i.e., hydrosphere, cryosphere, atmosphere, and lithosphere. The temporal mass variability in the Earth system observed by gravity satellites is crucial to understand climate change and anthropogenic activities, e.g., droughts and flooding, sea level change, groundwater depletion, glacier melting, etc.

ISSI and ISSI-BJ will offer us ideal facilities to establish fruitful interactions and meetings between the members in creating a framework of regular and dedicated science rendezvous. The international team involved in the project unified for the first time in the field of satellite gravity inversion and simulation from leading Chinese experts, and scientists from Switzerland, Germany, France, and Austria. The meetings supported by ISSI/ISSI-BJ will provide excellent opportunities for us to have concrete face-to-face discussions and build the foundation for further close international collaboration on satellite gravimetry.

Motivation & Goals

The Gravity Recovery and Climate Experiment (GRACE) mission and the GRACE Follow-On (GRACE-FO) mission have provided nearly two decades (2002-now) of continuous observations for monitoring the Earth’s mass transports, which have brought the unprecedented understanding of sea level change, mass melting of ice sheets and glaciers, and variations in terrestrial water storage. To more accurately quantify these effects, but in particular to cover smaller and thus also much more glaciers or marginal seas and river basins, developing higher-precision and resolution GRACE/GRACE-FO gravity field solutions becomes an urgent demand besides extending the gravity field time series. As such, refining data processing methods to account for the data characteristics of GRACE/GRACE-FO becomes more and more important.

The ISSI-BJ/ISSI team proposed in this project consists of most of research groups in Europe and China, who have long-term experience on GRACE data analysis. The GRACE products from these groups have been published on the website of ICGEM Service, and widely used by global researchers. The achievements from the international team have been acknowledged by global scientists and users.

Through over 15 years of efforts, various gravity field recovery methods have been proposed to derive different GRACE gravity field models. Currently, the main approaches for gravity field modelling include dynamic approach, short-arc approach, celestial mechanics approach, acceleration approach, and energy balance method (Jekeli 1999). Using the dynamic approach, the latest GRACE gravity field solutions, including CSR RL06 from Center for Space Research (CSR), GFZ RL06 from German Research Centre for Geosciences (GFZ), JPL RL06 from Jet Propulsion Laboratory (JPL), CNES/GRGS RL04, AIUB RL02 from Astronomical Institute University Bern (AIUB), and ITSG-Grace2018 from Graz University of Technology were determined. Note that GRACE solutions from USA were also considered with most European solutions via the established mechanisms of COST-G.

In recent years, Chinese researchers have released a series of GRACE solutions with the similar level of accuracy as the solutions from Europe and USA (Fig. 1), such as IGG solutions from the Institute of Geodesy and Geophysics, Chinese Academy of Sciences (IGG-CAS), Tongji solutions from Tongji University (TJU), Hust-Grace2016 from Huazhong University of Science and Technology (HUST), and WHU solutions from Wuhan University.

Figure1: Trend maps of long-term GRACE-derived mass variations over China and its surrounding regions from different solutions during 2003-2013, expressed in cm/yr

The goals of the team are as follows: (1) One goal is to add the contribution of Chinese research groups to generate combined time-variable gravity models, as an update of COST-G project. The involvement of more solutions from Chinese groups will lead to a more robust and reliable product of gravity change. (2) The other goal is to investigate the optimal orbit of inclined TianQin-2 by full-scale simulation of joint observing scheme, and evaluate the capability of retrieving mass change signals with higher spatial and temporal resolutions. A thorough preparation is needed to achieve these goals. It will consist of:

  • Creating the synergy between international teams working on gravity field modelling
  • Improving and homogenizing the modelling adopted by the Analysis Centers (AC)
  • Investigating the optimal constellation design by full-scale numerical simulation by the Simulation Centers (SC)
  • Creating combined reference solutions by the Combination Center (CC)
  • Assessing the reference solutions by a Validation Center (VC)

The ambition is (1) to extend the existing COST-G combination service and standards and operationally produce combined GRACE and GRACE-FO mass redistribution data including the new Chinese ACs, and (2) to illustrate the insights into potential joint international gravity missions or possible synergistic observing of satellites from Europe and China, and build the foundation for further close international collaboration on satellite gravimetry.