After a series of intentional Global Positioning System (GPS) jamming attacks impacted a large area of South Korea, the Ministry of Oceans and Fisheries of South Korea considers long-range navigation (Loran) and enhanced Loran (eLoran) as a maritime backup navigation system. Despite its robustness to signal jamming, the positioning accuracy of Loran/eLoran is lower than that of GPS. Because the signal delay due to the land path, which is called the additional secondary factor (ASF), is the largest unknown component of Loran/eLoran, it is necessary to account for temporal and spatial ASF errors to ensure high accuracy. The generation of ASF maps based on ASF survey data in a service area is the most convenient way to mitigate spatial ASF error, but the quality of ASF maps depends on the applied interpolation algorithm. It is desirable to generate high-quality ASF maps based on ASF measurements at only a few survey points, because extensive ASF surveys are expensive and time-consuming and require considerable effort. This paper proposes kriging methods for satisfying this objective and shows their superior performance during a field test in Incheon, Korea. In particular, universal kriging with the proposed drift model showed a better performance than linear interpolation, inverse distance weighing, and ordinary kriging when the test vehicle was close to a coastline. The positioning accuracy with the ASF maps generated by the proposed universal kriging along a 5-km route during the field test was 25.24 m (95%). The land vehicle used for the test experienced significant signal-to-noise ratio (SNR) degradation owing to the noise caused by its engine. A vessel without such SNR degradation is expected to achieve higher accuracy.

Intentional high-power GPS jamming is a significant threat for ships in the South Korean waters and has occurred multiple times in recent years. The South Korean government intends to utilize the existing Long Range Navigation (Loran) infrastructure to provide a backup navigation capability to maritime users. However, the observed accuracy of a conventional Loran positioning fix during a field test in Incheon, Korea, was 592.88 m, far from the 20 m accuracy that the South Korean government tries to provide. The largest error source for Loran is the additional secondary factor (ASF) delay. A conventional time of flight (TOF)-based ASF correction is not applicable in Northeast Asia because several transmitters are not synchronized to Universal Time Coordinated (UTC). Thus, we propose a time difference of arrival (TDOA)-based ASF correction method that is applicable to the existing Loran signals in Northeast Asia. The demonstrated accuracy with this correction was 32.12 m when using a single Loran chain for positioning. In order to utilize the full capability of the observed signals from the five transmitters of two Loran chains in the region, we further propose a novel multi-chain-based Loran positioning algorithm. By applying this algorithm together with the ASF correction method, we achieved a 15.32 m accuracy with 100% position availability. This result shows the potential of the existing Loran transmitters in Northeast Asia to provide a reliable and accurate backup maritime navigation service.

The vulnerabilities of global navigation satellite systems (GNSSs) to radio frequency jamming and spoofing have attracted significant research attention. In particular, the large-scale jamming incidents that occurred in South Korea substantiate the practical importance of implementing a complementary navigation system. This letter briefly summarizes the efforts of South Korea to deploy an enhanced long-range navigation (eLoran) system, which is a terrestrial low-frequency radio navigation system that can complement GNSSs. After four years of research and development, the Korean eLoran testbed system has been recently deployed and is operational since June 1, 2021. Although its initial performance at sea is satisfactory, navigation through a narrow waterway is still challenging because a complete survey of the additional secondary factor (ASF), which is the largest source of error for eLoran, is practically difficult in a narrow waterway. This letter proposes an alternative way to survey the ASF in a narrow waterway and improve the ASF map generation methods. Moreover, the performance of the proposed approach was validated experimentally.

Global navigation satellite system (GNSS) and localization technology have become highly compact and effective. Although GNSS has been proven to be vulnerable to jamming when the signal strength is weak, most vehicles rely on it for navigation. Compared to GNSS, enhanced Loran (eLoran) is more resistant to jamming and is recommended as a realistic alternative to GNSS. Therefore, South Korea embarked on an eLoran testbed development project to provide resilient navigation information for maritime users. The primary goal of this project was to demonstrate that the system can provide a 20-m position accuracy to the maritime user within a 30-km coverage of a differential Loran station. In this paper, we describe the development of the eLoran testbed implementation and the performance of its preliminary experimental results. To confirm the feasibility of the eLoran testbed performance, we installed an eLoran transmitter and two differential Loran stations and subsequently conducted a zero-baseline performance test at the Pyeongtaek differential Loran station with a new eLoran signal. We also conducted a survey to collect conventional Loran signals along the western and southern coastlines of South Korea. Our experimental results show that the eLoran system met the performance requirements of this project, as well as that the signal to noise ratio of the eLoran signals changed over the topographic characteristics of the propagation path.

The Global Positioning System (GPS) has become the most widely used positioning, navigation, and timing system. However, the vulnerability of GPS to radio frequency interference has attracted significant attention. After experiencing several incidents of intentional high-power GPS jamming trials by North Korea, South Korea decided to deploy the enhanced long-range navigation (eLoran) system, which is a high-power terrestrial radio-navigation system that can complement GPS. As the first phase of the South Korean eLoran program, an eLoran testbed system was recently developed and declared operational on June 1, 2021. Once its operational performance is determined to be satisfactory, South Korea plans to move to the second phase of the program, which is a nationwide eLoran system. For the optimal deployment of additional eLoran transmitters in a nationwide system, it is necessary to properly simulate the expected positioning accuracy of the said future system. In this study, we propose enhanced eLoran accuracy simulation methods based on a land cover map and transmitter jitter estimation. Using actual measurements over the country, the simulation accuracy of the proposed methods was confirmed to be approximately 10%–91% better than that of the existing Loran (i.e., Loran-C and eLoran) positioning accuracy simulators depending on the test locations.

With the increasing utilization of location information, attempts to improve the safety of absolute positioning coordinates, which have depended on global navigation satellite systems (GNSSs), such as the Global Positioning System (GPS), are underway. Among these, enhanced long range navigation (eLoran) is the most technically developed system. In Korea, related technologies have been developed since 2016, and a testbed for eLoran performance evaluation, which is currently in operation as a pilot service, was completed in 2021. We analyze the position accuracy of the eLoran pilot service to use it as an alternative when GNSS usage is challenging within Korea’s eLoran testbed. We evaluated the accuracy of the absolute position using the eLoran system by sailing up to 160 km away from the Incheon testbed transmitter according to four navigation stages (inland waterway, port approach, coastal, and ocean) classified by the International Maritime Organization (IMO). To validate the eLoran positioning performance in which an additional secondary factor (ASF) map is not provided, we propose a differential GPS (DGPS) position-based ASF estimation technique. Based on this study, Korea’s eLoran system can calculate the absolute position with an accuracy of approximately 15 m with 95% probability at the port-approach stage.