Additional Secondary Factor Research Articles

Map-aiding of the additional secondary factors isosurface by the spline approximation method as a condition of improving the e-Loran observations accuracy

The benefit of the Loran ground-based radio navigation system as an alternative to global navigation satellite systems is argued. The motivation of searching for a duplicate variant is caused by the necessity to ensure the reliability of positioning as a subject of countering spoofing and jamming risks when implementing the concept of cybernetic awareness on water transport. Hypothetically, the possibility of using e-Loran on the Northern Sea Route as a backup navigation system is considered. The experience of operating pilot projects of the enhanced Loran in the issue of the predicted potential of positioning accuracy is analyzed. The optimal method of compensation for the system error of geolocation based on the use of additional secondary factors is investigated. Taking into account the principle of the d-Loran functional, based on comparing the measured values of the radio signal passage delay with published analogues for transmitting clarifications to marine consumers, the necessity of using integrity as a criterion for a confidence assessment of the navigation information processing reliability has been determined. A critical analysis of the attempts effectiveness to use the linear interpolation method to calculate intermediate representative values of the delay in the radio signal transmission in order to simulate the map of additional secondary factors is performed. The hypothesis of the prospects for the isosurface corrections synthesis based on B-spline approximation is put forward. The South Korean experiment of synthesizing a chart of additional secondary factors with measured indicators of the radio signal propagation time delay in nanoseconds from the Pohang transmitting station has been repeated at the correct algorithmic level. In order to demonstrate the practical feasibility of the spline algorithm, a computer visualization of a cartographic fragment of additional secondary factors of Yongil Bay is performed. It is suggested that three-dimensional representation of the additional secondary factor for situational perception by the Officer on watch of map-aided correction as a process in alternative positioning in order to increase the reliability of location control through visual evaluation of proper use of corrections of the e-Loran differential variant. The prospect of using the developed package of applied Pascal-programs with implementations on display-type monitors as intellectual support for decision-making by the navigator in a posteriori assessment of the observation accuracy due to the visual representation of the correction field is noted.

Study on the regional ASF prediction method based on the ordinary kriging interpolation

The eLoran system functions as a robust backup to the Global Navigation Satellite System (GNSS), providing substantial signal power, robust anti-jamming capabilities, and an easily maintainable ground system. Nonetheless, the propagation time delay of the system, primarily driven by the Additional Secondary Phase Factor (ASF), significantly influences the positioning accuracy. Compensating for ASF can effectively enhance the positioning performance. Traditional propagation delay theories frequently yield substantial discrepancies between the predicted and measured values in regions characterized by extended propagation distances and varying topographic features. To address this issue, we conducted ASF measurements within a selected test area using a limited number of measurement points. We employed the ordinary Kriging interpolation method to predict ASF values across the entire test area, and used cross-validation to validate our predictions. The results confirmed the accuracy and effectiveness of the Kriging interpolation algorithm in predicting ASF values within specific regions. The cross-validation demonstrated that the errors remained within acceptable ranges. Furthermore, we applied Ordinary Kriging, Inverse Distance Weighting, and Radial Basis Function Interpolation methods to evaluate the positioning accuracy of the test area before and after ASF correction. Compared to other methods, using the Ordinary Kriging interpolation algorithm for predicting ASF values resulted in a corrected positioning accuracy of up to 68.8 m at various test locations. This approach effectively resolves the challenge of low accuracy in theoretical calculations in complex environments. By utilizing ordinary Kriging interpolation, we required measurements of only a few ASF values within a specific region to create an ASF correction map, addressing the challenges related to inaccurate theoretical calculations in complex pathways and avoiding time-consuming and labor-intensive large-scale measurements. The results of this study offer valuable theoretical support for improving the accuracy of land-based navigation systems.

Demonstration of the Feasibility of the Korean eLoran System as a Resilient PNT in a Testbed

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.

First Demonstration of the Korean eLoran Accuracy in a Narrow Waterway Using Improved ASF Maps

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.

Method for Loran-C Additional Secondary Factor Correction Based on Neural Network and Transfer Learning

An additional secondary factor (ASF) correction method is proposed to improve the accuracy of Loran-C navigation and positioning based on a combination of backward propagation neural network (BPNN) and transfer learning. The BPNN is used to train a network model based on the theoretical ASF data source calculated by the integral equation method. Although the trained BPNN model fitted well with the theoretical ASF results, it is not sufficient for ASF prediction in the actual environment involving uncertain topography and geology. To compensate for this deficiency, transfer learning is employed to further finely calibrate the BPNN model against the ASF measurements. By using the advantages of BPNN and transfer learning, the proposed method achieved higher accuracy than the purely theoretical method. Measurement results are used to validate the superiority of the proposed ASF correction method to the theoretical one. Particularly, the mean absolute error of the ASF prediction results decreased from 2.02 to 0.29 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> s after introducing the transfer learning in the trained BPNN model.

