Jean-Paul Boy, EOST École et Observatoire des Sciences de la Terre, Strasbourg Cedex, France, Frank G Lemoine, NASA Goddard SFC, Greenbelt, MD, United States, Douglas S Chinn, Stinger Ghaffarian Technologies Inc., Greenbelt, MD, United States, Nikita P Zelensky, University of Maryland, College Park, United States and Despina E Pavlis, University of Maryland, College Park, MD, United States