# Analysis of BDS-3 Onboard Clocks Based on GFZ Precise Clock Products
## 7 Conclusions  
The performance of BDS satellite onboard clocks was analyzed based on the precise clock offset products provided by GFZ from DOY 1, 2021 to DOY 300, 2021. The series of clock bias, frequency, drift rate, and fitting residuals were shown. Moreover, the frequency stability and the periodicity of the clock offsets were analyzed. The following conclusions can be drawn from the results obtained in this paper:  
1. In terms of the BDS-3 system, the performance of PHMs equipped on MEO satellites is slightly better than the performance of Rb clocks. The OADEV of MEO Rb clocks is \(4.3~{}\times~{}10^{-14}\) s/s at 1000 s, which is 7% higher than the \(4.03~{}\times~{}10^{-14}\) s/s of MEO PHMs. Furthermore, the RMS of fitting residuals of Rb clocks is 0.17 ns, which is 13% higher than that of PHMs.
2. The precise clock offset products of PHMs carried on BDS satellites show different performances when they operate in different types of orbits. The RMS values of the fitting residuals of PHMs on BDS-3 MEO, IGSO, and GEO satellites are 0.15, 0.28, and 0.46 ns, respectively. Moreover, the 1000-second stabilities of PHMs carried on MEO, IGSO, and GEO satellites are \(4.03~{}\times~{}10^{-14},1.10~{}\times~{}10^{-13}\), and \(2.38~{}\times~{}10^{-13}\) s/s, respectively.
3. Compared to BDS-2, the BDS-3 satellites show great improvements in clock quality. There are fewer frequency jumps in the long-term performance of BDS-3 onboard clocks. Moreover, the drift rates of BDS-3 satellites clocks varies within the range between \(-2~{}\times~{}10^{-18}\) and \(2~{}\times~{}10^{-18}\) s/s\({}^{2}\), which is less than the range between \(-4~{}\times~{}10^{-18}\) and \(4~{}\times~{}10^{-18}\) s/s\({}^{2}\) for BDS-2 clocks. Furthermore, the RMS of fitting residuals of BDS-2 onboard clocks in GEO, IGSO, and MEO satellites are 0.58, 0.53, and 0.33 ns, respectively, while those of BDS-3 onboard clocks in GEO, IGSO, and MEO satellites are 0.46, 0.28, and 0.16 ns, respectively. The frequency stability of BDS-2 clocks is also worse than that of BDS-3 clocks.
4. The analysis of the periodicity of BDS onboard clocks shows that there are significant periodic signals in BDS satellite clock offsets. The periodicity of the same types of satellite clocks is similar. BDS-3 with PHMs equipped on IGSO and GEO satellites show the same periodic characteristics as BDS-2 IGSO and GEO Rb clocks. The PHMsand Rb clocks carried on BDS-3 MEO satellites show different periodic characteristics. The main periods of the BDS-3 MEO Rb clocks are about 12.88 (1 CPR), 6.44 (2 CPR), and 4.29 h (3 CPR). The main periods of the BDS-3 MEO PHMs are about 6.44 (2 CPR) and 12.88 h (1 CPR).  
Conceptualization, T.G.; methodology, T.G., R.J. and Y.L.; validation, R.J. and T.G.; formal analysis, T.G. and R.J.; resources, T.G.; writing--original draft preparation, R.J.; writing--review and editing, Y.L., X.X. and T.G.; supervision, T.G. All authors have read and agreed to the published version of the manuscript.  
This research was financially supported by the National Natural Science Foundation of China (No. 41974036), and the Natural Science Foundation of Hubei Province (No. 2019 CFA051).  
Not applicable.  
The dock products are obtained from [[ftp://igs.ign.fr/pub/igs/products/mpex](ftp://igs.ign.fr/pub/igs/products/mpex)]([ftp://igs.ign.fr/pub/igs/products/mpex](ftp://igs.ign.fr/pub/igs/products/mpex)), accessed on 20 January 2022.  
International GNSS Service (IGS) and IGS Multi-GNSS Experiment (MGEX) are greatly acknowledged for providing clock products.  
The authors declare no conflict of interest.