An improved global gravity field model of the Earth derived from reprocessed GOCE observations with the time-wise approach

Abstract

ESA's dedicated gravity field mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) successfully completed its science operations in 2013 as the first Earth Explorer mission in orbit. In 2014, the fifth releases of GOCE based global gravity field models of the Earth were published. Already at that time, the entire mission data set was used. Until now, the release 5 models are the highest resolution models of the static Earth's gravity field derived by satellites. On the one hand, high-low satellite-to-satellite tracking observations of GOCE by GPS satellites is used to derive the long wavelength part of the gravity field. On the other hand, observations of the core instrument - a gravitational gradiometer - are sensitive to the medium and short wavelengths of the Earth gravity field. Combining both observation groups provides a global model of the gravity field with a spatial resolution of about 70 km. <br /> Recent studies have shown that the quality of the derived level 1B gravity gradients can be significantly improved, if a quadratic term is additionally considered in the calibration procedure. Consequently, ESA funded a reprocessing campaign by the High-level Processing Facility (HPF). Within that framework, global gravity field models are estimated and provided as higher level GOCE products. <br /> One approach which can be used for the processing of the GOCE observations is the so called time-wise approach. Within this approach, only GOCE observations are used to estimate the gravity field model in a least-squares sense. The highly correlated gravity gradients in the gradiometer reference frame are used to derive normal equations in terms of global spherical harmonics. A lot of effort is spend on the stochastic modeling of the noise characteristics of the gravity gradients, to deliver in addition to the high quality gravity field a meaningful covariance matrix. The gradiometry normal equations are combined with normal equations determined from high-low satellite to satellite tracking to better capture the long wavelength of the gravity field. Within this contribution, the release six of the official ESA time-wise model is presented. An improved time-wise processing is used, to estimate the new EGM_RIM_RL06 model form the reprocessed GOCE observations. An advanced detection of suspicious data is implemented and used in the estimation of decorrelation filters as well as for gravity field recovery. These decorrelation filters serve as a stochastic model for the gravity gradients in the least squares estimation of the spherical harmonic coefficients which represent the gravity field. <br /> Within this contribution, the processing used to derive the new release is summarized. The improvements with respect to the former 5th release are highlighted. The characteristics and the quality level of the new solution is discussed. Finally the model is validated with comparisons to independent data sets. As an outlook, the combination with GRACE models towards a combined GOCO06S is shown.