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Earth Gravitational Model . en Lemoine Kenyon Factor Trimmer Pavlis Chinn Cox Klosko Luthcke Torrence Wang Williamson Pavlis Rapp and Olson NASA Goddard Space Flight Center Greenbelt Maryland USA July nasa gov html Lemoine Kenyon Factor Trimmer Pavlis Chinn Cox Klosko Luthcke Torrence Wang Williamson Pavlis Rapp and Olson NASA