# Research on the Impact of BDS-2/3 Receiver ISB on LEO Satellite POD
## 5 Conclusions  
BDS-3 provides a new option for LEO POD and promotes the application of the multi-GNSS. It is unavoidable to estimate the GNSS receiver ISB in multi-GNSS applications. This paper preliminarily researched the impacts of GNSS receiver ISB for LEO POD in detail through the POD test with Tianping-1B onboard GPS and BDS data.  
This paper discussed the reason for the ISB between BDS-2 and BDS-3 and gave the measurement equations. Using GFZ multi-GNSS rapid products, the ISBs were estimated in the Tianping-1B POD based on the BDS-2, BDS-3 and GPS measurements. Using only onboard BDS-3 measurements, the orbit accuracy was 6.57 cm in the 3D direction, improved by 56% from the BDS-2-based POD. When the ISBs of BDS-2 and BDS-3 were estimated in the combined POD, the orbit accuracy of 5.37 cm was 18% better than the BDS-3-based POD, which was nearly two times larger than the improvement ratio of the BDS-2/3-based POD with no ISB.  
For the GPS and BDS combined POD, the orbit accuracy of GPS+BDS-3-based POD was higher. The 3D orbit RMS difference between the GPS+BDS-3-based POD solution and GPS-based POD solution was less than 3 cm, and its STD value was the smallest. These results illustrated that BDS-3 is more appropriate to join the GPS to determinate the LEO satellite orbit.  
In conclusion, the ISB estimated between BDS-2 and BDS-3 could improve the Tianping-1B orbit accuracy using the onboard BDS-2/3 measurements. GPS and BDS-3 combined POD could obtain a more consistent orbit with a GPS-based orbit. It is meaningful to estimate the ISB to improve the Tianping-1B satellite orbit accuracy when using BDS-2 and BDS-3 measurements together. At the same time, the POD experiments with GPS and BDS provide an important reference for LEO POD with onboard multi-GNSS measurements.  
Conceptualization, X.Z. and X.H.; Formal analysis, X.Z.; Funding acquisition, S.Z.; Methodology, S.Z. and X.Z.; Resources, X.H.; Software, J.C. and Z.W.; Validation, S.Z.; Writing--original draft, X.Z. and S.Z. and Writing--review and editing, X.Z. and J.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (No. 12173072) and Qian Xuesen Youth Innovation Fund of China Aerospace Science and Technology Corporation In 2021.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Test**} & \multirow{2}{*}{**Statistic**} & \multicolumn{4}{c}{**Overlapping**} & \multicolumn{4}{c}{**Compared with GPS-Based Orbit**} \\ \cline{3-10}  & & **3D** & **Radial** & **Along** & **Cross** & **3D** & **Radial** & **Along** & **Cross** \\ \hline \multirow{2}{*}{**GPS+BDS-based POD**} & MEAN & 3.15 & 0.91 & 2.77 & 1.09 & 3.20 & 0.99 & 2.41 & 1.59 \\  & STD & 1.25 & 0.27 & 1.09 & 0.70 & 1.13 & 0.47 & 1.19 & 0.81 \\ \multirow{2}{*}{**GPS+BDS-2-based POD**} & MEAN & 2.91 & 0.88 & 2.51 & 1.07 & 3.07 & 0.95 & 2.06 & 1.85 \\  & STD & 1.17 & 0.30 & 1.04 & 0.66 & 1.21 & 0.34 & 1.01 & 1.12 \\ \multirow{2}{*}{**GPS+BDS-3-based POD**} & MEAN & 3.03 & 0.91 & 2.66 & 1.05 & 2.57 & 0.92 & 1.98 & 1.24 \\  & STD & 0.68 & 0.22 & 0.69 & 0.34 & 0.95 & 0.34 & 0.84 & 0.63 \\ \hline \end{tabular}
\end{table}
Table 5: The orbit RMS difference statistics of the GPS and BDS combined different strategies (unit: cm).  
Data Availability Statement: The Tianping-1B onboard GPS and BDS-3 data are supported by the Xi'an Satellite Control Center.