Extensive ionospheric studies were conducted to support the initial phase of system design approval (SDA) for the existing SLS-4000 GBAS installed at Galeão Airport (GIG) in Rio de Janeiro, Brazil. This paper focuses on determining the broadcast value of the standard deviation of vertical ionospheric gradients (or sigma_vig) that is required to bound ionospheric spatial gradients at the Brazilian region under nominal conditions during daytime hours. The days for the analysis were selected by a combination of high levels of daytime or nighttime scintillation and/or severe values of the geomagnetic storm index. The time-step method, which is useful for gaining sufficient samples at distances less than the physical separation distance of ground stations, was utilized to estimate ionospheric spatial gradients. The results show that the bounding sigma_vig values are in the range of 8-13 mm/km, when daytime was defined to exist between 6 AM and 6 PM local time. Because the time-step method estimates gradients over a time interval instead of at a single time, the results include temporal as well as spatial gradients, meaning that they overestimate the spatial gradients that are the desired results. Thus, a new method called “geometric modeling” was proposed to estimate ionospheric temporal gradients and evaluate the temporal effect added to the bounding sigma_vig values. As a result, a sigma_vig of 13 mm/km including an approximately 2 mm/km temporal gradient contribution, is conservative enough to bound ionospheric spatial decorrelation for daytime GBAS operations in the Brazilian region.