The article presents a methodology for determining the positioning accuracy parameter of GPS with EGNOS and SDCM corrections in air navigation. Namely, the aircraft positioning accuracy parameter was determined as part of a joint GPS/EGNOS+GPS/SDCM solution for two GNSS satellite receivers. The calculation scheme itself was divided into two stages, i.e., first, the accuracy was estimated based on raw GPS/EGNOS+GPS/SDCM data for both GNSS receivers, and then a Kalman filter was applied to improve the determined position of the aircraft, and the accuracy was determined again. The article uses research material collected during a airplane flight around EPOD airport. Algorithms for the research methodology were written in script in language of Scilab v.6.1.1. During the research, accuracy term was better than 2.94 m. After Kalman filtering, position errors were reduced to 14% for horizontal components and 20% for ellipsoidal height. This allowed for a reduction in position errors below 2.35 m. The paper also calculated other statistical errors such as: MAE, RMS or standard deviation. In this paper, the characteristics of SBAS corrections were taken into account in terms of determining the accuracy parameter. In particular, the results of SBAS corrections such as: long-term and fast corrections, ionosphere and troposphere delay. During the research, in addition to accuracy, the availability, continuity, and integrity of GPS/EGNOS+GPS/SDCM positioning in aviation were also calculated. The presented research method can be used in aviation to increase the accuracy term using SBAS support systems.