Laboratory experiments on CO2 gas exchange with wave breaking

AbstractTheCO2gas transfer velocity () at air-sea interface is usually parameterized with the wind speed, but to a great extent is defined by waves and wave breaking. To investigate the direct relationship betweenand waves, laboratory experiments are conducted in a wind-wave flume. Three types of waves are forced in the flume: modulational wave trains generated by a wave maker, wind waves with 10-meter wind speed ranging from 4.5 m/s to 15.5 m/s, and (mechanically-generated) modulational wave trains coupled with superimposed wind force. The wave height and wave orbital velocity are found to be well correlated withwhile wind speed alone can not adequately describe. To reconcile the measurements, non-dimensional equations are established in which gas transfer velocity is expressed as a main function of wave parameters and an additional secondary factor to account for influence of the wind.

GPS and eLoran Integrated Navigation for Marine Applications Using Augmented Measurement Equation Based on Range Domain

Vulnerability to the interference of GPS (Global Positioning Systems) is one of the main obstacles to performing safe marine navigation. As eLoran (Enhanced Long Range Navigation), considered as the backup navigation system of GPS, has been developed specifically in several countries, the need for GPS / eLoran integrated navigation has emerged. This study represents the integrated algorithm for GPS and eLoran to overcome the vulnerability of GPS. The main research contents are as follows: a) Formula for integrated positioning of GPS / eLoran based on range domain is suggested. b) Essential ASF (Additional Secondary Factor) estimation filter to implement positioning by eLoran is presented in terms of real-time processing instead of employing ASF grid map, which should be pre-made at a local area and be distributed to marine users. c) Procedures for determining HPL (Horizontal Protection Level) are represented so that the reliability of the integrated navigation can be investigated. d) The marine test results are presented to validate the suggested methods based on the Loran-C signal. Test results show the integrated GPS and eLoran by the suggested methods can complement each other in terms of accuracy and reliability.

Universal Kriging for Loran ASF Map Generation

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.

An Analysis of Positioning Method in AIS Ranging Mode

Position, navigation and timing (PNT) resiliency is required in e-Navigation strategy, which is proposed by International Maritime Organization (IMO). The PNT is mainly provided by global navigation satellite system (GNSS) at sea for now. It is necessary to introduce an alternative position system using a different technique, as a complement to existing GNSS. The automatic identification system (AIS) is utilized far and wide for collision avoidance based on radio-communications at sea and is mandated by IMO. However, position information of AIS depends on GNSS currently. If AIS can estimate the position by itself independently of GNSS, it would be of great significance to ensure resilient PNT. AIS base stations transmit the ranging signals, while vessels monitor the signals and then calculate the position. This idea is called AIS ranging-mode (R-mode) in e-Navigation. AIS R-mode positioning system is a potential candidate system to resilient PNT in the World Wide Radio Navigation System (WWRNS). This paper focuses on positioning methods used in AIS R-mode. Two position estimation methods are investigated. One is a direct positioning method according to geodetic surveying. The other is a positioning method according to map projecting. Principles and mathematical models of these methods are presented respectively. Besides the theoretical analysis, the performance of these two positioning methods using in AIS R-mode is also evaluated by simulation both with the ideal condition and with the additional secondary factor (ASF).

Novel Multichain-Based Loran Positioning Algorithm for Resilient Navigation

Intentional high-power global positioning system 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, South 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 based ASF correction is not applicable in Northeast Asia because several transmitters are not synchronized to universal time coordinated. Thus, we propose a time difference of arrival 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 multichain-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.

Research on the Additional Secondary Phase Factor for Automatic Identification System Signals Transmitted over a Rough Sea Surface.

This paper investigates the Additional Secondary Phase Factor (ASF) characteristics of Automatic Identification System (AIS) signals spreading over a rough sea surface. According to the change of the ASFs for AIS signals in different signal form, the influences of the different propagation conditions on the ASFs are analyzed. The expression, numerical calculation, and simulation analysis of the ASFs of AIS signal are performed in the rough sea surface. The results contribute to the high-accuracy propagation delay measurement of AIS signals spreading over the rough sea surface as, well as providing a reference for reliable communication link design in marine engineering for Very High Frequency (VHF) signals.

