Impact of LEO configuration on GPS precise orbit determination with un-differenced ambiguity resolution

Un-differenced (UD) ambiguity resolution (AR) is widely applied since it can avoid wrong-fixing and fix the most of ambiguities. However, in the precise orbit determination (POD) solution using ground network only, the accuracy of float estimates is not sufficient for a direct UD AR, and thus double-differenced (DD) AR must be conducted first. It is demonstrated that integrating low-Earth orbit (LEO) satellites improves the accuracy of ambiguity-float solutions and consequently the improved ambiguity estimates enable successful UD AR. With more and more LEO satellites receiving Global Positioning System (GPS) observations, it is important to investigate the effect of different LEO configurations for LEO-augmented GPS UD AR. In this study, about 100 stations of the International GNSS Service (IGS), and eight LEO satellites with five orbital configurations, including two GRACE-FO, three SWARM, two SENTINEL-3, and one SENTINEL-6 satellites, are used for integrated processing of LEO satellites and ground network. The influence of different numbers of LEO satellites and orbital configurations on the performance of UD POD is investigated by analyzing the POD results of the ground network together with 35 different LEO-satellite schemes. We demonstrate that the performance of POD, in terms of GPS orbit accuracy and unsuccessful fixing in the UD AR solution, depends on the accuracy of float estimates. The number of unsuccessful fixings can be reduced by the increasing number of LEO satellites and orbital configurations. Moreover, including LEO satellites from more orbital configurations is more efficient for a given number of LEO satellites.