First GOCE gravity field models derived by three different approaches - Roland Pail; Sean Bruinsma; Federica Migliaccio; Christoph Förste; Helmut Goiginger; Wolf-Dieter Schuh; Eduard Höck; Mirko Reguzzoni; Jan Martin Brockmann; Oleg Abrikosov; Martin Veicherts; Thomas Fecher; et al. (Journal of Geodesy, 2011)