Analysis of the Development Status of eLoran Time Service System in China

Abstract

This article introduces the eLoran timing system principle, the characteristics of the eLoran and GNSS systems, and the current development status of eLoran in China. This article elaborates on the significance and scale of this high-precision ground time service system currently being constructed in China and describes the technical methods used in the high-precision ground time service system. Finally, it analyzes and elaborates on the signal and data channels of the eLoran time service system.

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 electromagnetic environments 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. Comparison of characteristics between GNSS and eLoran.

System	GNSS|eLoran
Frequency	High|Low
Power	Very Low|Very High
Transmissions	Space|Terrestrial
Jamming	Easy|Very Hard
Spoofing	Easy|Very Hard
Integrity	None|Built In
Data Channel	None|At least one
Reach	Global|Continental
Accuracy	Best|Good
Positioning	3D|2D
Propagation	Atmosphere|Ground

. 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. As IoT based services are becoming inevitable and getting deployed at very high rate, it would give more visibility. The inherent vulnerability of GNSS systems will limit the development of the Internet of Things. The combination of GNSS and eLoran is a very good complementary solution that will further promote and expand the application scenarios of the Internet of Things [15,16].

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 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 1PPS (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.

The operation control center sends standard time frequency signals to the time service broadcasting, time service monitoring, and differential enhancement stations to ensure the consistency of their time frequency signals. And it transmits the integrity monitoring, satellite ground fusion monitoring, and differential correction data generated by the time service monitoring and differential enhancement stations to the time service broadcasting station [5,6].

3. Current Development Status of eLoran Time Service in China

The earliest broadcasting station that adopted Loran-C in China was the BPL long wave time service system built by the National Time Service Center of Chinese Academy of Sciences in the 1970s, which is still operated and maintained, mainly providing time and frequency services.

The BPL Long Wave Time Service station was completed and put into use in 1983 and included in the operation and management of the National Science and Technology Project in 1989. The BPL long wave time service system is the only high-precision microsecond time service method for space launch missions in China in the 20th century. The National Time Service Center of Chinese Academy of Sciences modernized the BPL long wave time service station in 2008. The data channel uses Eurofix technology to broadcast time code information, making it capable of eLoran business [29,30].

The BPL long wave time service station is located in the hinterland of China, Pucheng, Weinan, Shaanxi Province. According to the characteristics of signal (long wave) ground wave propagation, BPL mainly covers the central and eastern regions of China. Other Loran broadcaster stations are mainly distributed in the coastal and nearby areas of China. Their ground wave signal mainly covers the coastal and surrounding areas, filling the blind spots of BPL ground wave signal coverage in the southern, southeastern, and northeastern regions of China. The coverage of the time service signals in existing eLoran stations in China is shown in Figure 2. However, the southwestern, western, and northwestern regions of China are still the blind spots of ground wave signal coverage in the long wave time service system, which are unable to use eLoran signal to achieve high-precision timing and frequency calibration.

4. Current Situation of the Construction of eLoran Time Service in China

In order to meet the needs of eLoran signal blind spot users in the western, northwestern, and southwestern regions of China and to consider the overall layout of the eLoran system in China, the high-precision ground-based time service system was approved in 2016. This specifies the construction of a high-precision ground-based time service system, which mainly utilizes the high reliability and high-precision technical features of the eLoran time service and fiber optic time service technologies to build a highly reliable and high-precision ground-based time service system that is relatively independent, integrated, shared, backup, complementary, and mutually reinforces satellite navigation.. Combined with the existing eLoran system, it basically achieves the national coverage of eLoran time service signals.

The project conducted theoretical calculations and actual measurements of the coverage range of the existing loran ground wave timing signals. The results showed that due to system layout reasons, there is a large coverage blind spot for the existing eLoran signal in the west, with a blind spot length of about 2100 km from east to west and a width of about 2300 km from north to south. At the same time, the western region is important, with high requirements for the safety and reliability of the time service. It is necessary to have an eLoran time service system that complements the satellite navigation system time service to provide a time service. It is necessary to add and build time service broadcasting stations to meet the coverage needs. Therefore, taking into account some factors, such as the current signal coverage status and construction costs in China, combined with the future navigation expansion needs, three eLoran time service broadcasting stations will be added and constructed in Dunhuang, Korla, and Naqu in the western region. The layout of eLoran system in China is shown in Figure 3.