The Additional Secondary Phase Correction System for AIS Signals.

This paper looks at the development and implementation of the additional secondary phase factor (ASF) real-time correction system for the Automatic Identification System (AIS) signal. A large number of test data were collected using the developed ASF correction system and the propagation characteristics of the AIS signal that transmits at sea and the ASF real-time correction algorithm of the AIS signal were analyzed and verified. Accounting for the different hardware of the receivers in the land-based positioning system and the variation of the actual environmental factors, the ASF correction system corrects original measurements of positioning receivers in real time and provides corrected positioning accuracy within 10 m.

Development of an Automatic Identification System Autonomous Positioning System.

In order to overcome the vulnerability of the global navigation satellite system (GNSS) and provide robust position, navigation and time (PNT) information in marine navigation, the autonomous positioning system based on ranging-mode Automatic Identification System (AIS) is presented in the paper. The principle of the AIS autonomous positioning system (AAPS) is investigated, including the position algorithm, the signal measurement technique, the geometric dilution of precision, the time synchronization technique and the additional secondary factor correction technique. In order to validate the proposed AAPS, a verification system has been established in the Xinghai sea region of Dalian (China). Static and dynamic positioning experiments are performed. The original function of the AIS in the AAPS is not influenced. The experimental results show that the positioning precision of the AAPS is better than 10 m in the area with good geometric dilution of precision (GDOP) by the additional secondary factor correction technology. This is the most economical solution for a land-based positioning system to complement the GNSS for the navigation safety of vessels sailing along coasts.

Issues in Hypertriglyceridemic Pancreatitis

Hypertriglyceridemia (HTG) is a well-established but underestimated cause of acute pancreatitis and recurrent acute pancreatitis. The clinical presentation of HTG-induced pancreatitis (HTG pancreatitis) is similar to other causes. Pancreatitis secondary to HTG is typically seen in the presence of one or more secondary factors (uncontrolled diabetes, alcoholism, medications, pregnancy) in a patient with an underlying common genetic abnormality of lipoprotein metabolism (familial combined hyperlipidemia or familial HTG). Less commonly, a patient with rare genetic abnormality (familial chylomicronemic syndrome) with or without an additional secondary factor is encountered. The risk of acute pancreatitis in patients with serum triglycerides >1000 and >2000 mg/dL is ∼ 5% and 10% to 20%, respectively. It is not clear whether HTG pancreatitis is more severe than when it is due to other causes. Clinical management of HTG pancreatitis is similar to that of other causes. Insulin infusion in diabetic patients with HTG can rapidly reduce triglyceride (TG) levels. Use of apheresis is still experimental and better designed studies are needed to clarify its role in the management of HTG pancreatitis. Diet, lifestyle changes, and control of secondary factors are key to the treatment, and medications are useful adjuncts to the long-term management of TG levels. Control of TG levels to 500 mg/dL or less can effectively prevent recurrences of pancreatitis.

A Design of LORAN Disciplined Oscillator

This article presents the design of long range navigation (LORAN)-disciplined oscillator (LDO), employing the timing information of the LORAN system, which was developed as a backup system that corrects the vulnerability of the global positioning system (GPS)-based timing information utilization. The LDO designed on the basis of hardware generates a timing source synchronized with reference to the timing information of the LORAN-C receiver. As for the LDO-based timing information measurement, the Kalman filter was applied to estimate the measurement of which variance was minimized so that the stability performance could be improved. The oven-controlled crystal oscillator (OCXO) was employed as the local oscillator of the LDO. The controller was operated by digital proportional-integral-derivative (PID) controlling method. The LDO performance evaluation environment that takes into account the additional secondary factor (ASF) of the LORAN signals allows for the relative ASF observation and data collection using the coordinated universal time (UTC). The collected observation data are used to analyze the effect of ASF on propagation delay. The LDO stability performance was presented by the results of the LDO frequency measurements from which the ASF was excluded.

