Combination of IGS Real-Time Global Ionospheric Maps at CAS: Method and Analysis

Benefiting from the global multi-frequency and multi-constellation GNSS measurements provided by the International GNSS Real-Time Service (IGS-RTS), real-time global ionospheric maps (RT-GIMs) have been generated by the Chinese Academy of Science (CAS), Centre National d’Etudes Spatiales (CNES) and Universitat Politècnica de Catalunya (UPC) since 2017, and late by Wuhan University (WHU) since late 2020. To provide a more stable real-time ionospheric correction stream for precise GNSS application and ionospheric space weather monitoring, an experimental combined RT-GIM has been generated at CAS since late-2021 using real-time streams from CNES, UPC, WHU, and CAS itself. CAS combined RT-GIM streams are transmitted in both RTCM-SSR (IONO01IGS0) and IGS-SSR (IONO01IGS1) standards, which are freely accessible from IGS (products.igs-ip.net:2101) and CAS (cas-ip.gipp.org.cn:2101) casters. Following the motivation for the RT-GIM combination, the method used to generate the CAS combined RT-GIM is described in detail. The performance of CAS combined RT-GIM is validated in both RT-GPS dSTEC and single-frequency precise point positioning (SF-PPP) domains by comparison with the existing combined RT-GIM provided by UPC. CAS combined RT-GIM is presently validated and analyzed for a short period (2-3 months), and the analysis covering different time spans (e.g., different seasons) is being done together with the RT-GIM combination.