At the same time, in order to improve the application accuracy of the existing and new eLoran systems, the high-precision ground-based time service system plans are to build multiple eLoran differential enhancement stations nationwide. However, due to the vastness of our country, there is a huge demand for differential station construction. At the same time, considering the economic cost of construction and the urgency of the demand, the layout of differential enhancement stations should first meet the high-precision time service needs of the key and critical areas in China. By using eLoran differential enhancement technology, a time service accuracy better than 100 ns can be achieved. The high-precision ground-based time service system is currently the most accurate large-scale land-based radio time service system. After the system has been built, it can improve the accuracy of the eLoran time service to a level equivalent to that of the satellite navigation system, and thus achieve a combined and enhanced space- and land-based integrated time service system.

The construction of the eLoran time service stations in Dunhuang, Korla, and Nagqu has begun, and the relevant equipment has been designed and is in production. It is expected to be in operation in 2026.

With the construction of the system, the eLoran system faces possible challenges that need to be paid attention. The number of users is smaller than that of GNSS, and the price for eLoran users is higher than that of GNNS receivers. These may hinder the development of eLoran in China.

5. Construction Content

The construction of a high-precision ground-based time service system eLoran time service broadcasting station mainly includes the transmission antenna, transmitter, and time frequency systems. The eLoran time service station is shown in Figure 4.

5.1. Transmission Antenna System

The transmission antenna adopts a single-tower umbrella-shaped antenna, mainly composed of an insulation tower with a height of 280 m and a top umbrella-shaped load. It is insulated with a grounding unit and a base insulator, and the insulation tower and top umbrella-shaped load serve as the main radiator of the antenna to achieve radiation transmission. The grounding unit reduces the ground loss resistance and ensures the antenna radiation efficiency. The main performance parameters of single-tower umbrella-shaped antenna are given in

Table 2. Main performance parameters of single-tower umbrella-shaped antenna.

The height of Single-Tower Antenna (m)	280
Center frequency (kHz)	100
Antenna efficiency (%)	76.3%
Input resistance (Ω)	8.08
Ground Loss Resistor Ω)	1.788
Static capacitance (nF)	7.28
Antenna bandwidth (kHz)	3.8
Power capacity (kW)	2929
Effective height (m)	189.4

, Figure 5 and Figure 6.

5.2. Transmitter System

The transmitter adopts a solid-state eLoran transmitter based on a magnetic pulse amplifier, which uses 64 half-cycle pulse generators to synthesize the power. The peak power output can reach 2000 kW, which is mainly composed of the local time reference, broadcasting control, power generation, power network, and main distribution units. The main performance parameters of the transmitter are given in

Table 3. Main performance parameters of transmitter.

Center frequency (kHz)	100
Broadcasting control accuracy (ns)	30
Peak effective power (kW)	2000
Transmitter efficiency (%)	70%
Signal coverage (km)	800
Timing accuracy (before differentia) (μs)	1
Timing accuracy (after differentia) (ns)	100

and Figure 7.

5.3. Time Frequency System

The deviation between the time signal generated by the eLoran Time Service station’s time frequency system and UTC (NTSC) is maintained within 5 ns. Multiple methods, such as fiber optic, GNSS CV/PPP, and satellite bidirectional comparison links, are integrated and complementary to achieve the traceability of time frequency signals to UTC (NTSC). The time and frequency system is equipped with three hydrogen atomic clocks to provide the main clock frequency signal of the system. Seamless switching between the main and backup is adopted to ensure the consistency and stability of the output time and frequency signals.

6. Signal System

The eLoran signal of the high-precision ground-based time service system adopts the standard Loran-C system, with a carrier frequency of 100 kHz.

The transmitter uses Eurofix data link and “9th Pulse Modulation” technology to modulate the encoded data information onto a standard Loran-C signal, forming an eLoran signal. Among them, the Eurofix data link is mainly used for modulating the time code, leap second, system time deviation, system state, and other data, while “9th Pulse Modulation” is mainly used for modulating the differential enhancement and emergency communication data. A schematic diagram of the modulated pulse is shown in Figure 8.

