Undifferenced Integer Ambiguity Resolution in GNSS Network Solutions: Benefits to Satellite Orbits, ERP, Geocenter, and Station Coordinates

Abstract Integer ambiguity resolution (IAR) is critical in high‐precision Global Navigation Satellite Systems (GNSS) data processing. While the legacy double‐differenced (DD) IAR is widely adopted by most analysis centers (ACs) of the International GNSS Service, undifferenced (UD) IAR is gaining popularity for global network solutions. Theoretically, DD‐IAR and UD‐IAR should achieve equivalent results. Practically, however, several studies show that the latter preserves better results, which is not clearly explained yet. In this study, we investigate the performance of serval commonly used DD‐IAR and UD‐IAR strategies across key GNSS products, including satellite orbits, station coordinates, earth rotation parameters (ERP), and geocenter coordinates. The results show that UD‐IAR consistently outperforms all conventional DD‐IAR strategies in both orbit and geodetic parameter estimation. Through detailed comparisons, we identify two primary effects that degrade the performance of DD‐IAR: incorrectly fixed ambiguities and insufficient independent fixable ambiguities. Compared to the UD‐IAR solution, about 40% of the differences in satellite orbits from DD‐IAR are linked to incorrectly fixed ambiguities, whereas approximately 45%–71% of the differences in geodetic parameters are associated with insufficient independent fixable ambiguities. When both issues are addressed, DD‐IAR and UD‐IAR yield comparable performance, confirming their theoretical equivalence. However, it is not practical to implement the perfect DD‐IAR free of the two issues mentioned above, particularly in large‐scale networks. In contrast, the UD‐IAR is more robust and efficient, making it is highly suitable for handle thousands of stations. As such, UD IAR is recommended for high‐precision GNSS applications.