Assessment of the added value of data from the GOCE satellite mission to time-varying gravity field modelling

Temporal changes of the Earth’s gravity field recovered from K-Band Ranging (KBR) data of the Gravity Recovery and Climate Experiment (GRACE) suffer from a number of limitations, including a limited spatial resolution and a relatively low accuracy of the East–West variations. With this manuscript, we investigate whether a retrieval of temporal gravity field changes can be improved by incorporating data from the Gravity field and steady-state Ocean Circulation Explorer (GOCE). To that end, we conduct twin experiments by producing monthly solutions up to spherical harmonic degree and order 120. First, we compute solutions from GRACE KBR data alone. Second, we produce solutions using a statistically optimal combination of GRACE KBR and GOCE data. The difference between these solutions allows conclusions to be drawn regarding the impact of GOCE data. Both unconstrained and optimally filtered solutions are considered in our analysis. The impact of GOCE data in these two cases is found to be radically different. In the case of unconstrained solutions, the usage of GOCE data allows the noise in these solutions to be reduced by 1–2 orders of magnitude. We demonstrate, however, that this reduction is due to a stabilization effect and not driven by the information content in GOCE data. Hence, it can be achieved without GOCE data, too. In the case of optimally filtered solutions, the impact of GOCE data stays, in average, at sub-millimeter level in terms of equivalent water heights. This is well below the GRACE noise level. The peak impacts reach about 1 cm. This holds true for the combined impact of GOCE satellite-to-satellite tracking data and its gravity gradients as well as for the impact of these data types individually. In relative terms, the peak impacts do not exceed 5–7 % of the signal amplitude, because they are always observed at locations where the time-varying gravity field signal is relatively strong. Nevertheless, we refrain from concluding that the added value of GOCE data to the retrieval of temporal gravity field variations is always negligible. A number of scenarios are discussed, in which the impact of GOCE data may exceed the level quantified in the study presented.