## 5 Conclusions  
In this study, we derived the double-difference observation model of the GPS/GLOAASS/BDS/Galileo/QZSS five-system ultra-rapid POD based on the one-step method. Additionally, using the framework of Bernese v5.2 and this model and method, we developed a new prototype software platform for five-system ultra-rapid precise orbit calculation. Subsequently, one month of real tracking data collected from 130 globally distributed IGS MGEX stations were used to test the software platform. The results show that this new prototype software platform has the preliminary function of processing five-system ultra-rapid orbits. In terms of the analysis of the orbit discontinuities at day boundaries, the average SDs in 3D were 0.022, 0.031, 0.139, 0.064, 0.028, and 0.465 m for GPS, GLONASS, BDS IGSO, BDS MEO, Galileo, and QZSS satellites, respectively. Besides, based on the one-step method, the double-difference observation model of five systems and the no-difference observation model of five systems have good consistency. Compared to the ultra-rapid orbits of IGGAC, in 3D direction of orbit consistency with the final MGEX orbit products of GFZ, the improved percentages of this new prototype software platform were 78.51%, 77.19%, and 85.82% for GPS, GLONASS, and Galileo satellites, respectively. The consistency in other directions has also improved significantly.  
[PERSON], [PERSON] provided the initial idea for this study; [PERSON], [PERSON], and [PERSON] developed the new prototype software platform; [PERSON], [PERSON], and [PERSON] conceived and designed the experiments; [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] analyzed the experiment results; [PERSON], [PERSON], [PERSON], and [PERSON] wrote the paper.  
This work was supported by the Collaborative Precision Positioning Project funded by the Ministry of Science and Technology of China (No. 2016 YFB0501900) and China Natural Science Funds (No. 41674022, 41574015, 41574033).  
The authors also acknowledge LU JIAXI International team program supported by the K.C. Wong Education Foundation and CAS. The authors would like to thank the IGS MGEX and Chinese iGMAS networks for providing the Multi-GNSS data. The authors would like to thank the CODE for providing the Bernese v5.2. Thanks to [PERSON] and [PERSON] for giving careful answers in developing software platform. Thanks to [PERSON], [PERSON] and [PERSON] for giving careful guidance in modifying this article.  
The authors declare no conflict of interest.
Figure 16: The differences between the six-hour predicted orbits of five-system ultra-rapid orbits and the orbits of GFZ for Galileo.  
Figure 17: The differences between the six-hour predicted orbits of five-system ultra-rapid orbits and the orbits of GFZ for Galileo.  
Figure A5: The differences between the six-hour predicted orbits of five-system ultra-rapid orbits and the orbits of GFZ for QZSS.  
Figure A10. The residuals of the orbit discontinuities at day boundaries for QZSS.  
Figure 11: The differences between the six-hour predicted orbits of IGGAC and the orbits of GFZ for GPS.  
Figure 12: The differences between the six-hour predicted orbits of IGGAC and the orbits of GFZ for GLONASS.