SC 2.3: Gravity Missions

Chair: David Wiese (USA)
Vice-Chair: João de Teixeira da Encarnacao (Netherlands)

Terms of Reference

The successful launches of the German CHAMP (2000), the US/German GRACE(2002), the ESA GOCE (2009) and US/German GRACE-FO (2018) missions have led to a revolution in global gravity field mapping using space-borne observation techniques. These missions have utilized an array of measurement concepts and technologies, including high-low satellite-to-satellite tracking (SST) using the GNSS constellation, low-low SST based on microwave and laser ranging technologies, satellite gravity gradiometry (SGG), as well as high-precision space-borne accelerometry, to provide direct measurements of the global mass distribution and transport within the Earth system. The GRACE (2002-2017) and GRACE-FO (2018-present) missions have combined to provide a 21-year (and still growing) record of Earth system mass change and have served as the catalyst for several new planned gravity missions, including the US/German GRACE-Continuity (GRACE-C; planned launch in late 2020s),and ESA’s Next Generation Gravity Mission NGGM; planned launch in early 2030s), to both continue and improve upon this vital climate data record. GOCE provided high- accuracy and high-resolution static gravity field models. In combination with complementary gravity field information from terrestrial data and satellite altimetry, including the newly launched SWOT mission, an even higher spatial resolution static gravity field model can be achieved. These satellite missions provide valuable contributions to a diversity of geoscientific application fields, such as geodesy, hydrology, oceanography, glaciology, solid Earth physics, and climate research.

Objectives

The focus of SC 2.3 will be to promote and stimulate the following activities:

• providing the scientific environment for the development of the next generation of static and temporal gravity field solutions based on observations from the satellite gravity missions CHAMP, GRACE, GOCE, and GRACE-FO, as well as optimum combination with complementary data types (SLR, terrestrial and air-borne data, satellite altimetry, GNSS tracking, etc.),
• developing alternative methods and new approaches for global gravity field processing with special emphasis on functional and stochastic models and optimum datacombination,
• fostering the exchange of knowledge and data among processing entities,
• communication and interfacing with gravity field model user communities (climatology, oceanography/altimetry, glaciology, solid Earth physics, geodesy, hydrology,etc.) as well as relevant IAG organizations such as the GGOS Committee on Satelliteand Space Missions and the GGOS Bureau of Products and Standards,
• supporting the implementation of currently planned satellite gravity missions,
• identification, investigation, and definition of enabling technologies for future gravityfield missions such as observation types, technologies or mission architectures, and
• triggering new gravity field mission proposals and supporting their implementation.

Program of Activities 

The Sub-commission will establish, if necessary, Working Groups on relevant topics.The Steering Committee will work closely with members and other IAG commis-sions and sub-commissions to obtain mutual goals. The Sub-commission will alsopromote and jointly sponsor special sessions at IAG Symposia and other workshopsand conferences.

Working Groups of Sub-Commission 2.3

JWG 2.3.1: Spatial Leakage Mitigation in Satellite Gravimetry

(joint with ICCC)

Chair: Eva Boergens (Germany)
Vice-Chair: Bramha Dutt Vishwakarma (India)

Terms of Reference

An inherent and well-known problem of satellite gravimetry is the limited spatial resolution of the sensors observing the functionals of the Earth’s gravity field. The problemis particularly present for mass change applications from GRACE and GRACE-FO. That is due to the limited amount of sensor data available to calculate monthly gravity fields, the signal attenuation due to the distance between the sensor (the satellites) and observable, and the typically applied global mathematical basis functions. The limited spatial resolution leads to apparent mass loss or gain in regions by mass signals from neighbouring regions and is called the spatial leakage effect. Although future gravity missions are planned to increase the spatial resolution with additional satellite pairs in inclined orbits (e.g. ESA’s MAGIC), the spatial leakage effect will only be reduced. 

Over the last 20 years of the GRACE and GRACE-FO mission lifetime, numerous methods have been proposed by various research groups to mitigate the disadvantageous effects of leakage. Some of these approaches focus on specific regions, while others provide a global mitigation scheme.

The approaches can be grouped into the following categories:

• Estimates obtained from Earth system models of the most prominent surface mass processes;
• Forward-modelling approaches, where the geometry of surface mass change is assumed a-priori and parameters are estimated to fit the gravity field data (in a least-squares sense);
• Data-driven approaches that only rely on the geodetic data themselves;
• Regional integration approaches that use apriori covariance information on the surface mass change signal and accordingly optimize the weight function (or sensitivity kernel) used for the regional integration;
• Methodologies based on (non-geodetic) satellite remote sensing data. 

Objectives

• Collect existing leakage approximation and mitigation methods;
• Develop a common and independent validation approach;
• Recommend best practices in communicating spatial leakage to non-geodetic users of surface mass change data derived from satellite gravimetry. Planned timeline with deliverables The planned time frame of the working group is four years.
• Year 1:
• Collection and review of existing leakage mitigation methods (A review paper);
• Definition of community-accepted validation criteria and requirements for validation data set (Tech report or a white paper);
• Apply for a grant/fellowship-scheme for a PhD student.
• Year 2:
• Generation of validation data set (Paper to a data journal).
• Year 3:
• Execution and evaluation of validation experiments.
• Year 4:
• Analysis of the validation experiments (Research paper);
• Recommendation to the user community (Tech report or a white paper).
  

Members

Eva Boergens (Germany); Chair
Bramha Dutt Vishwakarma (India); Vice-Chair
Christoph Dahle (Germany)
Balaji Devaraju (India)
Thorben Döhne (Germany)
Wei Feng (China)
Frank Flechtner (Germany)
Andreas Güntner (Germany)
Adrian Jäggi (Swizerland)
Felix Landerer (USA)
Bryant Loomis (USA)
Mahdiyeh Razeghi (Australia)
Ernst Schrama (Netherlands)
Michael Zemp (Swizerland)