RT‐FAIRS: Real‐Time Factor‐Adjusted Ionospheric Residual Statistics Approach for Generating Accuracy Information in Global Ionospheric Maps

Abstract Ionospheric delays remain a major error source in Global Navigation Satellite Systems (GNSS). The International GNSS Service (IGS) provides Global Ionospheric Maps (GIMs) that offer vertical total electron content (VTEC) estimates together with RMS maps as accuracy information. While such indicators are routinely available in post‐processed GIMs, real‐time GIMs (RT‐GIMs) still lack a corresponding RMS description of the correction errors. This information is essential for robust and integrity‐driven GNSS positioning in the IGS real‐time service community. We introduce the Real‐Time Factor‐Adjusted Ionospheric Residual Statistics (RT‐FAIRS) approach, which exploits residuals from contributing real‐time GNSS stations and applies adjustment factors that account for the non‐Gaussian characteristics of their error distributions. The method particularly improves estimates in data‐sparse regions, providing conservative but adaptive error bounds. RT‐FAIRS has been operationally implemented at the Chinese Academy of Sciences (CAS) for the routine generation and dissemination of real‐time accuracy information in CAS RT‐GIM products. Validation with differential slant TEC (dSTEC) residuals from independent GNSS stations shows that RMS maps in legacy CAS RT‐GIMs often underestimate GIM errors, whereas RT‐FAIRS yields statistically consistent bounds and substantially stronger correlations with observed variations. Comparisons with post‐processed GIMs further demonstrate that RT‐FAIRS ensures both reliability and responsiveness of the generated real‐time accuracy information. The approach provides a statistically consistent framework for RT‐GIM accuracy information assessment and supports the integrity analysis in real‐time GNSS applications.