6.1. Eurofix Principles

The Eurofix data link uses 3S-PPM (Tri-State Pulse Position Modulation); 3S-PPM modulates pulses 3–8 in each pulse group. The modulated signal consists of a 1 μs pulse transmission phase shift relative to the unmodulated pulse. The three possible states of modulation are given in

Table 4. States of 3S-PPM modulation.

Pulse State	Transmission Time Minus Time of Reference Pulse (μs)	Indication
Advanced pulse	−1	−
Prompt pulse	0	0
Delayed pulse	+1	+

and Figure 8 [18].

In the 3S-PPM pulse group, the numbers of advanced and delayed pulses in a channel are equal. The modulation of six pulses in a pulse group will produce 141 possible modulation modes, of which 128 modes represent valid data. The 3S-PPM mode is shown in Figure 9. Each of the 128 effective modulation modes uniquely represents a seven-bit binary data block.

The frame length of Eurofix message design is 210 bit, and a frame requires 30 GRIs to complete data modulation. The information body occupies 56 bit, the CRC code occupies 14 bit, and the RS code occupies 140 bit.

6.2. Ninth Pulse Modulation Principles

The ninth pulse technology for data transmission has a 32-ary PPM scheme. An additional pulse is inserted after the eighth pulse of the traditional LORAN pulse group. The thirty-two state PPM is used to represent the time delay from the zero symbol offset. Each signal position is defined by a five bit symbol, the first two signals are roughly delayed, and the last three digits indicate a small delay. In this way, the data transmission rate is 5 bits/GRI.

The zero sign offset of the additional pulse is 1000 µs. The delay time of the remaining 31 symbols relative to the zero symbol is shown in microseconds. The ideal delays are given by the formula:

$$d_i=1.25mod\left(i,8\right)+50.625floor\left(i/8\right)$$

(1)

The actual delays are the ideal values shifted to coincide with the ticks of a 5 MHz clock [17].

Figure 10 shows the ninth pulse modulation given in Ref. [30]. All the modulation messages and consist of three components: 4 bit for the type, 41 bit for the payload, and 75 bit for the parity component. The message sending rate is 5 bits/GRI. The time length of the messages is 24 GRI (maximum of approximately 2.4 s).

6.3. Signal System Test

According to the requirements of the high-precision ground-based time service system test, this article uses an eLoran signal simulator to conduct the compatibility testing of the ninth pulse modulation system signal with the existing receivers (UN152B).

The simulator generates the ninth pulse system signal by overlaying it with the existing Eurofix system 0 μs delay pulse or 151.875 μs delay pulse. This is shown in Figure 11 and Figure 12.

Through experimental observation, as shown in Figure 13, it can be seen that eLoran receivers can operate normally, and receivers with a Eurofix demodulation function can continuously output and correctly demodulate the data. Based on the above experimental data, it can be basically determined that the eLoran simulator generates a ninth pulse system signal superimposed on the existing Eurofix system, which does not affect the normal operation and performance of the existing receiver.

This experiment can demonstrate that the signal system of the high-precision ground-based time service system of eLoran system is compatible with the existing eLoran receivers.

7. Summary

eLoran is an internationally standardized positioning, navigation, and time service system. It is mainly based on the traditional Loran-C and combines technologies, such as all solid-state transmitters, data links, integrity monitoring, differential enhancement, and digital reception. It utilizes the characteristics of the stable phase and the precise prediction of ground wave propagation in long wave signals to achieve high reliability, accuracy, completeness, and the continuity of services. It has the ability to form a mutual backup with the GNSS system such that they can complement and enhance each other.

The combination of eLoran and GNSS has become a relatively safe time service model that has been widely adopted internationally. 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. Therefore, it is necessary to build multiple integrated time service resources to ensure the reliability of the time service. The eLoran time service system is a radio technology that China completely independently controls. In addition, eLoran and GNSS have many different physical characteristics, including that they are, respectively, ground- and satellite-based, had a low and high frequencies, higher and lower signal levels, perform regional and national control, etc. It is unlikely that the two will experience the same mode of failure at the same time. The application of satellite ground integration, mutual supplementation, and backup will leverage their respective advantages, providing more secure and reliable time services.

The eLoran system is an important national time service system in China, which can effectively improve the safety and reliability of major projects and important infrastructure, and enhance China’s economic operation and national security guarantee capabilities.