# 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 Orbit Products  
The accuracy of 6H and 1H orbit products for GPS, GLONASS, Galileo, and BDS is analyzed by comparing the reference productions from IGS/MGEX. This method compares the orbit positions between our orbit products with reference orbits at a 15 min sampling in radial, along-tracking, and cross-tracking direction, with seven-parameter Helmet rotation transformation. Moreover, the results are expressed as mean 1 DRMS values over the three components for different types of GNSS satellites, such as MEO, IGSO, and GEO, respectively. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Session**} & & **GPS** & **GLONASS** & **Galileo** & & **BDS** & \\ \cline{2-8}  & & **MEO** & **MEO** & **MEO** & **MEO** & **IGSO** & **GEO** \\ \hline Observation session & 24 h & 0\% & 0\% & 0\% & 0\% & 0\% & 0\% \\ \hline \multirow{8}{*}{Prediction session} & 1 st hour & 0\% & 15\% & 12\% & 20\% & 54\% & \(-\)7\% \\  & 2 nd hour & 9\% & 14\% & 28\% & 32\% & 59\% & \(-\)7\% \\ \cline{  
Figures 6 and 7 show the average RMS of the orbit difference including the observation and prediction session for 6H and 1H orbit with respect to the reference orbit for GPS, GLONASS, Galileo, and BDS over the period from the 041 st to 050 th days of the year in 2019 (from 10 February 2019 to 19 February 2019). Figure 5: Average root mean square (RMS) in along, cross, and radial directions of differences between 6-hourlyorbit products generated by the new and old method and IGS/MGEX final products for 24-h observation session and the 1 st–8 th hour prediction session. For GPS satellites, the average RMS of 6H was about 2 cm for the observation session. However, the RMS of the prediction orbit was from 3.5 to 4.7 cm for the 1 st-8 th hour prediction session. The accuracy of prediction shows a diminishing trend, with the prediction hour session increasing. The mean RMS of observation and 1 st-2 nd hour prediction session was 2.2 cm and 3.3-3.8 cm for 1H, which is better than 6H. Moreover, the higher renewal rate of 1H will provide the better accuracy of 1H orbit products, which can be provided to users during the 3 rd-8 th prediction hours of 6H. For GLONASS satellites, the average RMS for 6H was less than 4.7 cm for the observation session, while the RMS of the prediction session was from 5.8 to 8.1 cm, which is worse than GPS, due to the difficulty in GLONASS ambiguity fixing, and the bad tracking site distributed in some places, such as the Russian area. Moreover, the average RMS for 1H was about 3.2 cm and 3.9-4.3 cm for observation, and 1 st-2 nd hour prediction sessions, which is better than 6H. The high updated rate, recently updated observation data, and the improved strategy may benefit the 1H orbit. Figure 6: Average RMS of 6-hourly GNSS ultra-rapid orbit products with respect to IGS/MGEX final products for 24-h observation session and 1 st-8 th prediction session. (**a**) shows RMS of the MEO orbit for GPS/GLONASS/Galileo, and MEO/IGSO orbit for BDS, (**b**) displays RMS of GEO orbit for BDS for the observation session and prediction session. Figure 7: Average RMS of 1-hourly GNSS ultra-rapid orbit products with respect to IGS/MGEX final products for 24-h observation session and 1 st-2 nd prediction session. (**a**) shows RMS of the MEO orbit for GPS/GLONASS/Galileo, and MEO/IGSO for BDS, (**b**) displays RMS of GEO orbit for BDS for the observation session and prediction session. For Galileo satellites, the mean RMS of 6H was about 3.2 cm and 4.7-8.7 cm for observation and 1 st-8 th hour prediction session. While the mean RMS of 1H was about 2.8 cm and 3.6-4.2 cm for observation and 1 st-2 nd hour prediction session. For BDS satellites, the average RMS of 6H observation and 1 st-8 th hour prediction session for MEO satellites was about 5.7 cm and 8-12 cm, which is better than that of IGSO and GEO because of the limitation of tracking station distribution for the latter. While the mean RMS of 1H was about 5 cm and 6-7 cm for observation and 1 st-2 nd hour prediction session.