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In this paper we analyze and discuss the impact of ionospheric scintillations and multipath on the availability of the current single-frequency single-constellation Ground Based Augmentation System (GBAS). Scintillation effects, which usually occur around plasma bubbles, cause the receiver to lose lock of one or more satellites, leading to potentially unfavorable satellite geometries for an airborne user. We simulate different bubble dimensions and distinct locations of the bubble in the sky in order to illustrate that the use of a second constellation improves the performance of the system in terms of availability for the situations where the single constellation system is unavailable. Furthermore, we investigate the impact of multipath in the availability of the system. During the touch-down and roll-out of the aircraft on the runway, multipath coming from ground reflections becomes important, especially for low elevation satellites producing large position errors. The results show that the use of a second constellation allows the removal of low elevation satellites from the position solution, which are typically strongly affected by multipath. Elevation masks of 10° or even 15° do not degrade the availability of the dual-constellation system in any of the scenarios considered.