The landing of aircraft under low visibility conditions has always been a challenge even with conventional navigation systems like ILS (Instrument Landing System). The requirements for CAT III can not be reached with Ground Based Augmentation Systems (GBAS) for single frequency GPS only without relaxing the alarm limits and continuity requirements of air navigation. Large delay gradients between the GBAS ground station and the user caused by ionospheric anomalies, remain the main threat for GBAS. Using GBAS with both GPS and Galileo in a combined constellation will increase the robustness of the complete system. Galileo is providing promising features like the possibility offered to the aviation community to acquire 3 frequencies: L1, E5a and E5b in the Aeronautical Radio Navigation Service (ARNS) band. The consideration of phase observations allows the use of efficient smoothing techniques: the ionosphere free and the divergence free dual frequency smoothing algorithms which have been defined in , allow to mitigate or even to cancel the ionosphere gradient. Due to the different geometry characteristic of the extended constellation the Geometry Dilution of Precision (GDOP) is reduced. The low probability of satellite outages combined with the number of additionally available satellites will dramatically improve the availability of the combined GPS and Galileo system. The objective of this work is to analyze the impact of Galileo through the use of a combined constellation on the performance of GBAS under severe ionospheric gradients. The errors experienced by a user with a spatial separation of 5km and 20NM respectively to the GBAS ground station are evaluated. The simulation scenario considers an ionosphere anomaly with a gradient of 420mm per km between the ground station and the user – a value which has turned out to be a worst-case assumption as explained before in several publications . The dual frequency smoothing techniques mentioned above are applied. The simulation is performed over a period of several days to account for the effects of the changing satellite geometry. The models of the GBAS residual errors used in the preceding work  were considered to be Gaussian distributed individual errors. To give a more realistic representation of the individual errors used in the simulation, we use distributions of errors which are, in general not Gaussian. The distributions are derived from measurements or theoretical considerations pertaining to the origin of the error. Here, we consider the four major individual sources of pseudorange error in GBAS systems: receiver noise, ionosphere and troposphere, multipath. For this work, the impact of applying smoothing filter, averaging and position calculation are taken into account.