GRAVITY FIELD MISSIONS: STATUS & STUDIES FOR FUTURE MISSIONS

Visser, P.N.A.M.
Delft Institute of Earth Observation and Space Systems (DEOS) Delft University of Technology Kluyverweg 1, 2629 HS, Delft, The Netherlands

The ability of observing gravity from space with ever increasing spatial and temporal resolution has been and is being demonstrated by missions such as CHAMP (launch July 2000, proof of concept for GPS high-low satellite-to-satellite tracking - SST - in combination with a precise accelerometer), GRACE (launch March 2002, microwave low-low SST plus GPS and accelerometers), and GOCE (launch March 2009, ultra-precise accelerometers composing a gradiometer plus GPS and drag-free control).

The GRACE mission is already going strong for more than seven years (status August 2009) and is producing longer and longer time series of temporal gravity at typical spatial scales of 500-1000 km and temporal resolution of 10-30 days. Striking results are obtained in observing mass changes e.g. due to continental hydrology and due to ice mass changes in polar areas (Greenland in particular). In addition, the accumulation of a long time series of GRACE observations allows the modeling of a mission mean gravity field with increasing spatial resolution. GOCE is foreseen to enter the first operational observation phase soon (again status August 2009) and is ready to provide observations that will significantly further enhance the spatial resolution down to 100 km or better. The observation of gravity in terms of spatial and temporal resolution by missions like GRACE and GOCE is not only limited by the very nature of their sensor complement and orbital characteristics, but also by imperfections in the post-processing of the observations such as uncertainties in so-called de-aliasing products. Whereas the impact of the latter can be (and is) mitigated by ongoing efforts to improve models and processing schemes, the impact of the first can only be overcome by improving sensor systems and/or mission characteristics (e.g. tuning orbital parameters, flying more satellites simultaneously, etc.).

A large scientific and user community has expressed the need for further improving the temporal resolution of observing mass changes in the Earth system at enhanced spatial scales, significantly beyond the capabilities of the (combination of) GRACE and GOCE. Many activities are going on to study new sensor technologies (e.g. low-low SST by laser, improved accelerometers and drag-free control systems, etc.) and mission concepts (e.g. optimization of orbits, formations such as multiple GRACE-type tandems interlaced in time or space, cartwheel and pendulum). Typically, teams of researchers from universities and institutes, highly qualified engineers from industry and colleagues from space agencies work together to define and design new gravity field missions. Defining and designing future gravity field missions is a complicated process in which many trade-offs have to be made (e.g. minimum requirements for scientific and societal outcome, cost efficiency, technological feasibility, etc.).