# Accuracy Analysis of GNSS Hourly Ultra-Rapid Orbit and Clock Products from SHAO AC of iGMAS
## 3 Assessment and Analysis
### Assessment of 6H and 1H Clock Products  
In this study, the GNSS clock was estimated, relatively, with respect to a reference clock. Thus, the clock bias must be removed before comparing with the reference products from IGS/MGEX. A satellite clock of GPS is selected as the reference clock because GLONASS/Galileo/BDS are coupled with GPS in the running threads. Then the difference between other satellite clocks and the reference clock for each epoch are calculated to remove the clock bias. Moreover, the RMS and SD of the differences between our hourly clock productions and reference clock products from IGS/MGEX are summarized to assess the quality of the clock.  
Figure 8 shows the average SD values for 6H (top chart) clock versus reference products. The mean SD of the observation session for GNSS satellites are no more than 0.16 ns. The GPS clock shows the best accuracy, followed by BDS-MEO, GLONASS, and Galileo. Moreover, the SD of BDS MEO are better than IGSO/GEO, for more global tracking stations available for MEO. While the SD of the GNSS prediction clock is less than 0.18 ns, which is a little worse than the observation session, but can be available to users. The mean SD for 1H (bottom chart) clock is less than 0.16 ns for the observation session, or even the 1 st-2 nd hour prediction session, which benefits from the higher updated rate and shorter prediction session, and shows better performance than 6H.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Session**} & **GPS** & **GLONASS** & **Galileo** & **BDS–MEO** & **BDS–IGSO** & **BDS–GEO** \\ \cline{2-7}  & **MEO** & **MEO** & **MEO** & **MEO** & **IGSO** & **GEO** \\ \hline Observation & 1\% & 31\% & 13\% & 11\% & 23\% & 9\% \\ Prediction & 18\% & 43\% & 45\% & 34\% & 53\% & 15\% \\ \hline \end{tabular}
\end{table}
Table 3: Improved accuracy of 1H relative to 6H orbit for observation and prediction session.  
Figure 9 shows the mean RMS of 6H and 1H for GNSS clock with respect to reference clock products. The mean RMS of 6H for GPS is about 0.42 ns and 1.16-2.58 ns for observation and 1 st-8 th hour prediction session, which also shows better accuracy followed with Galileo, BDS, and GLONASS. The RMS of the prediction session shows anincreasing trend for GPS and Galileo, which means the accuracy decreases when the prediction hour augments. The mean RMS of BDS GEO clock is worse because of the station distribution limit and complicated satellite constellation, no better phase center correction (PCC) available, and the improper SRP model. The RMS of the GLONASS is the worst due to the possible bias caused from the FDMA signal transmit mechanism and the float ambiguity solutions. The mean RMS for the 1H clock is similar to the 6H, but the advantage of 1H is that it can provide the users clock products with higher updated rates, which is better than the poor accuracy of the broadcast clock and can be used as initial values for real-time or near-real-time applications.  
Figure 8: Average SD of 6-hourly (**a**) and 1-hourly (**b**) GNSS clock versus reference products from IGS/MGEX.