# Design and Performance Evaluation of eLoran Monitoring System
## 4 Far-Field Monitoring Station Performance Analysis  
The four far-field monitoring stations of the BPL monitoring system are located in the coverage area of the BPL ground-wave signals, and suitable locations are selected in different directions for layout and construction. According to the experimental test, the layout of the far-field monitoring stations is mainly set up in four different directions of the BPL station, and the distance of the four far-field monitoring stations from the BPL station varies from 500 to 900 km. The far-field monitoring stations are mainly used to monitor the changes in the time-of-arrival (TOA) value, and to analyze the performance of the transmitter-radiator through these worthwhile changes. The coordinate information and distribution map of the far-field monitoring stations are shown in Table 1 and Figure 5 [19; 26; 27; 28; 29; 30].  
The difference between the 1 PPS output from the BPL monitoring receiver and the 1 PPS output from the time-traceable keeping unit, which is subtracted from the propagation time and the receiver delay, is the TOA value, and the performance of the BPL signal is judged by analyzing the change in the TOA value and the standard deviation. Figures 6-9 show the trend of TOA values for Lanzhou, Chengdu, Wuhan, and Nanjing. Table 2 shows the standard deviation of TOA values of each monitoring station.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Serial Number** & **Location** & **Distance from BPL** \\ \hline
1 & Lanzhou & 527 km \\
2 & Chengdu & 700 km \\
3 & Wuhan & 675 km \\
4 & Nanjing & 861 km \\ \hline \hline \end{tabular}
\end{table}
Table 1: Coordinates of far-field monitoring stations.  
Figure 4: Transmission control precision.  
Figure 5: Distribution map of far-field monitoring stations.  
Figure 6: Trend of TOA in Lanzhou.  
Figure 7: Trend of TOA in Chengdu.  
From Figures 6-9, it can be seen that as the distance increases, the STD value of TOA becomes larger and larger, indicating that the jitter in the TOA value also increases.  
The Wuhan station is slightly closer to the BPL than the Chengdu station, but the STD of the TOA value is larger, mainly because the Wuhan station is located in the center of the city of Wuhan, which has a complex electromagnetic environment and a lower signal-to-noise ratio. The Chengdu station is located in the suburb of Chengdu, with a better electromagnetic environment and a higher signal-to-noise ratio. The receiver antenna installation environment is shown in Figure 10.  
BPL adopts the eLoran signal system, which belongs to a low frequency and long-wave signal. The increase in TOA jitter is mainly caused by the difference in the propagation path. However, in the same location, the trend of the TOA value is basically the same.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Serial Number** & **Location** & **Distance from BPL** & **STD of TOA** \\ \hline
1 & Lanzhou & 527 km & 16.2 ns \\
2 & Chengdu & 700 km & 65.4 ns \\
3 & Wuhan & 675 km & 66.6 ns \\
4 & Nanjing & 861 km & 70.1 ns \\ \hline \end{tabular}
\end{table}
Table 2: Standard deviation of TOA value.  
Figure 8: Trend of TOA in Wuhan.  
Figure 9: Trend of TOA in Nanjing.