Beyond GOCE, GRACE & Swarm

In an internationally coordinated initiative among the main user communities of gravity field products the science requirements for a future gravity field mission constellation (beyond GRACE-FO) have been reviewed and defined. This activity was realized as a joint initiative of the IAG (International Association of Geodesy) Sub-Commissions 2.3 and 2.6, the GGOS (Global Geodetic Observing System) Working Group on Satellite Missions, and the IUGG (International Union of Geodesy and Geophysics). After about one year of preparation, in a user workshop that was held in September 2014 consensus among the user communities of hydrology, ocean, cryosphere, solid Earth and atmosphere on consolidated science requirements could be achieved.

The consolidation of the user requirements became necessary, because several future gravity field studies have resulted in quite different performance numbers as a target for a future gravity mission (2025+). Based on limited number of mission scenarios which took also technical feasibility into account, a consolidated view on the science requirements among the international user communities was derived, research fields that could not be tackled by current gravity missions have been identified, and the added value (qualitatively and quantitatively) of these scenarios with respect to science return has been evaluated. The resulting document shall form the basis for further programmatic and technological developments.

In this contribution, the main results of this initiative will be presented. An overview of the specific requirements of the individual user groups, the consensus on consolidated science and user needs for observing global mass transport to understand global change and to benefit society, as well as the new research fields that have been identified during this process will be outlined and discussed.

Future gravity field missions are expected to use new generation instruments of unprecedented accuracy that will affect the error tree of spaceborne gravity field recovery methods as we know it today. Temporal aliasing remains one of the biggest contributors to the error budget, even if more complex satellite formations (e.g. two pairs instead of one) are deployed. This study, which was part of the ESA project “Assessment of Satellite Constellations for Monitoring the Variations in Earth Gravity Field (SC4MGV)”, investigates the impact of temporal aliasing effects in case of a so-called Bender-type constellation, as one of the best candidates of satellite formations to be launched in the future. An error assessment is performed, and the effect of different sources of aliasing is highlighted.

A method called the Wiese approach (Wiese et al, 2013) that results in reduction of temporal aliasing effects is investigated for a Bender-type constellation. This method employs a gravity field processing technique that co-estimates low resolution gravity fields at short time intervals together with the higher resolution gravity field which is sampled at a longer time interval. The approach opens a big search space for parameterization choices, which is thoroughly investigated. The effect of each choice is assessed, and the long-term as well as the short-term fields are validated. The method is also extended in order to retrieve even more signal variability at higher frequencies, by parameterizing in multiple time periods instead of single (Daras, 2015). A fine-tuning is performed in terms of temporal sampling and maximum resolution of the short-term gravity fields that leads to the biggest reduction of aliasing errors. The contribution of individual aliasing components to the error-budget is assessed, and the effectivity of the Wiese parameterization to reduce their effects is demonstrated. An attempt is also made to extract possible correlations between the short-term fields and other co-estimated parameters (e.g. boundary arc conditions, empirical accelerations), and to analyze their impact on the gravity field solutions.

Finally, it is demonstrated that future gravity satellite missions of a Bender-type constellation processed with the proposed parameterization, have the possibility to estimate the full spectrum of geophysical processes that comprise system Earth, including variations of atmosphere and ocean. The averaged long-term fields, together with the short-term low-resolution fields (new bi-product) of this content, comprise an innovative product that may open doors to new fields of application.

The success of the geopotential mapping missions GOCE, GRACE and Swarm, including new application areas in geospace research and aeronomy, paves the way for continued endeavour in these exciting areas of science and services. In fact, the International Union of Geodesy and Geophysics has clearly identified (constellation) missions dealing with measurements of mass variations (mass transport) and the magnetic field as being a key priority for the community, and has directly addressed stakeholders, including the major space agencies, in order to encourage long-term perspectives for such types of missions.   The present contribution addresses the current situation in terms of data exploitation for the three missions GOCE, GRACE and Swarm as well as programmatic preparations that are on-going at the European Space Agency and elsewhere in order to facilitate and pave way for future missions, and/or additions to the current fleet of satellites. 

