oa Second-Order Effect of the Ionosphere During Geomagnetic Storms

One of the most complex atmospheric effects is the response of ionospheric regions to geomagnetic storms. The ionospheric response during the same storm can vary in time in different locations, which can introduce significant errors/displacement (meter-level) in single-frequency relative GNSS positioning (DGNSS technology) due to the inability of the differential process to eliminate the effect of the ionospheric delay between the reference station and the user. The residual effect can be somewhat mitigated by using dual- or multi-frequency GNSS (RTK/RTN technology), but dual frequency is not a guarantee against degradation of RTK results, especially during significant geomagnetic storms. In this regard, PPP (absolute positioning) technology can be effective. However, another atmospheric effect - ionospheric scintillation (rapid fluctuation of the amplitude and phase of GNSS signals) can have a significant impact on the accuracy of both GNSS positioning approaches. The main goal of this study was to analyze the effect of second-order ionospheric delay during geomagnetic storms and ionospheric scintillations on GNSS positioning using the PPP method. GNSS data corrected and uncorrected for higher-order ionospheric delay, respectively, were processed by the static PPP-AR method using the PRIDE-PPPAR ver.2.2.6 software for the selected periods of geomagnetic storms. Two approaches were used to estimate the second-order ionospheric delay correction: STEC derived from the CODE measurements at L1 and L2 and TPP derived from the global ionospheric maps (GIM) from CODE were used together with the IGRF geomagnetic field model (IGRF12). The sloped ionospheric delay from the PPP decoupling achieves slightly higher accuracy than the values modeled by the GIM, but for the second order this was not significant. From the analysis of the influence of second-order ionospheric errors, it follows that their values can reach almost 4 cm for first-frequency signals under different states of ionospheric disturbances for the GPS constellation and almost an order of magnitude less for the GNSS quadroconstellation. The appearance of stronger geomagnetic storms increases the second-order ionospheric errors by several millimeters.