era, which describes s- tematically the orbit theory with analytical solutions, with respect to all of extrat- restrial and geopotential disturbances of the second order, and the solutions are free of singularity. Based on such a theory, the algorithms of orbit determination can be renewed; deeper insight into the physics of disturbances becomes possible; the way to a variety of new applications and re?nements is opened. My primary knowledge of the orbit theory came from my education of mat- matics while studying physics and theoretical mechanics (1981). My ?rst practical experience with orbit came from the research activity at the Technical University (TU) Berlin on orbit corrections of the satellite altimetry data (1988-1992). The extensive experience on orbit came from the GPS/Galileo software development for orbit determination and geopotential mapping at the GFZ (2001-2004).