Abstract: The DOP is one of the ways to assessment the performance of satellite navigation and positioning. Based on the mathematical and surveying meanings of GDOP, two methods to get the minimum of GDOP were given, and the study on the minimum was made theoretically. The conclusion was drawn that, the traditional idea that DOP is just a factor to decrease the navigation & positioning precision was limited, since all DOPs including GDOP would certainly be less than one and improve the precision as a result with the change of geometrical relationship between satellites and users. On the basis of the above conclusions, CGF (Constellation Geometry Factor), which could be used to assessment the spatial geometrical layout characteristics of constellation, was given. The Test with GPS and Galileo constellations had shown that CGF could give an effective evaluation the spatial geometrical layout characteristics of constellation. The Test on the navigation precision performance in near-earth space with GPS constellation proved that DOP has the attribute to improve precision.