Spectral characteristics of the Hellenic vertical network - Validation over Central and Northern Greece using GOCE/GRACE global geopotential models Vassilios D. Andritsanos 1 , George S. Vergos 2 , Vassilios N. Grigoriadis 2 , Vassilios Pagounis 1 and Ilias N. Tziavos 2 1 Department of Civil Engineering and Surveying and Geoinformatics Engineering, Technological and Educational Institute of Athens, Greece 2 Department of Geodesy and Surveying, Aristotle University of Thessaloniki, Greece European Geosciences Union – General Assembly 2014 Vienna – Austria – April, 27 th to May, 2 nd 2014 Session G4.2 Satellite Gravimetry: GRACE, GOCE and Future Gravity missions