# Analysis of the Development Status of eLoran Time Service System in China
## 1 Introduction  
With the development of radio technology, radio navigation is widely used worldwide. Enhanced Long-Range Navigation (eLoran) is an evolution of Loran-C (Long-Range Navigation) and an internationally standardized medium-to-long-range land-based radio navigation and time service system. During early World War II, the United States developed the Loran-A hyperbolic navigation system for use at sea, but its application range and accuracy were limited [1; 2; 3]. Therefore, during late World War II, the United States developed the Loran-B, Loran-C, and Loran-D systems. The Loran-C system operates at a frequency of 100 kHz and adopts a pulse signal system, which has a wide signal coverage range and higher application accuracy and is widely used. Since the 1970s, with the application of large-scale electronic technology, microcomputers, and high-power solid-state transmitter technology, the Loran-C system has been upgraded and transformed, and its broadcasting technology has been largely improved upon. In 1989, Delft University of Technology in the Netherlands proposed the concept of \"Eurofix\", which implemented the Loran-C additional data channel (LDC) using the Pulse Phase Modulation (PPM) method. The aim was to use Loran-C to broadcast GPS differential correction data and integrity information, forming the prototype for the eLoran system. In 2007, the International Loran Association (ILA) released the \"Enhanced (eLoran) Definition File\", which pointed out that the eLoran system is a standardized PNT service system [2]. At the same time, combining the latest antenna, transmitter, data communication, differential correction, and new signal reception and processing technologies to enhance the data, the eLoran system has higher performance indicators. After differential correction, the typical positioning accuracy value is 10-20 m (95%) [4; 5; 6]. After the 1980s, with the emergence and development of the Global Navigation Satellite System (GNSS), which has obvious advantages in terms of performance and reliability, the development of the eLoran system has suffered serious challenges. The U.S. announced the cessation of the eLoran system support in 2000 and gradually closed down some eLoran stations, which made the direction of international eLoran development uncertain. The eLoran systems of other countries were not substantially developed. However, with the weak reception signal, poor penetration ability, and susceptibility to interference with GNSS, its application in underground, underwater, indoor, and complex electromagneticenvironments is insufficient, and the risk of relying solely on GNSS is gradually exposed. eLoran, as a well-functioning PNT system, has the advantages of a long range of action, phase stability, is highly repeatable, and has a very strong relationship with GNSS in terms of the operating frequency and service form. It is also strongly complementary with GNSS in terms of the working frequency, service form, signal strength, etc. [7; 8; 9; 10; 11; 12; 13]. A comparison of the characteristics of GNSS and eLoran is given in Table 1. When they suffer from intentional or unintentional interference, they will not fail and will greatly reduce the risk of relying on GNSS alone, so many countries in the world are now developing and perfecting their own eLoran system. The inherent shortcomings and fragility of GNSS time service system restrict the availability and robustness of its time service, posing security risks and application limitations. The New York Stock Exchange in the United States has tested the use of the eLoran system as a backup of the GPS for time synchronization, and South Korea is testing the use of a differential eLoran system as a backup for the GPS system for precision navigation in ports. China has also added eLoran timing systems in similar financial institutions and communications fields to improve the security of critical infrastructure [14]. Internet of Things (IoT) technologies based on GNSS for localization and positioning are being proposed and used.