1. a Laboratory for Space Geodesy and Remote Sensing Research, The Ohio State University, Columbus, Ohio, USA
2. b Byrd Polar Research Center, The Ohio State University, Columbus, Ohio, USA
3. c Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, Centre National d'Etudes Spatiales, Toulouse, France
4. d Center for Space Research, The University of Texas, Austin, Texas, USA