## 3 Results and Discussion
### Case-Study of Geomagnetic Storm on 25-26 August 2018
The maps also show the position of GOES-16 (large black dot), the GOES RO tangent point projections (magenta dots), and the line-of-sight link in the direction between the GEOS GPS receiver and GPS transmitter (black line). existence over sub-auroral latitudes in Northern America during these periods, and the GOES RO tangent points were located in close proximity to areas affected by ionospheric irregularities, as specified by the ground-based GNSS ROTI. Later, into the storm recovery phase, two GOES RO events occurred at \(\sim\)16:46 UT and 19:27 UT in the Atlantic sector in the Southern Hemisphere and another one at 23:34 UT in Northern Europe (Figure 11). For the first two cases, the signatures of the ionospheric irregularities on the GOES-GPS link were detected in the topside ionosphere within the altitudinal range of tangent points of 800-1000 km and 700-1000 km, respectively. The GNSS ROTI maps corresponding to these periods of time depicted the presence of ionospheric irregularities in the auroral zone in the general line-of-sight direction from the GOES to GPS and relatively close to the locations of the tangent point projection of these occultations. Figure 9: The same as Figure 8 but for (**a**–**c**) several occultation events for the conditions after the storm onset on 25 August 2018. The last RO case at 23:34 UT (Figure 11c) shows minimal TEC fluctuations along the GOES-GPS link, with slight intensification of the ROT near \(\sim\)120 km altitude (ionospheric E layer) and in the topside ionosphere within a 700-1000 km interval, but here the ROT values did not surpass 0.1 TECU/min. The ground-based ROTI map corresponding to this temporal interval demonstrates that extremely weak ionospheric irregularities persisted at the high latitudes. During this time, the geomagnetic indicators (Bz, Ae, and SYM-H on Figure 6) did not exhibit any significant disturbances and the ionospheric state was transitioning back to normal conditions. The presented GOES RO results emphasize the known and important problem of geolocating ionospheric irregularities in remote sensing. The ionospheric plasma irregularities can be reliably detected along the line of sight between the GNSS transmitter and the receiver. But determining the precise location of ionospheric irregularities along this direction is still a challenging task. For the traditional LEO RO geometry, the simplest and widely used approach is to assign the location of the detected ionospheric irregularities to the tangent point (the point along the line closest to the surface), e.g., [71; 72]. For several cases that we examined for GOES RO (e.g., Figure 10a), this assumption does not work well  
Figure 10: (**a–c**) The same as Figure 8 but for 26 August 2018 (part 1). when the irregularities assigned to the tangent point location were found to be rather distant from the area of the irregularities detected from the ground-based observations. There exist several approaches for more accurate localization of the ionospheric irregularities along the line-of-sight GNSS transmitter-receiver, which are based on the phase screen [73] or back propagation method [59; 74]. But all these methods require high-rate 50-100 Hz sampling of the RO signal to capture the spectrum of scintillation. The GPS receivers currently used onboard the GOES satellites were not designed for high-rate RO signal tracking, and GOES-based GPS observations do not allow us to apply these approaches for accurate localization of the ionospheric irregularities. However, incorporating this option to the future versions of the GOES payload will open up new possibilities not only to detect but also accurately localize ionospheric plasma irregularities on links between the geostationary GOES GPS receiver and GPS transmitter on the opposite side of the Earth. Figure 11: **(a–c)** The same as Figure 8 but for 26 August 2018 (part 2).