## 5 Discussion and Conclusions  
The utilization of LEO satellites is anticipated to enhance various aspects of traditional GNSS-based PNT services. These enhancements include improvements in coverage, signal strength, reliability, initialization time, and landing power. The improved performance of LEO satellites positions them as valuable assets for future real-time high-precision PNT \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**DOY**} & \multirow{2}{*}{\(\Delta T\) **(s)**} & \multirow{2}{*}{\(T_{pre}\) **(s)**} & \multirow{2}{*}{**Fitting Time (s)**} & \multicolumn{2}{c}{**STD of Near-Real-Time**} & \multirow{2}{*}{**Precision Loss (ns)**} & \multirow{2}{*}{**STD (vs. R) (ns)**} \\ \cline{3-3} \cline{5-6}  & & & & & \\ \hline \multirow{4}{*}{227} & 60 & 90 & 120 & 0.219 & 0.064 & 0.229 \\ \cline{2-6}  & 120 & 150 & 180 & 0.219 & 0.084 & 0.246 \\ \cline{2-6}  & 180 & 210 & 180 & 0.222 & 0.106 & 0.247 \\ \cline{2-6}  & 240 & 270 & 270 & 0.211 & 0.127 & 0.264 \\ \cline{2-6}  & 300 & 330 & 300 & 0.223 & 0.156 & 0.297 \\ \hline \multirow{4}{*}{228} & 60 & 90 & 120 & 0.304 & 0.069 & 0.311 \\ \cline{2-6}  & 120 & 150 & 180 & 0.301 & 0.092 & 0.326 \\ \cline{2-6}  & 180 & 210 & 180 & 0.301 & 0.115 & 0.329 \\ \cline{2-6}  & 240 & 270 & 270 & 0.299 & 0.138 & 0.341 \\ \cline{2-6}  & 300 & 330 & 300 & 0.303 & 0.169 & 0.342 \\ \hline \multirow{4}{*}{229} & 60 & 90 & 120 & 0.247 & 0.068 & 0.266 \\ \cline{2-6}  & 120 & 150 & 180 & 0.245 & 0.090 & 0.279 \\ \cline{1-1} \cline{2-6}  & 180 & 210 & 180 & 0.246 & 0.127 & 0.295 \\ \cline{1-1} \cline{2-6}  & 240 & 270 & 270 & 0.243 & 0.136 & 0.312 \\ \cline{1-1} \cline{2-6}  & 300 & 330 & 300 & 0.228 & 0.173 & 0.327 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The daily STDs of the connected real-time clocks according to different \(\Delta T\) (R denotes the daily reduced-dynamic clocks based on batch least-squares adjustment using the CODE final products). Figure 9: Error budget of the real-time LEO satellite clocks for Sentinel-3B from DOY 227 to 229, 2018.  
users, particularly in applications like autonomous driving and unmanned aerial vehicles. However, in practical applications, LEO satellites also face some challenges. For instance, the determination of real-time LEO satellite clocks still awaits further investigation due to challenges in mid- to long-term LEO satellite clock prediction. To address this need, this study introduces real-time LEO satellite clock determination based on efficient high-precision near-real-time clock determination and ultra-short-term clock prediction. The method connects the most updated clock estimates from different rounds of Kalman-filter-based solutions and performs ultra-short-term prediction using the clock estimates of the last few minutes, hampering unnecessary bias propagation and accumulations of ancient epochs. Using Sentinel-3B GPS L1/L2 observations and the CNES real-time GNSS products, near-real-time satellite clocks were determined and assessed. The results showed that the precision of the near-real-time LEO satellite clocks is generally at 0.2 to 0.3 ns. A processing arc of 6 h can be selected considering the efficiency and the precision. The time interval \(\Delta T\) between subsequent processing sessions affects the prediction time windows needed for real-time applications. When setting the time interval to 60 s, it is possible to obtain real-time LEO satellites with a precision of about 0.3 ns, with a precision loss within 0.07 ns during a prediction time of 90 s. A longer \(\Delta T\) induces larger prediction errors due to the longer prediction time needed. Although notable precision is achieved in estimating LEO satellite clocks in this study, there are still challenges in real-world applications. For instance, the timely acquisition of observation data and GNSS real-time products is essential, as the corresponding latencies would directly add to the prediction time required to catch up with the real-time applications, which requires further research in the clock prediction of a longer time.