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Multi GNSS system increases the GNSS positioning accuracy, efficiently improve the satellite geometry strength and further enhances precise point positioning (PPP) performance. In this study, positioning coordinate accuracy of GPS, GLONASS, Galileo and BeiDou dual frequency observations is estimated and comparatively analysed. Ten days of dataset from nine International GNSS service sites are adopted for eight different GNSS PPP scenarios. Position in east, north and up components and convergence speed test for single system GPS, GLONASS, BeiDou and Galileo, dual system GPS/GLONASS and Galileo/BeiDou, combined triple system GPS/GLONASS/BeiDou and GPS/GLONASS/Galileo is investigated. Results demonstrate that PPP solutions of GPS show an improvement in east, north and up components over GLONASS, BeiDou and Galileo PPP solutions. GPS PPP solutions reach to 2.88, 2.32 and 6.10 cm in east north and up components, respectively. Difference of standard deviation (STD) values between GPS and GLONASS PPP results is 4, 3 and 2 cm, in east, north and up direction, respectively. Moreover, difference of STD between GPS and Galileo PPP is >1cm in all three components. Furthermore, BeiDou only PPP results reach to 15, 10 and 20 cm in east, north and up direction in Asia Pacific, respectively. Horizontal component for combine Galileo/BeiDou PPP and GPS/GLONASS PPP solutions reach to 3.24 and 3.02 cm, respectively. Calculation results of 3D positioning show that combined GPS/GLONASS/BeiDou PPP solutions improve by 5.59 % and 17.72 % over GPS/GLONASS and Galileo/BeiDou PPP solutions, respectively. Furthermore, STD for 3D positioning of triple system GPS/GLONASS/Galileo shows an improvement of 47.53 %, 31.56 % and 24.90 % over Galileo/BeiDou, GPS/GLONASS and GPS/GLONASS/BeiDou PPP results, respectively. Two different convergence time tests are undertaken. Results of GPS-only PPP solutions show fastest convergence speed to achieve accuracy level of 1.0 cm over GLONASS-only, BeiDou-only, Galileo-only, and Galileo/BeiDou PPP solutions. Combine dual system GPS/GLONASS PPP convergence time show an improvement of 56.46 % over GPS-only solutions. The contribution of BeiDou to reducing the convergence time of the combine GPS/GLONASS PPP improve by 27.53 % over combine GPS/GLONASS PPP convergence time. Moreover, GPS/GLONASS/Galileo PPP convergence speed show an improvement of 20.06 % over convergent sessions of GPS/GLONASS/BeiDou PPP. Furthermore, to achieve accuracy level of 5.0 cm, combine three system GPS/GLONASS/BeiDou PPP reduces the convergence time than the GPS/GLONASS/Galileo convergent sessions length. ARTICLE INFO

Analysis of position coordinate accuracy of triple GNSS system by post-processing dual frequency observations using open source GAMP: A case study