During the last decade, geodesy in general and gravity field modelling in particular, have experienced a notable swing in both their recognition and end-product use by many scientific disciplines. This was mainly due to the advent of the dedicated satellite gravity missions CHAMP, GRACE and GOCE, which have brought new insights to both the static and time-variable representations of the Earth’s gravity field. Future satellite missions such as GRACE-Follow On and the generation beyond, complemented by enhanced processing technologies, shall substantially improve this knowledge with further increased spatial and temporal resolution. Alongside came advancements in traditional terrestrial, airborne and shipborne gravity instrumentation which offer considerable improvements in gravity networks and the monitoring of gravity variations. Moreover, the continuous availability of altimeters on-board satellites and the improvements in the representation of the marine geoid have provided higher accuracies to shorter spatial scales of the ocean topography and circulation. The collocated use of the new datasets and models offered by gravity-field related research can improve the knowledge of the Earth system, its sub-systems, coupling processes among them, which are important indicators for climate change. They provide the breadboard for innovative investigations of the solid Earth giving new details of crust and mantle and their variation in time. Finally, they can lead to the linkage of local vertical datums to a global one, the latter being fundamental for monitoring sea level variations, as well as engineering and hydrological and cryospheric studies.
The GGHS2016 “Gravity, Geoid and Height Systems 2016” Meeting is the first Joint Commission2 and IGFS Symposium. Its main focus is on methods for observing, estimating and interpreting the Earth gravity field as well as its applications.
Session 1: Current and future satellite gravity missions
Satellite gravimetry, i.e. observing the Earth’s gravity field and its temporal variations from space, is the only measurement concept capable of providing a global view of mass distribution and mass variation in the system Earth. While the Gravity Recovery and Climate Experiment (GRACE) mission has paved the way for observing the time variable gravity field from space over the last decade, the Gravity Field and Ocean Circulation Explorer (GOCE) mission has provided a consistent global model of the static gravity field with a resolution of 100 km and 1-2 cm accuracy in terms of geoid heights. These complementary missions have triggered a large number of new applications in the fields of oceanography, continental hydrology, polar and mountain glacier ice mass monitoring, solid Earth geophysics, geodetic height systems and others. The increased length of the GRACE data record (now 14 years) has made satellite gravimetry indispensable for climate studies, and clearly paints the need for continuing these observations. The first continuity mission, GRACE-Follow On (GRACE-FO) is planned for launch in 2017, and is more or less identical to GRACE with the exception of flying a technology demonstration laser interferometer to more accurately measure the distance variations between the two spacecraft. The community is additionally preparing for a future gravity mission after GRACE-FO, with various initiatives studying things such as a reassessment of mission requirements, improved instrumentation, innovative satellite concepts, and new data processing strategies. Because the observation concepts intrinsically assume that the satellite as a whole is the gravity sensor, there is a strong dependency between each of these components, requiring that a joint scientific-technological methodology be used to prepare for future satellite gravimetry missions.
This session solicits contributions about (1) processing and filtering of current satellite gravimetry mission data, including GRACE, GOCE, and other LEO GPS satellites, and (2) mission concepts, instrumentation and processing strategies for future gravity field missions, including innovative strategies for utilizing the laser interferometer data on GRACE-FO.
Session 2: Global gravity Field Modelling
Gravity field models of global extent and very high resolution encapsulate critical information for a wide range of applications, from a unified World Height System to Inertial Navigation. The development of such gravity models requires the effective combination of ground and satellite-derived gravity information, especially from dedicated space missions (e.g., CHAMP/GRACE/GOCE/SWARM and follow-on), with the information contained in gravity data of uniformly high accuracy and resolution and covering ideally the entire Earth. An essential element of such model developments is the availability of global digital topographic models, which are necessary for both the processing of raw gravity observations and for providing supplemental gravity information implied by the topography, over areas or bandwidth where gravity observations are unavailable or their use is spectrally restricted. The error assessment of any global gravity solutions is of equal importance to the signal information that the model furnishes. Very high resolution global gravity models require a rather formidable number of parameters (e.g., spherical harmonic coefficients) for their definition. This makes “compressive” representation of model functionals and of their error properties appealing to many applications.
This session solicits contributions that focus on all aspects of global high resolution gravity model developments and assessment, from methodological issues to modeling results and applications. We also seek contributions that focus on effective parsimonious representations of global gravity model functionals and their errors.