ESA’s Swarm mission aims at studying all sources of Earth’s magnetic field. It consists of two satellites (Alpha and Charlie), which fly side-by-side on near polar orbits at an altitude of slightly less than 500 km, and of a third satellite (Bravo) on a similar but slightly more polar and higher orbit, which progressively drifts with respect to that of Alpha and Charlie. This orbital configuration has proven extremely valuable, as evidenced by the many results already obtained from the first two years of the mission. These results, however, also reveal that geomagnetic field modeling and investigation efforts are now hampered by the still limited local time coverage provided by this constellation. This affects our ability to accurately characterize time changes in the ionospheric and magnetospheric field contributions, and to model the electrical conductivity of the Earth’s mantle. It also indirectly limits our ability to model the core and lithospheric field. More generally, many of the “residual signals” detected in the very accurate magnetic data of the Swarm mission can still not fully be exploited. Further increasing the scientific return of the Swarm mission by squeezing more out of these data, however, would be possible if a fourth “Delta” satellite were to be launched soon enough to join the constellation at a similar altitude but much lower inclination orbit (such as 60°). Such a satellite would provide less geographical coverage but a much faster mapping of all local times over these latitudes. In this presentation we will present the rational for such a Delta mission and discuss the benefit it would bring.

Geothermal energy is a valuable renewable energy source the exploitation of which contributes to the worldwide reduction of consumption of fossil fuels oil and gas. The exploitation of geothermal energy is facilitated where the thermal gradient is higher than average leading to increased surface heat flow. Apart from the hydrologic circulation properties which depend on rock fractures and are important due to the heat transportation from the hotter layers to the surface, essential properties that increase the thermal gradient are crustal thinning and radiogenic heat producing rocks. Crustal thickness and rock composition form the link to the exploration with the satellite derived gravity field, because both induce subsurface mass changes that generate observable gravity anomalies. The recognition of gravity as a useful investigation tool for geothermal energy lead to a cooperation with ESA and the International Renewable Energy Agency (IRENA) that included the GOCE derived gravity field in the online geothermal energy investigation tool of the IRENA database. The relation between the gravity field products as the free air gravity anomaly, the Bouguer and isostatic anomalies and the heat flow values is though not straightforward and has not a unique relationship. It is complicated by the fact that it depends on the geodynamical context, on the geologic context and the age of the crustal rocks. Globally the geological context and geodynamical history of an area is known close to everywhere, so that a specific known relationship between gravity and geothermal potential can be applied. In this study we show the results of a systematic analysis of the problem, including some simulations of the key factors. The study relies on the data of GOCE and the resolution and accuracy of this satellite. We also give conclusions on the improved exploration power of a gravity mission with higher spatial resolution and reduced data error, as could be achieved in principle by flying an atom interferometer sensor on board a satellite.

More information can be found here:

IRENA online tool: http://irena.masdar.ac.ae/?map=1046%20

GOCE and Geology: 

Braitenberg C. (2015). Exploration of tectonic structures with GOCE in Africa and across-continents. International Journal of Applied Earth Observation and Geoinformation, 35, 88-95, doi:10.1016/j.jag.2014.013,

Braitenberg C., Mariani P. (2015). Geological implications from complete Gondwana GOCE-products reconstructions and link to lithospheric roots. Proceedings of '5th International GOCE User Workshop', Paris, France, 25-28 November 2014, (ESA SP-728, March 2015), ISBN:978-92-9221-292-6, ISSN:1609-042X.

Mariani P., Braitenberg C. (2015). GOCE gradient tensor characterization of the coupled Parana' (South America) and Etendeka (Africa) magmatic provinces. Proceedings of '5th International GOCE User Workshop', Paris, France, 25-28 November 2014, (ESA SP-728, March 2015), ISBN:978-92-9221-292-6, ISSN:1609-042X,.

Shin Y. H., Shum C. K., Braitenberg C., Lee S. M., Na S.-H., Choi K. S., Hsu H., Park Y.-S., Lim M. 2015. Moho topography, ranges and folds of Tibet by analysis of global gravity models and GOCE data. SCIENTIFIC REPORTS, 5, 11681; doi: 10.1038/srep11681, 1-7.