The ground-based augmentation system must make provisions to being sufficiently robustness to ionospheric anomalies through the development of an ionospheric anomaly threat model. For developing the threat model in Brazil, earlier work found that ionospheric spatial decorrelations larger than those in the midlatitude regions were frequently observed during the peak of Solar Cycle #24 (current cycle). We provide details of a study of the extreme ionospheric spatial decorrelation observed over Brazil during the March 1, 2014, equatorial plasma bubble (EPB) event. As viewed by two Brazilian GNSS reference stations in Sao Jose dos Campos, PRN 03 descended to an elevation angle of about 19 degrees in the northern sky. A spatial decorrelation of 850.7 mm/km at the GPS L1 signal at 01:04:00 UT between the two stations SJCU (23.21 degrees S, 45.96 degrees W) and SSJC (23.20 degrees S, 45.86 degrees W) over a baseline of 9.72 km was discovered, when the line of sight of PRN 03 passed through the transition zone of the EPB. Since the EPB-induced ionospheric scintillation can corrupt the ionospheric gradient estimates, multiple gradient observations were made from multiple stations and satellites to verify the largest gradient observation. Severe gradients discovered at other station-satellite pairs support that the event of PRN 03 is a real anomaly as opposed to a receiver fault or the result of post-processing errors. Since the availability loss was estimated to be 41.7% with the Brazilian threat model, remedies to reduce over-estimated ionospheric impact when evaluating and mitigating ionospheric integrity risk are presented.