Geoid for Austria - Regional gravity FIELD improved (GARFIELD)

Within this project a new high-quality gravity field solution should be generated for Austria, which overcomes the inconsistencies between previous geoid solutions and geoid heights derived from GPS/leveling campaigns. The computation will optimally combine the complementary data types of satellite observations and all available terrestrial gravity field measurements in Austria and neighbouring countries. For this purpose, the Remove- Compute-Restore technique will be adapted in order to avoid a double consideration of the topographic-isostatic masses when performing long- and short-wavelength signal reductions. Furthermore, the atmospheric mass effects on gravity observations will be considered within this project. The Least Squares Collocation (LSC) approach will serve as reference method for the gravity field computation. Improvements are aimed for with regards to the choice of a proper universal covariance function and the optimal weighting of the different observation types. In this context, further developments concerning the integration of the now available full variance/covariance information of the recent global gravity field models will be necessary. The unique gravity gradient observation type of GOCE`s gradiometer instrument delivers information of the gravity field particularly in the medium wavelengths, where the spectral overlap of global satellite data and local terrestrial data occurs. For an optimum regional gravity field determination it is therefore reasonable to use the GOCE gradients as in-situ observations and directly combine them with terrestrial data. However, there are still some issues that are aimed to be clarified on the course of this project for a practical applicability using GOCE gradients as in-situ observations. Alternatively to LSC, a Gauss-Markov model with parametrization as Radial Basis Functions will be implemented. The optimum weighting of the observation groups will be performed by Variance Component Estimation (VCE). By the new methodological developments of this proposed project, an increasing number of observations can be included in the calculations and a downsampling of the available data, as it is required in LSC due to computational limitations, will no longer be necessary. Compared to the standard LSC approach, the information content which can be incorporated in a new geoid solution can therefore be increased drastically. The achieved results will be verified by cross-validation methods and by comparing with independent GPS/leveling observations.

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission was operational from 2009 to 2013 and has been designed for the determination of the static, global Earth gravity field. Based on second derivatives of the measured GOCE gravitational potential, the so called GOCE gradients, several regional capabilities in the field of Geodesy and Geophysics were investigated in this project. It could be demonstrated how these gradients, although measured at a height of about 250 km can contribute significantly to an improved local Austrian gravity field solution. Another more regional application which is reflecting the multi-faceted applicability of these measurements is the determination of the boundary layer between Earth's crust and mantle, which is referred to as the Mohorovi?i? discontinuity (Moho). From a geological point of view, this boundary layer is of particular interest as it reflects both the discontinuity of the speed of seismic waves as well as the transition zone between different mineralogical rock compositions. The validation of these geophysical applications with several seismic studies showed that the results from the GOCO gradient based Moho computed in this project is much more precise in revealing geophysical meaningful features than other models covering similar spatial extents.Another project aim was the optimum combination of global, satellite derived Earth gravity fields with complimentary local terrestrial gravity field quantities such as gravity anomalies or deflections of the vertical in order to obtain a new and consistent equipotential surface which is called geoid. The problem here was the synthesis of different quantities involved, covering in turn a different bandwidth of the gravitational spectrum. Through an appropriate modeling and consideration of a 2D surface density model in combination with a least squares approach and associated radial basis functions, the accuracy of an Austrian geoid solution which is purely based on gravity measurements could be further increased. Finally, the corresponding validation with independent GPS/Levelling data, provided by the Austrian Federal Office of Metrology and Surveying shows a root mean square difference of 2.75 cm.In conclusion, it can be stated that the findings gained and methodological developments within this project have significantly contributed to improved gravity field solutions. Moreover, it should be possible to transfer these insights from their local Austrian meaning to a larger scale.