DORIS NRT data: an independent data source for GNSS-based ionospheric maps validation and combination

Information on the distribution of free electrons in the Earth's ionosphere is needed for many applications, including the mitigation of single-frequency range delay errors and the monitoring of space weather. Most of the existing ionospheric models are generated using ground-based GNSS measurements, e.g., the Global Ionospheric Maps (GIM) provided by the International GNSS Service (IGS). However, the quality assessment of GIM is presently still an open question. In addition to altimetry Vertical Total Electron Content (VTEC) information over the oceanic regions, limited external data sources are available today to perform a fully independent validation of GNSS-based ionospheric models. The high-quality dual-frequency phase measurements of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system provide valuable opportunities to examine the Earth’s ionosphere. In this work, we analyzed the feasibility of using DORIS data to estimate the accuracy of GNSS-generated ionospheric models. To this end, the concept of DORIS differential Slant Total Electron Content (dSTEC) assessment is proposed. Using Jason-3 Near-Real-Time (NRT) DORIS data of the International DORIS Service (IDS), the accuracy of different Real-Time Global Ionospheric Maps (RT-GIM) as well as the IGS combined one is evaluated. The consistency between DORIS and GNSS dSTEC assessments in the quality analysis of RT-GIMs is also checked, and the overall Pearson correlation coefficient reaches 0.81 during the one-year test period. The DORIS dSTEC assessment can be used not only to estimate the accuracy of individual GIMs, but also to determine their weighting within a combination strategy. The performance of DORIS-dSTEC and GNSS-dSTEC combined GIMs is assessed by comparison to Jason-3 VTEC from the mission altimeter. The standard deviations are 4.71 TECu and 4.80 TECu for DORIS-dSTEC and GNSS-dSTEC combined GIMs, indicating the slightly better performance of DORIS-dSTEC combined RT-GIM in Jason-3 VTEC assessment. It was shown that NRT DORIS data can be used to independently validate and combine GNSS-derived ionospheric maps. In the future, it is also envisaged that DORIS data can be directly incorporated into ionosphere modeling. To this end, the provision of NRT data from other DORIS missions is planned (e.g., Sentinel-3).