## 3 Precise Orbit and Clock Product Introduction and Analysis  
In order to validate that the XRS product is competent to evaluate space clock stability, the XRS product precision is first analyzed. The multi-GNSS XRS precise orbit and clock product is calculated by the software Satellite Positioning and Orbit Determination System (SPODS) [27]. About 80 MGEX stations and 20 iGMAS stations with GPS, BDS, Galileo, and GLONASS satellite data are used to process precise orbit determination, including ultrarapid (XRU), rapid (XRR), and final (XRS) products in the daily routine. The BDS-3 observations are from B1I and B3I signals. Figure 1 shows the used ground stations, where there are about 50 stations tracking BDS-3 signals (i.e., the blue circles). With orbit and clock products from XRS, WUM, and IGS, the series of difference values between the orbits of two precise products on each day in radial (R), along-track (T), cross-track (N), and three-dimensional (3D) directions are computed. Then, the root mean square (RMS) of the difference values series is derived with the reference frame differences being eliminated by using Helmert transformation. As for the clocks, the RMS and STD are computed with the time series difference values between two precise products. Table 2 gives the averaged orbit RMS in R, T, N, and 3D directions and the RMS and STD of clocks over the processing time period. \"GPS(XRS)\" and \"GPS(WUM)\" denote the results of XRS and WUM compared with IGS products, respectively. The results of Galileo and BDS satellites are calculated by comparing XRS products with WUM products. Figure 2 shows the daily averaged RMS of all GPS satellites in R, T, and N directions for XRS and WUM products compared with IGS products. The orbit accuracy of XRS and WUM is basically consistent, with the RMS in R and 3D directions being about 1.2 and 2.3 cm, respectively. The RMS of XRS products in radial direction is slightly better than that of WUM products, while in T and N directions, it is slightly worse. Figure 3 presents the RMS and STD of the clock comparison results. The STD of XRS products is slightly better than that of WUM products, while the RMS is slightly worse, with the RMS and STD for XRS products being about 0.24 and 0.05 ns, respectively. The GPS comparison results show the XRS product is excellent and comparable to the IGS analysis center. Figure 4 presents the comparison results of Galileo satellites between XRS and WUM products. The accuracy of the two products is consistent, with the orbit RMS in R, T, N, and 3D directions being 1.8, 2.4, 3.0, and 4.3 cm, respectively, and the clock RMS and STD being 0.34 and 0.08 ns, respectively. Figures 5 and 6 show the orbit and clock comparison results of BDS-3 satellites between XRS and WUM products. The orbit 3D RMS of BDS-2 GEO satellites is at meter level, while for IGSO and MEO, it is at 3 and 4 decimeter levels, respectively. The STD of clocks is about 0.2 ns. The orbit RMS in R and 3D directions is 0.118 and 0.351 m for BDS-3 RAFS satellites and 0.119 and 0.322 m for BDS-3 PHM satellites, respectively. The clock STD of BDS-3 RAFS and PHM is 0.17 and 0.21 ns, respectively. The clock STD of C34 and C35 with PHM is 0.33 and 0.48 ns, respectively, while the mean value of other PHM satellites is 0.14 ns. Besides, the clock and orbit accuracy of C37 is relatively bad, with clock STD of 0.36 ns and orbit 3D RMS of 0.53 m. The specific reason needs to be further studied. Compared with the results of GPS and Galileo, the orbit and clock of BDS satellites are worse, especially the RMS of clocks. Hence, the RMS of clocks is not listed in Table 1. The possible reasons are as follows. First, for BDS-2 satellites, the antenna phase center product is inconsistent, where XRS uses the reprocessed products [29] and WUM may use the products released by Wuhan University [30] or IGS product [31]. The antenna phase center offsets of BDS-3 satellites are also not absolutely equal.