## 4 Conclusions  
1. We designed and developed the occultation equipment that supports the GPS/BD systems, and tested its performance based on mountain-based experiments. It is concluded that B1/BD observation data can obtain atmospheric retrieval accuracy similar to L1C/GPS observation. Since the 2 nd and 3 rd generations of the BD constellations have more than 50 satellites in orbit, the equipment will be deployed on low-orbit satellites in the future to obtain more occultation observations.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No** & **GPS PRN** & **Time** & **Ascending/Descending Occultation** & **BIAS (\%)** & **MSER (\%)** \\ \hline
1. & 30 & 2020/9/01 & Down & 0.10 & 0.19 \\
2. & 5 & 2020/9/01 & Up & 0.29 & 2.36 \\
3. & 17 & 2020/9/02 & Down & 0.24 & 1.92 \\
4. & 5 & 2020/9/02 & Up & 0.22 & 1.32 \\
5. & 28 & 2020/9/03 & Down & 0.23 & 2.14 \\
6. & 13 & 2020/9/03 & Up & 0.30 & 4.01 \\
7. & 13 & 2020/9/04 & Up & 0.23 & 3.06 \\
8. & 17 & 2020/9/04 & Down & 0.21 & 1.22 \\
9. & 24 & 2020/9/06 & Up & 0.25 & 2.35 \\
10. & 29 & 2020/9/06 & Down & 0.10 & 0.10 \\
11. & 18 & 2020/9/06 & Down & 0.20 & 1.37 \\
12. & 17 & 2020/9/07 & Down & 0.11 & 1.02 \\
13. & 19 & 2020/9/07 & Down & 0.27 & 2.94 \\
14. & 24 & 2020/9/07 & Up & 0.21 & 1.82 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The relative refractive index deviation standard deviation statics of each atmosphere profile.  
2. Traditional OL processing relies on external data (GPS bit) to determine the initial sign of the RP. We use internal processing to recover the phase of the full OL tracking observation data for mountain-based occultation inversion. Due to the lack of external data, it is difficult to determine the initial RP sign. We use the correlation between the mountain-based closed-loop observation data of elevation angle above 3 degrees and the RP data to determine the initial sign of RP, and accomplish the full OL data recovery process based on closed-loop observations automatically.  
3. To determine the full OL tracking and capturing effect, we have performed statistics on the data for 7 consecutive days. The statistical results showed that in the case of counting all angles, the OL improves the capture angle value by nearly 20%, and when only the negative angle is calculated, the OL improves the capture angle value by nearly 89%. The full-open-loop capture mode greatly improves the capture effect, and there is no specific coupling relationship between OL tracking and closed-loop tracking, but the overall effect is better than closed-loop. The reason why there is no specific coupling relationship is due to the difference between the two capture methods.  
4. The atmosphere profiles retrieved from observations in OL tracking mode are evaluated with the local observations of temperature, humidity and pressure provided by the Beijing Meteorological Bureau. It can be concluded that the error of the OL tracking method is within \(\sim\)4% in the mean square error of relative error (MSER), which meets the accuracy of its applications (\(<\)5%, in MSER).  
Conceptualization, F.L., L.K., N.F., M.W., C.H. and Z.W.; methodology, F.L., L.K. and N.F.; formal analysis, L.K.; investigation, L.K.; data curation, F.L.; writing--original draft preparation, F.L.; writing--review and editing, F.L. and L.K.; data analysis and discrimination, F.L., N.F. and L.K.; supervision, F.L.; project administration, L.K.; and funding acquisition, F.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported in part by the National Natural Science Foundation of China under Grant Nos. 61520106002 and 61731003.  
The validation data was provided by Beijing Meteorological Administration, and the experiments team of Tianjin Yunyao Aerospace Technology Co., Ltd. We also thank reviewers for their suggestions on improvement in the article.  
The authors declare no conflict of interest.