## 3 Broadcast Ephemeris Performance  
### Broadcast Ephemeris Clock Offset Performance  
The broadcast ephemeris clock offset accuracy of GPS, BDS-2, BDS-3, GLONASS, Galileo, and QZSS satellites from DOY 283 to 289 in 2021 are depicted in Figure 5, and the mean clock offset accuracy is listed in Table 3. The clock offset accuracy assessment method is referred to [PERSON] et al. [23]. For GPS, the clock offset accuracy of the G08 satellite is 2.57 ns, which shows the poorest performance among all GPS satellites since the cesium atomic clock was installed on it. Furthermore, except for the G03, G17, G28, and G29 satellite clock, the clock offset accuracy of other satellites is better than 1 ns. The mean accuracy is 0.65 ns. The broadcast ephemeris clock offset accuracy is 0.98 ns, 1.72 ns, and 2.11 ns for BDS-2 GEO, IGSO, and MEO satellites, respectively. The clock offset accuracy of GEO satellites outperforms that of IGSO and MEO satellites, the reason is that the stations applied to estimate broadcast ephemeris clock offset are mainly located in China. Compared to the IGSO and MEO satellites, the observation arc of GEO satellites is longer, and the data used for predicting clock offset are more, resulting in the clock offset accuracy being higher, whereas the observation data of MEO satellites are few, leading to inferior clock offset accuracy. Moreover, the frequency stability of BDS-2 onboard satellite clocks is poorer, which has a negative impact on broadcast ephemeris clock offset accuracy. The broadcast ephemeris clock offset accuracy of the BDS-3 satellites is about 1 ns apart from the C38, C39, and C40 satellites, the operation period of these three satellites is shorter, and the stations can receive the signal of these three satellites are few. In a word, the mean accuracy is 1.04 ns, its accuracy is improved compared to that of BDS-2 satellites, which can be attributed to the following reasons: Firstly, the improved rubidium atomic clocks and high-performance Passive Hydrogen Masers (PHM) are equipped on BDS-3 satellites, the frequency stability is extremely improved compared to BDS-2 satellites; secondly, since the Inter-Satellite Link (ISL) technology is employed to estimate BDS-3 broadcast ephemeris clock offset [24], the broadcast ephemeris clock offset accuracy can be significantly improved. Figure 4: The MP series of GPS/QZSS/GLONASS/Galileo for DOY 283 on JFNG station. (The red wave indicates the sequence of elevation angle). \begin{table}
\begin{tabular}{c c} \hline \hline
**System** & **Clock Offset STD** \\ \hline GPS & 0.65 \\ BDS-2 & 1.72 \\ BDS-3 & 1.04 \\ GLONASS & 3.10 \\ Galileo & 0.61 \\ QZSS & 0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Clock offset accuracy of each system (units: ns). The cesium atomic clocks are installed on GLONASS satellites, previous studies have demonstrated that the performance of cesium atomic clocks is poorer than that of rubidium atomic clocks and PHM [25]. Compared to other systems, the broadcast ephemeris clock offset accuracy of GLONASS is worse, and the broadcast ephemeris clock offset accuracy of R13, R16, and R22 satellites is poorer than 4 ns. The mean clock offset accuracy is 3.10 ns. In terms of Galileo satellites, the broadcast ephemeris clock offset accuracy of each satellite is better than 1 ns, and the mean is 0.61 ns, which shows superior performance; the reason may be that the high-precision rubidium atomic clocks and PHM are employed on Galileo satellites [26]. Moreover, the update frequency of Galileo satellites is high, it is updated every 10 min, which also guarantees the high-precision performance of broadcast ephemeris clock offset. Each QZSS satellite clock offset accuracy is better than 1 ns, and the mean is 0.75 ns, the atomic clocks equipped on QZSS satellites are the same as that of GPS, while the clock offset accuracy is poorer than that of GPS, which may be related to the satellite orbit type.