Performance of BDGIM, Klobuchar, NTCM-G and NeQuick-G models during 25TH solar cycle

The ionospheric delay in Global Navigation Satellite System (GNSS) constitutes the majority of error sources in navigation and positioning and affects the accuracy for single-frequency (SF) users. Unlike dual-frequency users who reduce ionospheric delay by using the ionospheric elimination combination, SF users need to use external data sources such as the global broadcast ionospheric model to reduce the ionospheric delay. Moreover, with the arrival of 25th solar cycle, the increasing of solar activity level is further affecting the performance of ionospheric correction. To conduct a comprehensive evaluation of ionospheric models during the 25th solar cycle, we assessed four models: three operational models recommended by interface control documents (BDGIM, GPS Klobuchar, and NeQuick-G), as well as the NTCM-G model, which is based on Galileo broadcast coefficients. Our analysis utilized datasets from both continental and oceanic regions over the period from January 2020 to December 2023. For continental regions, the GNSS TEC data obtained from 33 individual monitoring stations are used as a reference data source. For oceanic regions, the TEC data obtained from JASON-3 altimeters are selected as the reference value. According to the comparison between JASON-3 TEC dataset and TEC dataset obtained from the four tested ionospheric models, the assessment demonstrates that the performances of tested models are strongly affected by solar activity. Specifically, BDGIM, Klobuchar, NTCM-G and NeQuick-G exhibit the RMS values of 4.36, 5.67, 5.06 and 5.84 TECU in low solar activity in 2020, and 9.99, 13.44, 10.64 and 9.50 TECU in high solar condition in 2023, respectively. In addition, as the validation against International GNSS Service (IGS) final Global Ionospheric Maps (GIMs) products shows, the average global RMS values of BDGIM, Klobuchar, NTCM-G and NeQuick-G are 5.23, 8.43, 4.95 and 6.22 TECU, respectively. BDGIM performs the best in low latitudes, NeQuick-G shows the highest accuracy in the northern mid to high latitude regions, NTCM-G exhibits the best performance in southern mid and high latitudes, and Klobuchar performs the worst across all latitudes.