Combination of Gravimetry, Altimetry and GOCE Data for Geoid Determination in the Mediterranean: Evaluation by Simulation R. Barzaghi (1) , A. Maggi (1) , N. Tselfes (2) , D. Tsoulis (3) , I. N. Tziavos (3) , G. S. Vergos (3) (1) DIIAR - Politecnico di Milano - Piazza Leonardo da Vinci, 32 - 20133 Milano - Italy (2) DIIAR - Politecnico di Milano, Polo Regionale di Como - Via Valleggio, 11 - 22100 Como - Italy (3) Department of Geodesy and Surveying, Aristotle University of Thessaloniki, University Box440, 54124 Thessaloniki - Greece Abstract. Local geoid determination is traditionally carried out on land and at sea areas using gravity anomaly and altimetry data. This determination can be aided and improved by the data of missions such as GOCE. In order to assess the performance of the combination of heterogeneous data for local geoid determination, simulated data for the area of the central Mediterranean Sea are analyzed. These data include gravity anomaly, altimetry, and GOCE observations processed with the space-wise approach. The results show that GOCE data improve the results for areas not well covered with other data types, while also accounting for any long wavelength errors of the adopted reference model. Even when the ground gravity data are dense, data from GOCE improve the error standard deviation and eliminate biases. At sea, the altimetry data give the dominant geoid information. However the geoid accuracy is sensitive to orbit calibration errors and the unmodelled mean sea surface topography. If such effects are present the GOCE data can account for them. Keywords. Mediterranean geoid, GOCE mission, gravity, altimetry. 1 Introduction GOCE (Gravity field and steady-state Ocean Circulation Explorer) is a satellite mission (ESA, 1999) designed by ESA (European Space Agency), which will be launched in 2008. The goal of this mission is the determination of the stationary part of the gravity field to a high degree of accuracy and spatial resolution. The main instrument on board the satellite will be the “gradiometer”, composed by six accelerometers, and measuring the second derivatives of the potential (the full tensor) along the satellite orbit (the so-called gradients). Additional information on the gravity field will be derived from the tracking of the satellite orbit, by means of a GPS receiver, and the accelerometers measurements of the non-gravitational forces.. Three different approaches will be applied for the determination of the global gravity field models from GOCE: the direct approach (Bruinsma et al., 2004), the time-wise approach (Pail et al., 2005) and the space-wise approach (Migliaccio et al., 2004). In a previous study (Maggi et al, 2007), the contribution of GOCE filtered data from the space- wise approach and a GOCE geo-potential model, were evaluated for local geoid determination in a combination scheme with terrestrial data employing least squares collocation in a simulation. It was found that the benefit from the GOCE long wave- length information will be very significant. The present paper, refers to a larger region incorporating satellite altimetry data as well. No topographic information (Arabelos and Tscherning, 1990) or bathymetric information (Vergos and Sideris, 2003) will be used for data reduction, even though this is possible with real data. 2 Simulation of data In the frame of the EGG-C (European GOCE Gravity Consortium) (Balmino, 2001) activities for the preparation of the GOCE mission, full simulation solutions are computed so that methodology and software efficiency are ensured (Migliaccio et al., 2006). The latest simulated data set available by EGG-C is used and the data are processed. These data include 60 days (note that at least 1 year is expected) of: gradients with in-flight calibration noise (including instrumental errors, satellite errors, etc.), signal simulated from EGM96 (Lemoine et al, 1998), orbit positions and velocities, common mode accelerations, rotations and attitude information. The orbit positions and velocities are used to obtain