1. Institute of Geophysics and Meteorology, University of Cologne Cologne Germany
2. School of Electrical Engineering, KTH Royal Institute of Technology Stockholm Sweden
3. Laboratoire de Physique Atmosphérique et Planétaire, STAR Institute, University of Liège Liège Belgium
4. Department of Earth and Planetary Science, and Department of Physics and Astronomy, The Johns Hopkins University Baltimore MD USA
5. Southwest Research Institute San Antonio TX USA
6. University of Texas at San Antonio San Antonio TX USA