Estimation of Klobuchar Coefficients for Japan and Performance Evaluation Compared with QZSS Coefficients

In this study, Klobuchar coefficients optimized for the Japanese region, referred to as the Klobuchar Regional (KR) model, were estimated using the International GNSS Service (IGS) Global Ionosphere Map (GIM) and evaluated against the Klobuchar GPS (KG) and Klobuchar QZSS (KQ) models. The grid points were defined based on ionospheric pierce points (IPPs) derived from Japanese GNSS stations, and the coefficients were estimated via a nonlinear least-squares approach using the vertical total electron content (VTEC) within the defined region (12.5-52.5° N, 115-170° E) under both solar minimum (2019) and solar maximum (2024) conditions. During the solar minimum, the number of distinct KG correction coefficient values was limited to fewer than 16, and intermodel correlations were found to vary with solar activity level, with the KQ-KR pair exhibiting the highest correlation for α₀ (r = 0.85). Vertical Total Electron Contents (VTEC) root mean square error (RMSE) analysis indicated that KR achieved an approximately 50% improvement in accuracy relative to both KG and KQ. Furthermore, standard point positioning (SPP) results at three Japanese stations demonstrated that KR and KQ exhibited comparable performance within 0.1 m during the solar minimum, whereas KR generally outperformed KQ during the solar maximum, with the standard deviation of KR approximately half that of KQ, confirming superior stability and reliability in positioning performance.