# Characteristics of BeiDou-3 Experimental Satellite Clocks
## 4 Stability Analysis
### Comparison with Other Global Navigation Systems  
In order to compare the frequency stability with the GPS, GLONASS, and Galileo, the OADEV of each of them has been presented with a time span from DOY 254 to DOY 281, of the year 2017. The IGS final product has been selected for estimating the OADEV of the GPS, and the GBM final product has been selected for GLONASS and Galileo. The satellites chosen here represent the general performance of this type of satellite or atomic clock. The results of the Allan deviation are depicted in the Figure 6.  
Figure 6: OADEV of Global Positioning System (GPS), GLObal NAvigation Satellite System (GLONASS), and Galileo from DOY 254 to 281, of the year 2017, with intervals of 300 s.  
Although the pronounced bumps stand out at an average interval of \(1.1\times 10^{4}\) s, it is not difficult to find that the Rb AFS on Block IIF has a competitive performance among the PHM clock of BeiDou and Galileo. However, in spite of the worse performance, between 300 s and \(1\times 10^{4}\) s, the old Rb clocks on Block IIR and Block IIR-M have no obvious fluctuations and are more stable than Block IIF between the interval of \(10^{4}\) s and \(10^{5}\) s.  
Galileo has launched many satellites since 2015, so E01 and E04, launched in 2016, have been chosen to assess the AFS on these new satellites. From the bottom left plot, we can see that whether FOC or IOV satellites show excellent stability of PHM. The stability of most PHM is superior to that of Rb AFS, except for some satellites, such as E08 shown in figure. Moreover, there is no significant difference between PHM clock performance on new satellites and old satellites. The OADEVs of PHM are less than \(2\times 10^{-14}\) s at averaging time of \(10^{4}\) s and smoother than that of BeiDou-3 PHM and GPS Block IIF Rb AFS, leading to a better clock prediction accuracy in broadcast applications. Compared with Galileo PHM, the BeiDou PHM has a more obvious no-power-law behavior near \(1.05\times 10^{4}\) s, which results from the orbit error.  
The majority of the GLONASS constellation currently consists of GLONASS-M satellites with a Cesium (Cs) clock [30], but the Russia government has started to launch the enhanced GLONASS-K series to gradually replace the existing satellites. As we can see from Figure 6, there is a correlation between the frequency stability and satellite lifetime. The stability of the atomic clock of most of the GLONASS satellites with shorter service times is better than that of the atomic clock of satellites with long service times [31]. On the other hand, the frequency standard equipped on GLONASS-K1 is worse than that of the old GLONASS-M series. Further studies on these new series of satellites are required in order to explain the specific characteristics. The current performance of the GLONASS clock is consistent with BeiDou-2, and better than that of the GPS Cs clock, regardless of life expectancy.  
For the above analysis, it follows that the new AFS operated on BeiDou-3 has greatly improved over the old frequency standard adopted by BeiDou-2, especially at the interval from \(3\times 10^{2}\) s to \(3\times 10^{3}\) s. The new PHM currently achieves a level of stability comparable to that of the PHM on Galileo and to the Rb clocks on GPS Block IIF. Considering the orbital error affects due to the limited number and uneven distribution of tracking stations, the stability of the improved Rb AFS on the BeiDou-3 experimental satellites is comparable to that of the Rb AFS on Galileo, and better than that of the rest GPS and GLONASS clocks.