Tumuhairwe Bruce2026-04-172026-04-172025Tumuhairwe Bruce. (2025). Analysis of the impact of geomagnetic storm onset time on GNSS signal reliability [Dissertation]. Busitema University.https://bdears.busitema.ac.ug/handle/123456789/1434This study investigates the effects of geomagnetic storm onset time on the performance of Global Navigation Satellite System (GNSS). Unlike previous studies focusing primarily on storm intensity, this work establishes storm onset time as a key predictive parame- ter for ionospheric scintillation occurrence. The analysis uses geomagnetic storm data and Global Positioning System (GPS) scintillation data from two low-latitude stations, Makerere and Mbarara, spanning 2011 to 2016. A total of 1,082 geomagnetic storms were identified and classified by their intensity into weak (−50 nT ≤ Dst < −25 nT), moderate (−100 < Dst ≤ −50 nT), and severe (Dst ≤ −100 nT) categories. Only a minority of storms triggered ionospheric scintillation, indicating that storm intensity alone is insufficient to predict scintillation events. Scintillation was identified using the amplitude index (S4), with thresholds set between 0.2 and 1.2. Using the epoch of minimum IMF Bz (bow shock time) as the precise storm onset marker, results show that scintillation predominantly occurred during night hours. Strong scintillation was observed consistently when the bow shock time fell between 15:00 UT and 00:00 UT (evening to nighttime in East Africa’s low-latitude region), with weaker events between 12:00 UT and 15:00 UT. No scintillation was detected when storm onset occurred from 00:00 UT to 12:00 UT, even during severe storms. These findings highlight the critical role of storm commencement timing in ionospheric irregularity development and subsequent GNSS signal degradation, which is crucial for mitigating risks in aviation, communication, and navigation systems in low-latitude regionsen-USThesis