Suitability of Low-Frequency Navigation Systems for Artillery Positioning in a GNSS Denied Environment

In an environment where Global Positioning System (GPS) jamming is present, the Dutch army faces all sorts of challenges in such a deployment area, e.g., for the determination of gun locations. Nowadays, an Inertial Navigation System (INS) is used as back-up positioning system. However, to use this back-up tool within specified accuracy limits, additional procedures are needed that limit deployment. An alternative positioning method such as a low-frequency navigation system could be a solution to these problems. An example of a low-frequency navigation system that is currently operational in certain regions is eLORAN (enhanced LOng RAnge Navigation). Hypothetically, a system like this could be realised in a similar way for positioning, navigation and timing solutions for large scale military operations. That is if the performance of eLORAN meets the accuracy requirements. In this paper, eLORAN performances are examined using experiments conducted at a military heath land area in the Netherlands. Moreover, eLORAN Additional Secondary Factor (ASF) correction maps have been computed for that particular area, based on differences in position between Real-Time Kinematic (RTK) GPS and eLORAN measurements at a fixed number of points in that area. Different interpolation methods are used to create these ASF maps. The maps, obtained from static measurements, are used to enhance eLORAN's performance in dynamic measurements using a military platform. The content of this paper has been presented during the European Navigation Conference 2011 in London.

Equivalent Ground Conductivity Inversion in Maritime ASF Correction

This letter presents an efficient method for the maritime Loran-C additional secondary factor (ASF) correction based on equivalent ground conductivity inversion. Using the proposed method, the accuracy of Loran-C system on maritime positioning, navigation, and timing (PNT) can be improved significantly with a limited number of surveys. Comparison with measured ASF results shows a root-mean-square error (RMSE) of less than 100 ns in most areas. key words: Loran-C, ASF, equivalent ground conductivity

9930M국 로란-C 신호를 이용한 내륙 ASF 측정 연구

The LORAN system had been used widely and it was an essential navigation aid for ships in the ocean until the GPS is adopted actively. In particular, it was essential functionality for the ships to sail the oceans. According to the advancement of industry, however, the current accuracy of traditional Loran is insufficient for the utilization of harbour approach, land navigation, and the field of survey and timing. Therefore it is necessary that the study on the improvement of the positioning accuracy of Loran. The one of the improving methods is to measure and compensate the propagation time delay between the transmitter and user`s receiver, which is called as additional secondary factor (ASF). In this study, we measured the ASF between the Pohang master transmitting station (9930M) and four points where locate within 33 km apart from the transmitting station, using the measuring technique of the absolute time delay without a time of coincidence (TOC) table. As the result of measurement, the ranging error caused by the propagation delay was about 210 m at 33 km, however it can be reduced up to 40 m with ASF compensation.

Developing and Validating the Loran Temporal ASF Bound Model for Aviation

For Loran to be used for aviation navigation, it must meet strict integrity requirements. A key integrity requisite is the ability to bound position errors to a high degree of confidence. This, in turn, means bounding range domain errors and variations. One major source of variation is the temporal variation of propagation delay known as the additional secondary factor (ASF). This paper presents the development and validation analysis of the model for bounding the temporal variation of ASF. It examines the performance of the range and position domain bounds using recently collected data. For each signal, the magnitude and distribution of the bounds into correlated and uncorrelated components is assessed. Position domain bounds and errors are also analyzed. The results show the bounds to be sufficient with exceptions attributable to non-ASF related error sources. The objective is to form the foundation for proving the integrity of the ASF bound methodology.

Time trends of mortality from Crohn's disease and ulcerative colitis

The present study served to test whether Crohn's disease and ulcerative colitis showed similarities in the temporal variation of their mortality rates among different countries. Mortality data from 21 different countries between 1951 and 2005 were analysed, including Argentina, Australia, Austria, Belgium, Canada, Chile, Denmark, England, Finland, France, Germany, Italy, Japan, Mexico, Netherlands, Scotland, Spain, Sweden, Switzerland, Taiwan and USA. The age-specific death rates of each individual country, as well as the average age-specific rates of all countries, were plotted against the period of death. Death rates from ulcerative colitis were initially 6-fold higher than those of Crohn's disease. Mortality from ulcerative colitis decreased continuously during the past 50 years. Mortality from Crohn's disease increased from 1951 to 1975 until reaching a similar level as mortality from ulcerative colitis. Since then the death rates of both diseases have followed a parallel time course. A same type of behaviour was found in the time trends of each individual age-group. The data from most countries revealed similar temporal patterns. The relationships between the temporal changes of mortality from Crohn's disease and ulcerative colitis might suggest the presence of one primary risk factor responsible for the occurrence of both diseases, and at the existence of one additional secondary risk factor, responsible for the expression of Crohn's disease alone.