# Evaluation of BDS-2 and BDS-3 Satellite Atomic Clock Products and Their Effects on Positioning
## 4 BDS-2/BDS-3 Clock Stabilities and Detum Discrepancy  
### BDS-2/BDS-3 Clock Stabilities  
All of BDS-2 satellites are manufactured by China Aerospace Science and Technology Corporation (CASC), on which Rb atomic clocks made by Switzerland and Chinese companies are installed. Unlike BDS-2, Rb and PHM clocks used by BDS-3 satellites are all developed by China independently. Besides CASC, Shanghai Engineering Center for Microsatellites (SECM) also manufactured a part of BDS-3 satellites. More details for BDS-2 and BDS-3 satellites are listed in Table 1. There are few precise products of BDS-3 GEO satellites and the last GEO satellite C61 launched on 23 June 2020 is still under in-orbit testing. Therefore, 3 BDS-3 GEO satellites are excluded from the analysis in this paper ([[http://www.csno-tarc.cn/en/system/constellation](http://www.csno-tarc.cn/en/system/constellation)]([http://www.csno-tarc.cn/en/system/constellation](http://www.csno-tarc.cn/en/system/constellation)), accessed on 10 December 2021). Figure 1: Distribution of iGMAS and MGEX stations used for ISB calculation and PPP, where the color of circle indicates the station from which tracking network (red—iGMAS; blue—MGEX). Figure 2 presents the overlapping Allan deviation of all BDS satellites, all GPS Block IIF satellites Rb and all Galileo Full Operational Capability (FOC) satellites PHM clocks over averaging intervals from 30 to 30,600 s. The Block IIF satellite Rb clocks (IIF-Rb) and FOC satellite PHM (FOC-PHM) clocks were selected because they have higher stabilities as the latest generation clocks for GPS and Galileo [19]. Since all IIF-Rb clocks have similar performance, we calculated the average Allan deviation values of all GPS Block IIF satellites [41]. The same was applied to FOC satellites [41; 42]. Average values per satellite were derived from single-day results for the period MJD 58815-59214. The top and bottom plots were based on the precise clock products generated by GFZ and WHU, respectively, in which results were grouped according to the satellite clock types or navigation systems and depicted in different colors. To make the comparison more obvious and visible, the FOC-PHM results of GFZ products are plotted in the bottom as a reference. There was a good concordance between the two sets of results for BDS-2 satellite clocks. However, for BDS-3 Rb and PHM clocks, the Allan deviation based on WHU clock products is significantly higher for averaging intervals less than 1000 s. At an averaging interval of 30 s, the Allan deviation for all BDS-3 clocks based on GFZ products is between 1.7 and 2.8 \(\times\) 10\({}^{-13}\), while the value derived from WHU products is between 2.8 and 3.5 \(\times\) 10\({}^{-13}\). For averaging intervals beyond 1000 s, almost all BDS-3 PHM clocks show similar performance between the two sets of ADEV values. However, the performance of BDS-3 Rb clock stabilities for GFZ products is still better than WHU products between 1000 and 5000 s and comparable for longer intervals. Similar phenomenon also exists in IIF-Rb and FOC-PHM. Overall, these results indicate that GFZ clock products has better performance for the ADEV of the BDS-3 clock. For simplicity, the following comparison of different system clocks is based on the results calculated from GFZ products, until there is a special note. Comparing the results of BSD-2 and BDS-3, it can be seen that the BDS-3 Rb and PHM clocks obviously have improved stability compared with the Rb clocks for BDS-2 in both plots. For the interval of 30 s, the BDS-2 Rb clocks show an Allan deviation between 3.4 and 11.0 \(\times\) 10\({}^{-13}\), approximately 1.2-3.9 times of the worst ADEV for the BDS-3 clocks.