1. Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark
2. Department of Geoscience, Aarhus University, Aarhus, Denmark
3. Istituto Nazionale di Ricerca Metrologica, Turin, Italy
4. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California
5. Department of Astronomy, Cornell University, Ithaca, New York
6. Physikalisches Institut, University of Bern, Bern, Switzerland