Session 3: Local/regional geoid determination methods and models
This session will focus on the practical solution of various formulations of geodetic boundary-value problems to yield precise local and regional high-resolution geoid models. Contributions describing recent developments in theory, processing methods, downward continuation of satellite and airborne data, treatment of altimetry and shipborne data, terrain modeling, software development and the combination of gravity data with other signals of the gravity field for a precise local and regional gravity field determination are welcome. Topics such as the comparison of methods and results, the interpretation of residuals as well as geoid applications to satellite altimetry, oceanography, vertical datums and local and regional geospatial height registration are of a special interest. Reports of the IAG sub-commissions for the determination of the geoid on the continental scale will be given.
Session 4: Absolute and Relative gravity: observations and methods
Gravimetric measurement and analysis techniques are addressed by considerable research efforts in geodesy and beyond. In particular, the development of quantum gravimeters is advancing in many groups worldwide, with the potential of providing sensors for more accurate, faster and more reliable gravity measurements. But also gravity campaigns and networks using absolute, superconducting and other relative gravity sensors continue to provide valuable and often complementary results, e.g., on hydrological or tectonic processes as well as for the exploration of natural resources. Scientific contributions showing results, methods and perspectives on these fields are strongly encouraged.
Session 5: Height systems and vertical datum unification
Session 5 focuses on the unification of the existing height systems around the world. This can be achieved through the realization of an international vertical reference system that supports geometrical (ellipsoidal) and physical (normal, orthometric) heights with centimeter precision in a global frame, enables the unification of all existing physical height systems, and provides high-accuracy and long-term stability of the vertical coordinates. A first step towards the establishment of a worldwide unified (standardized) height system was the release of an IAG resolution for the definition and realization of an International Height Reference System (IHRS) that was issued during the 2015 IUGG General Assembly. This resolution outlines the conventions for the definition of the IHRS in terms of potential parameters: the vertical coordinates are geopotential numbers referring to an equipotential surface of the Earth’s gravity field realized by a conventional Wo value. At present, the main challenge is the realization of the IHRS, i.e., the establishment of the International Height Reference Frame (IHRF). It is expected that the IHRF follows the same structure as the ITRF: a global network with regional and national densifications, with known geopotential numbers referring to the global IHRS. To guarantee a precise combination of physical and geometric parameters and to support the vertical datum unification worldwide, this reference network should be collocated with fundamental geodetic observatories, geometrical reference stations, reference tide gauges, local levelling networks, and gravity reference stations. Within this framework, contributions related to the following topics are invited:
- Refinement of standards and conventions for the definition and realization of vertical reference systems, in particular those standards needed for the realization of the IHRS.
- Strategies for the establishment of precise vertical reference frames, in particular the establishment of the IHRF.
- Precise determination and modelling of the time-dependent changes of the vertical coordinates and the datum itself.
- Theory and methodology for height system unification, in particular the connection/transformation of existing height systems to the IHRS/IHRF.
- Recent results on vertical datum unification, including the connection of vertical datums over oceans.
- Strategies for collocation of vertical reference stations (i.e., IHRF stations) with existing reference frames (GGOS core stations, ITRF, gravity stations, existing levelling networks, etc.).
- Development of a registry (metadata) containing the existing height systems and their connections to the IHRS/IHRF.
Session 6: Satellite altimetry and climate-relevant processes
Melting of ice sheets and glaciers, ocean circulation and sea level variations, river runoff, changes in precipitation, evapotranspiration, soil moisture and groundwater, post-glacial rebound and flow in the Earth mantle, all these processes cause transport and re-distribution of mass. Using extremely precise sensors, geodetic observing systems provide essential information necessary for understanding these phenomena. For example, satellite altimetry represents a valuable tool for observing sea level variations and has also demonstrated its potential to monitor surface water elevation changes of terrestrial water bodies. And the satellite mission GRACE has triggered a wide spectrum of new Earth science applications by measuring changes in the Earth’s gravitational field and thereby mass changes in both oceanic and terrestrial storages. Joint data analysis and integration with physical modeling improves our knowledge about processes within the system Earth. At the same time new innovative altimeters are being launched bringing new usage of satellite altimetry and increasing the resolution of altimetric observations into the sub-km resolution.
In this session, we invite presentations dealing with the interpretation and innovative use of satellite altimetry and gravimetry and of complimentary data such as obtained from global GNSS networks, terrestrial gravimetry, InSAR, SLR, tide gauges and ocean bottom pressure sensors. We welcome contributions on the use of geodetic data sets for the improvement of geophysical and climate models – both in terms of model evaluation, calibration and data assimilation.