1. D. J. Wingham, A. J. Ridout, R. J. Arthern, Department of Space and Climate Physics, University College London, 17-19 Gordon Street, London WC1H 0AH, UK. R. Scharroo, Delft Institute for Earth-Oriented Space Research, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, Netherlands. C. K. Shum, Departments of Civil and Engineering Science and Geodetic Science, Ohio State University, 2070 Neil Avenue, Columbus, OH 43210, USA.