Abstract: In this paper,We have analysed the differences in the definition of coordinate transformation models between different types of 3-D positioning networks(abbreviated as space networks) for the combination of them and strictly defined the systematic scale biases of space networks from the point of probability and statistics.The properties of the scale biases,inherent in the space networks of different type,are discussed and identified.Basing upon the theoretical transformation model put forward in this paper,we have also analysed the properties of various practical transformation models as well as their biases in the combination of different types of space networks.Finally,a comprehensive model for combining different space networks(VLBI,SLR,GPS and terrestrial network) has been proposed.