Matija Herceg

GOCE Data for Local Geoid Enhancement

The GOCE gradients, having a spatially dense data distribution, may potentially provide better predictions of the regional gravity field than those obtained using a spherical harmonic Earth Geopotential Model. The aim of this study is to develop a methodology to improve the use of GOCE gradients and to determine the Earth’s gravity field with better accuracy than by using global models, which have been truncated at a specific harmonic degree and order. The method makes use of all available GOCE gradient data in addition to the global models and aims at improving the determination of Earth’s gravitational field in regional areas. Subsequently, the calculated geoid is used together with measurements of sea surface height in a calculation of the Mean Dynamic Topography.In regional geoid recovery from GOCE gradients, two methods are used, one of them being Least-Squares Collocation (LSC). The second method is developed as a part of this study, and it is based on the Reduced Point Mass (RPM) response. The results show that the RPM method and LSC method give very similar results when using the same data, i.e. the difference is insignificant when compared to the EGM2008 results. However, when all of the available GOCE gradient data are used with the RPM method, an improvement in the gravitational field determination is achieved. The enhanced geoid by the RPM method is then used for the improvement of the MDT in the North Atlantic region.

Using ground gravity to improve ice mass change estimation from GOCE gravity gradients in mid-west Greenland

Vertical gravity gradient anomalies from the Gravity and steady-state Ocean Circulation Explorer (GOCE) DIR-3 model have been used to determine gravity anomalies in mid-west Greenland by using Least-Squares Collocation (LSC) and the Reduced Point Mass (RPM) method. The two methods give nearly identical results. However, compared to LSC, the RPM method needs less computational time as the number of equations to be solved in LSC equals the number of observations. The advantage of the LSC, however, is the acquired error estimates. The observation periods are winter 2009 and summer 2012. In order to enhance the accuracy of the calculated gravity anomalies, ground gravity data from West Greenland is used over locations where the gravity change resulting from ice mass changes is negligible, i.e. over solid rock. In the period considered, the gravity anomaly change due to changes in ice mass varies from −5 mGal to 4 mGal. It is negative over the outlet glacier Jacobshavn Isbræ, where the mass loss corresponds to a gravity change of approximately −4 mGal. When using only GOCE vertical gravity gradients, the error estimates range from 5 mGal at the coast to 17 mGal over the ice sheet. Introducing the ground gravity data from West Greenland in the prediction reduces the errors to range from 2 to 10 mGal.