# Characteristics of BeiDou-3 Experimental Satellite Clocks
## 4 Stability Analysis
### Comparison with Regional Solution  
The OADEVs of the BeiDou regional and global clock offsets are shown in Figure 5. The four plots in Figure 5, from left to right and from top to bottom, are the OADEV graphs of GEO, IGSO, MEO, and  
Figure 4: Time series of BeiDou clock frequency after pre-processing varies from day-of-year (DOY) 254 to 281, of the year 2017.  
BeiDou-3, respectively. The subscripts \"G\" and \"R\" in the legends represent the global solution and regional solution. The two auxiliary lines, \(1/\sqrt{\tau}\) and \(1/\tau\), correspond to the white phase noise and white frequency noise, respectively.  
As it is depicted in the bottom right plot, the stability for the experimental satellites of the global solution are similar to that of the regional solution, because of using of the same tracking stations, (i.e., the red stations plotted in Figure 1). For the new PHM carried on C32, the two distinctly sharp minima that occurred near the \(8.64\times 10^{4}\) s and \(1.728\times 10^{5}\) clearly reflect the presence of a 24-h period signal. Between 0 s and \(8.64\times 10^{4}\) s, there are two other steep minima that may be caused by the signal superposition of the 12-h and 8-h cycles related to the orbit period, according to the detail frequency analysis in the latter part. The frequency stability of C32 is approximately \(2.6\times 10^{-14}\) at an interval of \(10^{4}\), and \(1.4\times 10^{-15}\) at interval of \(8.64\times 10^{4}\). Nevertheless, C32 equipped with PHM has undoubtedly the lowest OADEV for any of the averaging intervals. The improved Rb AFS on C31 shows a better performance than the corresponding BeiDou-2 IGSO satellites. The rest of the MEO satellites (C33 and C34) that are equipped with improved Rb clocks are also superior to BeiDou-2 in the interval of \(3\times 10^{2}\) s to \(3\times 10^{3}\) s. The OADEVs of the BeiDou-3 clock show a white phase noise behavior from averaging intervals of \(3\times 10^{2}\) s to \(3\times 10^{3}\) s. This is a good supplement to the stability analysis of the TWSTFT method in this interval range, where the real clock stability is buried in the K-band measurement noise [13].  
Furthermore, according to Figure 5, for BeiDou-2, the number and distribution of the tracking stations have an impact on the estimated OADEV of the different types of satellites. Especially, the  
Figure 5: Overlapping Allan Deviation (OADEV) comparison of BeiDou regional and global clock offsets.  
bumps shown in MEO regional clock offsets are notably flattened from \(3\times 10^{3}\) s to \(8.64\times 10^{4}\) s in global clock offsets. Considering that the orbital error affects the stability between \(10^{3}\) s and \(10^{5}\) s [14], we believe that the stability of the BeiDou-3 MEO satellites shown in the figure should be different from the actual results. That is, the curve at \(10^{4}\) s should be smoother like the BeiDou-2 global solution in bottom left plot. In the same way, the bumps of the BeiDou-3 IGSO satellites at the interval of \(10^{4}\) s are larger than that of the BeiDou-2 IGSO satellites. Therefore, the clock stability of the BeiDou-3 experimental satellites is more affected by the orbit error because of the limited number and the uneven distribution of the stations.  
In addition, there are some anomalies in Figure 5. The frequency abnormalities in Figure 4 can explain the poor stability of C04. The abnormal fluctuation of C11 is consistent with the literature [12; 29], and the bump here is smoother than that from the literature [12].