Status of UnDifferenced and Uncombined GNSS Data Processing Activities in China

With the continued development of multiple Global Navigation Satellite Systems (GNSS) and the emergence of various frequencies, UnDifferenced and UnCombined (UDUC) data processing has become an increasingly attractive option. In this contribution, we provide an overview of the current status of UDUC GNSS data processing activities in China. These activities encompass the formulation of Precise Point Positioning (PPP) models and PPP-Real-Time Kinematic (PPP-RTK) models for processing single-station and multi-station GNSS data, respectively. Regarding single-station data processing, we discuss the advancements in PPP models, particularly the extension from a single system to multiple systems, and from dual frequencies to single and multiple frequencies. Additionally, we introduce the modified PPP model, which accounts for the time variation of receiver code biases, a departure from the conventional PPP model that typically assumes these biases to be time-constant. In the realm of multi-station PPP-RTK data processing, we introduce the ionosphere-weighted PPP-RTK model, which enhances the model strength by considering the spatial correlation of ionospheric delays. We also review the phase-only PPP-RTK model, designed to mitigate the impact of unmodelled code-related errors. Furthermore, we explore GLONASS PPP-RTK, achieved through the application of the integer-estimable model. For large-scale network data processing, we introduce the all-in-view PPP-RTK model, which alleviates the strict common-view requirement at all receivers. Moreover, we present the decentralized PPP-RTK data processing strategy, designed to improve computational efficiency. Overall, this work highlights the various advancements in UDUC GNSS data processing, providing insights into the state-of-the-art techniques employed in China to achieve precise GNSS applications.