Change of Geometric Dilution of Precision (GDOP) for integrated system

With the development of the Global Navigation Satellite System (GNSS), the integrated system including two or more individual constellations will be the main navigation mode in the future. For integrated system, the Geometric Dilution of Precision (GDOP) is widely used for the selection of satellites and the evaluation of positioning accuracy. The change of GDOP is related with the number of the satellites. In this paper, the impact of the number of satellites on the change of GDOP is theoretically derived. The result shows that, adding satellites from the existing tracked constellations into the integrated system, the GDOP always decreases when the number of satellites increases.