## 4 Conclusions  
This paper focuses on evaluating the space-borne GPS data and achieving POD for COSMIC-2. Based on TEQC, the observation data quality evaluation for COSMIC-2 was carried out from aspects of satellite visibility, utilization of GPS observations, SNR, cycle slip ratio, multipath error and \(IOD\). The results show that the proportion of 7-10 GPS satellites tracked by the six space-borne GPS receivers in each epoch is more than 79%, the average value of SNR is greater than 300, the data is less affected by the multipath effect, and SNR1 is better than SNR2 in signal-noise ratio. However, it is found that the utilization of the observation data is low and cycle slips occur frequently. The analysis of the change rate of ionospheric delay proves that the carrier-phase data is often interrupted.  
The POD was carried out based on the RD method using the five-day space-borne GPS observation of COSMIC-2, and the accuracy of the solved orbit is evaluated by three methods: carrier-phase residual analysis, overlapping orbit comparison and reference orbit comparison. (1) According to the residual analysis of carrier-phase observations, it is found that the residual RMS of carrier-phase observations for the RD orbit determination of six COSMIC-2 satellites is between 6.0 mm and 7.5 mm. (2) 4 h overlapping orbits are compared, and the test results show that the RMS values of the differences between the 4 h overlapping orbits in the three directions of R, T and N in five days are below 3 cm, and the mean RMS of six COSMIC-2 satellites in 3D are better than 2 cm, which indicates that this result is better than the overlapping orbit verification accuracy released by TACC. (3) According to the comparison between the solved orbit and the reference orbit, the results show that the RMS values of the orbit differences between the above two orbits of the six COSMIC-2 satellites in the R, T and N directions are all within 8 cm, and the mean RMS values of the 3D of the six COSMIC-2 satellites are smaller than 9 cm, which meets the orbit determination accuracy requirements of COSMIC-2. It can be seen that we have achieved the POD for COSMIC-2, and the orbit determination results can meet the requirements of the application of its science data.  
Conceptualization, writing--original draft preparation, writing--review and editing and methodology, Q.K., Y.C. and W.F.; software, Y.C.; validation, Y.C. and W.F.; data curation, G.W., C.L. and T.W.; formal analysis, J.H. and Q.B.; funding acquisition, Q.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (Grant No. 41704015, 41774001), the Shandong Natural Science Foundation of China (Grant No. ZR2017 MD032, ZR2021 MD030), a Project of Shandong Province Higher Education Science and Technology Program (Grant No. J17 KA077), Talent introduction plan for Youth Innovation Team in universities of Shandong Province (innovation team of satellite positioning and navigation).  
Our sincere thanks go to the Central Weather Bureau for providing space-borne GPS data for COSMIC-2; CODE Analysis Center at AIUB for providing GPS satellite orbits, clocks and Earth rotation parameters.  
We thank the Astronomical Institute of the University of Bern (AIUB) for providing Bernese GNSS Software, and the UNAVCO Facility for providing TEQC Software.  
The authors declare no conflict of interest.