## 4 Conclusions  
In this paper, we present the promising results of ionospheric irregularity detection using new data from GPS receivers operating on the geostationary orbit GOES-16. The GPS receivers onboard GOES satellites can track GPS signals propagated through the Earth's ionosphere and can be used to analyze rapid variations in the TEC on these links, which can be caused by ionospheric plasma gradients and ionospheric irregularities. Using the novel geostationary GOES GPS observations, we examined the signatures of the ionospheric irregularities' occurrence for two major geomagnetic storms that occurred in September 2017 and August 2018. The presence of ionospheric plasma density irregularities in the vicinity of the GOES GPS RO sounding field of view during the main phases of both geomagnetic storms and their absence during the quiet pre-storm conditions were confirmed by the ROTI maps constructed with multi-station ground-based GNSS observations. The ROTI maps depicted the generation and evolution of the storm-induced ionospheric irregularities and their occurrence near the GOES GPS RO sounding tangent points in the examined events. The actual GPS receivers operated on GOES satellites were not designed for RO ionospheric observations and have limitations in the numbers of tracking channels and data outputs available. But even such limited observations from \"signals of opportunity\" allowed us to detect signatures of the ionospheric irregularities by applying techniques proven for irregularity detection tasks with the ground-based GNSS observations. Taking into consideration the unique geometry of the GEO-GPS RO links from the geostationary orbit, with a repeatable pattern of ionospheric sounding from day to day, this type of observation can provide advantages for continuous irregularity monitoring over specific areas where traditional facilities cannot operate. There is great potential of RO observations from geostationary orbit to provide unique spatio-temporal coverage for ionospheric irregularity monitoring that can be realized in the future in upcoming geostationary satellites with new generations of receivers able to track different GNSS constellations. In combination with other Space Environment payloads, these new RO observations will allow for extending the GOES capabilities for ionosphere and space weather monitoring. Conceptualization and methodology, I.C., I.Z., S.G. and D.H.; software, I.C., I.Z. and D.H; visualization and investigation, I.C. and I.Z. ; validation and analysis, I.C., I.Z. and D.H.; writing--original draft preparation, I.C., I.Z., S.G. and D.H.; writing--review and editing, I.C. ; funding acquisition, S.G. and I.C. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Aeronautics and Space Administration (NASA) Research Opportunities in Space and Earth Science (ROSES) 2019 under Grant 80 NSSC20K1733. The GNSS data collection, generation, and analysis of multi-station ground-based GNSS ROTI maps was funded by the National Science Centre, Poland, grant No. 2022/47/B/ST10/01766. Not applicable. The geophysical data are available at NASA/GSFC's Space Physics Data Facility's OMNIWeb service for geophysical parameters data ([[https://omniweb.gsfc.nasa.gov/ow_min.html](https://omniweb.gsfc.nasa.gov/ow_min.html)]([https://omniweb.gsfc.nasa.gov/ow_min.html](https://omniweb.gsfc.nasa.gov/ow_min.html))).