Improving GNSS data analysis with undifferenced integer ambiguity resolution

Along with other space geodetics, the Global Navigation Satellite Systems (GNSS) technique has a profound impact on the realization of terrestrial reference frames and the provision of geodetic and geophysical parameters such as Earth Rotation Parameters (ERP) and geocenter (GCC) coordinates. In GNSS data processing ambiguity resolution (AR) is critical to achieve high-precision estimates. Due to the uncalibrated phase delay (UPD), currently most International GNSS Service (IGS) Analysis Centers (ACs) adopt the double-differenced (DD) ambiguity resolution strategy, which eliminates UPD by between-satellite and between-station differencing. Undifferenced (UD) ambiguity resolution, which recovers the integer characteristics of the UD ambiguities by directly estimating the UPD, was originally developed for Precise Point Positioning and nowadays is also applied to network processing, for example for Precise Orbit Determination (POD). In this study, we conducted a comparative study on the UD and DD ambiguity resolution for GNSS POD data processing. We demonstrated that the UD strategy significantly improves the solution compared to the DD solutions. The GPS orbit precision is improved by 20%, 12%, and 5% in the along, cross, and radial component, respectively. Moreover, the ERP are significantly improved, and the agreement with the IGS final products is improved by 28% and 23% for the offsets of x-pole and y-pole, respectively, and by 23%, and 17% for the corresponding rates. Additionally, the repeatability of station coordinates and GCC coordinates is also slightly improved. More important, we confirmed that the two strategies are theoretically equivalent and the outperformance of UD strategy could be explained by its strong robustness to wrong-fixings and its capability to fix the most UD ambiguities, while wrong-fixings and missing fixings are hardly avoidable in the DD strategy due to the requirement of forming DD-ambiguities.