1. Ph.D. Student, Institute of Geodesy and Geoinformation, University of Bonn, 53115 Bonn, Germany (corresponding author).
2. Researcher, Institute of Geodesy and Geoinformation, University of Bonn, 53115 Bonn, Germany.
3. Professor, Institute of Geodesy and Geoinformation, University of Bonn, 53115 Bonn, Germany.