Using DORIS data for validating real-time GNSS ionosphere maps

The ionospheric information retrieved from the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system provides valuable and external data sources to examine the quality of existing Global Navigation Satellite System (GNSS) generated ionospheric models. The concept of DORIS differential Slant Total Electron Content (dSTEC) analysis is presented, to check the feasibility of DORIS data in the quality assessment of GNSS derived ionosphere models. Thanks to the large relative frequency ratio between the two frequencies of DORIS, the theoretical precision of DORIS dSTEC is at the level of 0.028 TECu, which is about 10 times better than that of GNSS derived dSTEC. Using 48 co-located DORIS beacon sites of the International DORIS service (IDS) and GNSS stations of the International GNSS Service (IGS), the comparison between DORIS and GNSS dSTEC assessments of Real-Time Global Ionospheric Maps (RT-GIMs) from different analysis centers is performed in this paper. The analysis is performed during the first 110 days of 2022. Based on the analysis results of more than 18,000,000 Jason-3 DORIS ionospheric observables, no systematical deviation is found between DORIS and RT-GIM derived dSTECs, with a mean bias of 0.14 TECu. Compared to DORIS dSTEC, the root-mean-square (RMS) of those RT-GIMs reaches 4.5–5.4 TECu at low-latitudes, which is 2.2–3.6 TECu in mid- and high-latitude regions. The latitudinal variation of RT-GIM errors is clearly observed in DORIS dSTEC analysis. In addition, the Pearson correlation coefficient between Jason-3 DORIS dSTEC RMS and GPS-plus-GLONASS dSTEC RMS is 0.81, and no significant dependence on GPS-only or GLONASS-only data is found. It is suggested to perform the dSTEC analysis with higher satellite elevation cutoff angle, e.g., 45°, to ensure a better consistency between DORIS and GNSS dSTEC assessments. Overall, DORIS dSTEC assessment provides an independent reference for the validation of GNSS based ionosphere maps.