The accurate determination of the gravity field and its temporal variations is one of the three fundamental pillars of modern geodesy (besides of geometry/kinematics and Earth rotation). This is essential for applications in positioning and navigation, civil engineering, metrology, but also for many geoscientific disciplines, because the Earth’s gravity field reflects the mass distribution and its transport in the Earth’s interior and on its surface.
Main scientific (yellow) and societal (blue) challenges addressed by a future sustained gravity observing system.
The high-resolution static gravity field, represented by the geoid, serves as a unique physical reference surface. It is used to define height systems and for the prediction of satellite orbits. Since the geoid represents the surface of an ideal ocean at rest, in oceanography it is compared with the actual ocean surface, which can be derived by satellite altimetry. Thus, the so-called mean dynamic topography (MDT) can be computed, from which geostrophic ocean surface currents can be derived. These ocean currents are, beside the atmosphere, the second largest mechanism for global heat transport through the Earth system. High-resolution static gravity field models also provide boundary values for geophysical models of lithospheric suctures and dynamic processes in the Earth’s mantle and crust.
Temporal gravity variations are a direct measure of variations in the Earth system related to mass transport processes in land hydrology, cryosphere, and the ocean. In fact, gravimetry is the only available measurement technique that is directly sensitive to mass and mass change, and by this is complementary to geometrical techniques such as precise positioning with global navigation satellite systems (GNSS), remote sensing or satellite altimetry.
Since 2000, the era of dedicated satellite gravity missions such as CHAMP, GRACE and GOCE has revolutionized our knowledge on the Earth’s gravity field and its changes in time. Temporal gravity measurements quantify the rates of ice mass melting of the large ice sheets of Greenland and Antarctica and their contribution to ongoing sea level rise. They also provide global observations of seasonal, inter-annual and long-term water storage variations for large and medium size catchments, which supports the closure of the terrestrial water budget of the global water cycle. Additionally, mass displacement in connection with large earthquakes events can be measured, which constrain the physical modelling of earthquake mechanisms.
Based on data of these satellite missions, global Earth’s gravity field models with homogeneous accuracy and increasingly high spatial resolution are derived, but due to signal attenuation with satellite altitude they are still limited to spatial wavelengths down to 70-80 km. Therefore, complementary detail information from terrestrial, air-borne and ship-borne gravimetry has not become obsolete, but in contrast is nowadays even more important to complete the gravity field picture on a local to global scale. In parallel, new and innovative measurement concepts and satellite systems, which shall provide even more accurate gravity measurements in the near future, are under development and investigation. This also imposes new challenges to develop methodologies for optimally combining different gravity data types of different signal content and with different specific features, and finally to derive gravity field and geoid models on all spatial scales. Figure 1 summarizes the main scientific (yellow) and societal (blue) challenges that shall be tackled by a future sustained gravity observing system as integral part of the Global Geodetic Observing System (GGOS).
In the term 2015-2019, Commission 2 will continue working to develop cooperation in observation, theory, methodology and computation of Earth’s gravity field, and promoting several activies such as symposia and collaborative works. The next international symposium will be the joint Comm. 2 & IGFS meeting Gravity, Geoid and Height Systems, to be held 19-23 September 2016 in Thessaloniki, Greece.
Currently, Commission 2 consists of 6 Sub-Commissions, 7 Joint Study Groups and 4 Joint Working Groups.
The Sub-Commsions are:
- SC 2.1: Gravimetry and Gravity Networks (Chair: L. Vitushkin, Russia),
- SC 2.2: Methodology for Geoid and Physical Height Systems (Chair: J. Agren, Sweden),
- SC 2.3: Satellite Gravity Missions (Chair: A. Jäggi, Switzerland),
- SC 2.4: Regional Geoid Determination (Chair: M.C. Pacino, Argentina),
- SC 2.5: Satellite Altimetry (Chair: X. Deng, Australia),
- SC 2.6: Gravity and Mass Transport in the Earth System (Chair: J. Kusche, Germany).
Among the manifold tasks of Commission 2 in the term 2015-19, much emphasis will be given to support the realization of several recent IUGG and IAG resolutions. They address the establishment of a global absolute gravity reference system (GAGRS) to replace the International Gravity Standardization Net 1971 (IGSN71), the realization of an International Height Reference System (IHRS), and the realization of an Global Geodetic Reference System (GGRS), aiming at a consistent integration of geometry and gravity.
Written by Roland Pail, President of Commission 2.