## 6 Conclusions  
This paper discussed two key issues with regard to a large LEO-constellation-augmented BDS POD with simulated spaceborne and ground observations. BDS-3 and five types of LEO constellations, namely the Walker 2/1/0, 6/2/1, 12/3/1, 24/4/2, and 60/6/4 constellations, were designed. The actual spaceborne measurement noise level tends to be different from that of the ground receivers and needs to be analyzed and calibrated. Therefore, the measurement noise from Swarm-A, GRACE-A, and Sentinel-3A spaceborne observations were calibrated for by observation simulations with different LEO constellations.  
Another issue addressed here is that of the spaceborne observational errors. Unlike ground observations, in which the observational errors primarily result from atmospheric delay and the multipath effect, spaceborne observation errors are primarily caused by ionospheric scintillations or thermal atmosphere, which cannot be completely eliminated using the conventional POD method. This study examines the effect of large spaceborne observational errors on the BDS POD augmented by different LEO constellation configurations. Using an experiment, the actual observation residuals calculated from the Swarm-A and GRACE-A observations were analyzed and it was found that the impaction times and magnitudes of a solar storm varied for LEO satellites at different altitudes and in different orbital planes. The BDS orbit accuracy can be seriously degraded by spaceborne observational errors when using a Walker 12/3/1 LEO-constellation-augmented BDS POD. For a Walker 60/6/4 constellation, the BDS satellites can be tracked by as many as 10-25 LEO satellites at one time and both the rapid change in an LEO satellite and the many LEO spaceborne observations involved in the adjustment can effectively compensate for and reduce the spaceborne observational errors. The results indicate that even 3-4 LEO satellites are sufficient to augment the BDS POD with a regional ground network, allowing the 3D orbit accuracy of GEO satellites to reach 20-40 cm. However, a large LEO constellation with 10-25 LEO satellites tracking the BDS satellites is still necessary in the case of large spaceborne observational errors caused by extreme space weather, i.e., solar storms.  
[PERSON], [PERSON] and [PERSON] conceived the experiments. [PERSON] and [PERSON] processed the data and drew the pictures. [PERSON] and [PERSON] investigated the results. [PERSON] wrote the whole manuscript. All authors have read and agreed to the published version of the manuscript.  
This study was funded by the National Key Research and Development Program of China (2020 YFB050800), National Natural Science Foundation of China (grant number 41874032), and Postdoctoral Science Foundation of China (2021M691902).  
Not applicable.  
We are very grateful to the International GNSS Service (IGS) for providing the multi-GNSS precise orbit and clock products ([[ftp://igs.ign.fr/pub/igs/](ftp://igs.ign.fr/pub/igs/)]([ftp://igs.ign.fr/pub/igs/](ftp://igs.ign.fr/pub/igs/)) accessed on 20 June 2021). The Swarm, GRACE, and Sentinel-3 spaceborne GPS data provided by the European Space Agency (ESA) as part of the Copernicus program and GFZ are greatly acknowledged.  
The authors declare no conflict of interest.