A major threat for Ground Based Augmentation System (GBAS) is ionospheric anomalies caused by spatial or temporal decorrelation. With the introduction of GPS L1/L5 and Galileo E1/E5 signals, GBAS is evolving from single-frequency to dual-frequency implementation. This paper provides simulation analysis to assess the impact of ionospheric anomalies on dual-frequency GBAS with divergence-free smoothing technique. Based on middle-latitude and low-latitude ionospheric threat models, the analysis is designed to confirm that the performance requirements could be met by the combination of ground Wide-Lane Ionospheric Gradient Monitor (WL IGM) and airborne Dual Smoothing Ionosphere Gradient Monitor Algorithm (DSIGMA). The analysis and simulation results demonstrate that the system successfully satisfy the performance requirements under these threat models. Furthermore, the largest differential range error under wedge gradient and plasma bubble threat models with Probability of Missed Detection (PMD) larger than 10?9 are 2.19 m and 2.29 m, respectively. The differential range error reach maximum when the direction from ground facility (GF) to landing threshold point (LTP) is parallel to the wedge gradient or plasma bubble moving direction.