# GNSS Timing Performance Assessment and Results Analysis
## 3 Assessment Result and Analysis  
The receiver time offset UTC(NTSC)-UTC\({}_{\text{GNSS}}\) was generated at NTSC with 1 day time interval, involving BDS, GPS, GLONASS, and Galileo, in 2021. Figure 3 shows the comparison the time offset UTC(NTSC)-UTC\({}_{\text{GNSS}}\) with UTC(NTSC)-UTC\({}_{\text{true}}\). The red plus represents UTC(NTSC)-UTC\({}_{\text{GNSS}}\), while the blue point represents UTC(NTSC)-UTC\({}_{\text{true}}\). Figure 4 shows the comparison time transfer error sequence UTC\({}_{\text{GNSS}}\)-UTC\({}_{\text{true}}\) for BDS, GPS, GLONASS, and Galileo. Some conclusions can be drawn from Figures 3 and 4. During the assessment period, the time offset UTC(NTSC)-UTC\({}_{\text{GNSS}}\) of GLONASS had large fluctuations, caused by the fluctuation of UTC from the GLONASS navigation message. There was a fixed time offset between UTC(NTSC)-UTC\({}_{\text{GNSS}}\) and UTC(NTSC)-UTC\({}_{\text{true}}\) of BDS and GLONASS, which did not \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**GNSS** & **BDS** & **GPS** & **GLONASS** & **Galileo** \\ \hline SISE (m) & 0.7 & 1.0 & 2.3 & 0.8 \\ UEE (m) & 1.6 & 1.6 & 1.6 & 1.6 \\ UERE (m) & 1.7 & 1.9 & 2.8 & 1.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: UERE evaluation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**GNSS** & **BDS** & **GPS** & **GLONASS** & **Galileo** \\ \hline \(\sigma_{\text{GNSST}}\) (ns) & 1.8 & 2.2 & 3.8 & 2.4 \\ \(\sigma_{\text{UTCO}}\) (ns) & 3.0 & 1.3 & 1.7 & 0.5 \\ UTCr–UTC(NTSC) & 0.7 & 0.7 & 0.7 & 0.7 \\ UTCr–UTC(k) & / & 0.2 & 0.4 & / \\ \(\sigma\) (ns) & 3.6 & 2.7 & 4.2 & 2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The uncertainty budget for GNSS time transfer accuracy assessment. exist in GPS and Galileo. The offset reflects the accuracy of UTC\({}_{\text{GNSS}}\) relative to UTC\({}_{\text{true}}\), which is not caused by the GNSS user. The fluctuation range of GLONASS time transfer error was \(-35\) ns to \(5\) ns, while the fluctuation ranges of BDS, GPS, and Galileo were less than \(10\) ns. Table 3 shows the assessment results of time transfer accuracy from BDS, GPS, GLONASS, and Galileo, including 95%, RMS, AVG, STD, and MAX. For BDS, GPS, GLONASS, and Galileo, the time transfer accuracy (with 95%) was 13.8 ns, 4.5 ns, 16.8 ns, and 4.2 ns, respectively. Galileo's time transfer accuracy was the best, while that of GLONASS was the worst. The AVG values of BDS and GLONASS results reflect the fixed time offset in Figures 3 and 4, which was about 10 ns. However, the STD of the GLONASS time transfer accuracy was the largest, about 3.6 ns. Figure 5 shows the time transfer accuracy of BDS, GPS, GLONASS, and Galileo per month in 2021 (with 95%). In Figure 5, the assessment results of BDS, GPS, GLONASS, and Galileo are expressed in green, blue, red, and black, respectively. The red dashed \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **BDS** & **GPS** & **GLONASS** & **Galileo** \\ \hline
95\% (ns) & 13.8 & 4.5 & 16.8 & 4.2 \\ AVG (ns) & \(-10.2\) & 2.6 & \(-11.9\) & 1.6 \\ STD (ns) & 2.0 & 1.2 & 3.6 & 2.0 \\ MAX (ns) & 15.4 & 6.3 & 31.2 & 5.8 \\ RMS (ns) & 10.4 & 2.9 & 12.5 & 2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The assessment results of GNSS time transfer accuracy. Figure 4: The comparison time transfer error UTC\({}_{\text{GNSS}}\)-UT\({}_{\text{Ctrue}}\) for BDS, GPS, GLONASS, and Galileo. Figure 3: Comparison UTC(NTSC)-UT\({}_{\text{C GNSS}}\) with UTC(NTSC)-UT\({}_{\text{true}}\) from GNSS. line indicates that the time transfer accuracy requirement in GLONASS was 40 ns with open service performance specification, while the blue one indicates that it was 30 ns in GPS. The results shown in Figure 5 reveal that the monthly time transfer accuracy met the requirements of GNSS timing performance. The time transfer accuracy of GPS and Galileo was less than 5 ns, that of BDS was between 9 ns and 15 ns, and that of GLONASS was between 11 ns and 25 ns. The BDS and GLONASS timing performance can still be further improved.