1. Astronomical Institute, University of Bern , 3012 Bern , Switzerland
2. GFZ German Research Centre for Geosciences , 14473 Potsdam , Germany
3. Graz University of Technology, Institut für Geodäsie , 8010 Graz , Austria
4. Leibniz University Hannover, Institut für Erdmessung , 30167 Hannover , Germany