1. Post-Doctoral Researcher, Geodesy and Geodynamics Laboratory, Institute of Geodesy and Photogrammetry, ETH Zurich, Schafmattstrasse 34, HPV F51, 8093 Zurich, Switzerland (corresponding author).
2. Professor, Geodesy Laboratory, Dept. of Civil Engineering, Univ. of Patras, Patras 26500, Greece.