Abstract：Aiming at optimal construction of pseudolites in GNSS/Pseudolites(PLs) combined system, this paper pointed out the necessity of PLs’ joining in GNSS,and built a fourdimensional(4D) model based on azimuth angel, elevation angel and observed epochs.Then, an experiment was taken to test the model,and the results show that the increasing number of visible satellites and the changing information of lowelevation satellites can enhance the reliability of the system and improve positioning accuracy.Panorama shows that variations of the PDOP value and the reliability value within the optimal region are similar but the distributions are different.The weight constrained by the PDOP, internal reliability, cost and other factors can be effective coordinate indexes for selecting optimal system and provide reliable basis for effectively solving the problem of locating PLs for GNSS/PLs optimization.\
