# Analysis of the Development Status of eLoran Time Service System in China
## 2 Principle of eLoran Time Service System  
The eLoran time service system is mainly based on the traditional international standard Loran-C signal system, and it uses the latest Loran data link communication technology combined with large-scale, low-frequency, solid-state transmitter, antenna efficient radiation, time service monitoring and evaluation, differential enhancement, and digital reception technologies. It utilizes the characteristics of stable ground wave propagation and the accurate prediction of the propagation delay of long wave signals for a highly precise service. It also undertakes GNSS timing backup, supplementation, and enhancement, as well as satellite ground fusion timing applications [17; 18; 19; 20; 21]. The eLoran time service system includes a time service broadcasting station, a time service monitoring station, differential enhancement stations, and an operation control center. The working principle of the eLoran time service system is shown in Figure 1. Among them, the time service broadcasting station adopts solid-state transmitter technology and \begin{table}
\begin{tabular}{c c} \hline System & GNSS \(\mid\)eLoran \\ \hline Frequency & High \(\mid\)Low \\ \hline Power & Very Low \(\mid\)Very High \\ \hline Transmissions & Space \(\mid\)Terrestrial \\ \hline Jamming & Easy \(\mid\)Very Hard \\ \hline Spoofing & Easy \(\mid\)Very Hard \\ \hline Integrity & None \(\mid\)Built In \\ \hline Data Channel & None \(\mid\)At least one \\ \hline Reach & Global \(\mid\)Continental \\ \hline Accuracy & Best \(\mid\)Good \\ \hline Positioning & 3D \(\mid\)2D \\ \hline Propagation & Atmosphere \(\mid\)Ground \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of characteristics between GNSS and eLoran. data link technology to achieve high-precision time service signal broadcasting; sets up a time service monitoring station to ensure the reliability and completeness of time service broadcasting; sets up differential enhancement stations to improve the timing accuracy; sets up an operation control center to provide standard time frequency and differential correction data for time service broadcasting stations [21; 22; 23; 24; 25; 26]. The main function of the time service broadcasting station is to transmit the eLoran time service signal, which is modulated to carry time service data (Coordinated Universal Time, UTC time), integrity monitoring, differential correction, and satellite ground fusion time service data. The time service broadcasting station is based on high-precision time frequency signals distributed by the operation control center and traced back to UTC (NTSC) (UTC time is maintained by the National Time Service Center). It controls solid-state transmitters to generate high-power eLoran time service signals suitable for wide coverage and high-precision reception. After being efficiently radiated by large eLoran transmission lines, timing terminals are uses to receive and decode UTC time information and 1 PPS (pulse per second) signals through signal reception and demodulation [27; 28]. The main function of the time service monitoring station is to ensure the safe operation of the eLoran time service, implement the monitoring, comparison, and evaluation of timing signals, and ensure the timely detection of key timing signals and abnormalities in the working status of auxiliary systems [23]. The eLoran differential enhancement system requires each differential station to accurately measure the propagation delay of the eLoran signal based on high-precision time-frequency signals traced back to UTC (NTSC). The eLoran timing algorithm is used to generate real-time differential corrections for the propagation delay, and the differential corrections are transmitted and broadcasted to the relevant users using relevant data dissemination methods, thereby achieving the high-precision timing of the eLoran system.