Using Near-Real-Time DORIS Data for Validating Real-Time GNSS Ionospheric Maps

The high-quality dual-frequency phase measurements from Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system provide a valuable opportunity to examine the Earth’s ionosphere. In this paper, the ionospheric information retrieved from the DORIS system is used as an external and independent data source to validate the performance of GNSS-generated ionospheric models. The concept of DORIS differential Slant Total Electron Content (dSTEC) analysis is presented, to examine 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 four months 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 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., 45o, 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.

 

This study only uses co-located DORIS/GNSS stations in order to ensure a fair comparison between DORIS and GNSS dSTEC validation. However, for future validation activities, all DORIS beacons can and should be used. This will lead to a more homogeneous data distribution covering also more ocean areas. Moreover, the DORIS NRT measurements are fully independent of the data used for RT-GIM generation, and as such is a perfect source for validation. In addition to Jason-3 altimetry, more satellite missions providing NRT DORIS data are planned, which will indeed extend the coverage of DORIS ionospheric observables and benefit the ionospheric associated analysis. In the next step, we will continue the work on the analysis of dSTEC consistency between DORIS and those new GNSS constellations, e.g., BDS and Galileo. The feasibility of using DORIS data in the generation of real-time global ionospheric maps will also be analyzed.