The trans-ionospheric propagation of Global Navigation Satellite System (GNSS) signals is significantly influenced by ionospheric irregularities, particularly at equatorial and low-latitude regions. This study investigates the effect of the Fresnel First Zone a fundamental concept in wave diffraction on GNSS signal scintillation. The Fresnel scale defines the spatial dimensions of ionospheric irregularities that most strongly scatter GNSS signals, causing amplitude and phase scintillations that degrade signal quality. Using theoretical models and observational data, including S4 index measurements, the research analyzes how variations in electron density and irregularity drift affect signal propagation. The findings underscore the importance of the Fresnel First Zone in understanding diffractive scattering processes and provide insight into the mitigation of GNSS signal disruptions, especially in space weather-sensitive regions. This work contributes to enhancing GNSS reliability for communication and navigation applications.