Global Mapping of Ionospheric ROTI Index and Its Preliminary Application in Analysis of Precise Positioning Degradation

The Rate Of change of total electron content (TEC) Index (ROTI), derived from the GNSS dual-frequency carrier phase combination, has been widely used as a key indicator of ionospheric disturbances. In this paper, we propose an ordinary kriging approach to generating ionospheric ROTI maps with the aim of characterizing ionospheric irregularities and assessing their impacts on precise point positioning (PPP) at a global scale. In the kriging-based mapping of the ROTI index, we exploit three semi-variogram models (i.e., Gaussian, Spherical and Exponential) and evaluate their performance during the September 6–9, 2017 geomagnetic storm by taking advantage of more than 500 GNSS stations. Results suggest that (1) The overall RMSE of kriging cross-validation for all three models is generally less than 0.4 TECu/min, and the spherical semi-variogram performs best by analyzing the semi-variogram fitting and interpolation accuracy. (2) The ROTI maps can effectively represent the spatial-temporal evolution and intensity of global ionospheric irregularities during the geomagnetic storm. (3) There is a strong positive correlation between the PPP errors and ROTI values. The maximum, average and RMS of 3D positioning errors are 0.33 m, 0.14 m and 0.40 m, respectively, with the level of ROTI values less than 0.5 TECu/min, while they can reach 7.06 m, 2.80 m and 3.63 m, respectively, when the ROTI increases above 2.5 TECu/min. The ground positioning risk derived from the ROTI maps can effectively indicate the degradation of positioning performance.