Session descriptions

Session 1: Current and future satellite gravity missions 
Session chairs: Thomas Gruber, Germany; David Wiese, USA

Satellite gravimetry 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 2018, 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, including mission requirements, improved instrumentation, innovative satellite concepts, and new data processing strategies. 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.

The session solicits contributions about processing and filtering of current satellite gravimetry mission data, including GRACE, GOCE, and other LEO GPS satellites; and future mission concepts, instrumentation and processing strategies, including innovative strategies for utilizing the GRACE-FO laser interferometer data.

Session 2: Global gravity Field Modelling 
Session chairs: Dan Roman, USA; Shuanggen Jin, China

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 - 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 gravity field modelling
Session chairs: Jonas Agren, Sweden; Hussein Abd-Elmotaal, Egypt

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, Relative and Airborne Gravity - observations and methods 
Session chairs: Leonid Vituskin, Russia; Rene Forsberg, Denmark

Gravimetric measurement and analysis techniques are addressed by considerable research efforts in geodesy and beyond. In particular, the development of quantum and cold-atom gravimeters is advancing in many groups worldwide, with the potential of providing sensors for more accurate, faster and more reliable gravity measurements, for both static and kinematic applications. New inertial sensor systems for improving airborne gravimetry accuracy and robustness are also developing, potentially making airborne gravity campaigns less expensive and eventually portable to drones. 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, glacial isostatic adjustment and rheology, as well as for the exploration of natural resources. We encourage for the session scientific presentations on technology tests, results, gravimetric methods, and novel perspectives on gravimetry applications for tectonics and hydrography.

Session 5: Height systems and vertical datum unification 
Session chairs: Michael Sideris, Canada; Laura Sanchez, Germany

The session 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 W0 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. 

Contributions related to the topics are invited, in particular those related to the International Height Reference Frame and System (IHRF/IHRS): Refinement of standards and conventions for the definition and realization of vertical reference systems; strategies for the establishment of precise vertical reference frames, precise determination and modelling of the time-dependent changes of the vertical coordinates and the datum itself; and theory and methodology for height system unification. In addition topics such as vertical datum unification, including the connection of vertical datums over oceans, strategies for collocation of vertical reference stations with existing reference frames (GGOS core stations, ITRF, gravity stations, existing levelling networks, etc.), and developments of height system metadata are invited.

Session 6: Satellite altimetry and applications 
Session chairs: Ole B Andersen, Denmark; Xiaoli Deng, Australia

The long-term time series of altimeter measurements has revolutionized the knowledge of many interdisciplinary scientific research fields including the marine gravity field, oceanic dynamics, terrestrial hydrology, ice sheet mass balance, sea level changes, and solid Earth geodynamics. New missions employing innovative technological improvements will provide higher resolution observations of the cryosphere, sea-ice, ice-covered oceans, open oceans and inland water bodies. The session solicits contributions that focus on validation and (cross-)calibration of satellite altimeter data, new technologies, improvement in global marine gravity field from new altimetry data, derivation of sea surface height and mean dynamic topography, improvements in near-coastal areas (retracking), derivation of sea level and sea level extremes, determination of inland water levels from conventional altimetry and SAR, connection to height systems.

Session 7: Mass transport and climate-relevant processes 
Session chairs: Carmen Boening, USA; Annette Eicker, Germany

Melting of ice sheets and glaciers, 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. 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. Special focus shall be also given to the contributions of geodetic observations to climate modelling and climate model validation.
In this session, we invite presentations dealing with the interpretation and innovative use of gravimetry and of complimentary data such as obtained from global GNSS networks, terrestrial gravimetry, InSAR, SLR, VLBI, 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.