Loran-C Signal Research Articles

ロランC時間差測定値の修正に関する考察-II : ECDによる伝搬時間の修正

The non-homogeneous earth surface causes warpage of the Loran-C signal and induces a bias error on the measurements for the time of arrival. The purpose of this paper is to decrease this error, using the correction value calculated from the Envelope to Cycle Difference of the received pulse signal itself. In our previous study, we assumed the ECD variation model for the mixed propagation path. To appreciate that model and to indicate the advantage of the correction values, the experiment was carried out in the domain of the 9970 Loran-C chain. Using the results of this experiment, we can describe as follows, 1. The ECD had a correspond value at the same position, in proof of repeatability. 2. There were significant changes in the ECD values over the land propagation. 3. The fixed position by the corrected time differences from above method were closer to the fixed position by NNSS than case of non-corrected one.

LORAN-C Positioning Errors Caused by Scattering from Wires Above the Earth

In this paper, the scattering of a LORAN-C navigation signal by a powerline is calculated. The results of the derivation can be used to predict positioning errors for land-based applications of LORAN-C, such as vehicle-location schemes. The only assumptions are that the line spacing is small compared to its height and that the LORAN-C signal is narrow band, and not incident at a grazing angle. From this general result, a very simple theory for predicting positioning errors is derived which is valid for most parameters of interest. It is found that, when a LORAN-C receiver is located near a powerline, position errors of 1 km are possible. This value is in accord with published experimental results. Further, it is shown that position errors are significantly greater when the LORAN-C signal approaches the transmission line at a small angle.

The Evolution of Loran-C Coverage

This paper addresses the evolution of Loran-C signal coverage worldwide with comments on where such coverages may be expected to be implemented in the near future. Also addressed are privately operated Loran-C type systems and new system applications for Loran-C such as the Suez Canal Vessel Traffic System which may provide impetus for further expansion.

Weather Effect on LORAN-C Propagation

A few simple and fundamental theories are offered to help explain some observed weather effects on Loran-C signal. For instance, Loran-C data for the U.S. East Coast in winter indicate that the dry term of the refractive modulus, N-dry, behaves approximately as an exponential function of height. This finding of exponential behavior is based on the physics of propagation together with Loran-C data and not on a profile of N with height. Suggestions for future work are given so that these theories can be verified in general.

The Theory of Loran-C Ground Wave Propagation-A Review

The essential portion of a received Loran-C signal of 100 kHz consists of groundwave only. The phase velocity of the groundwave which is related to TOA (time of arrival) and TD (time difference) values depends on various parameters, such as ground conductivity (or surface impedance), terrain variation, and the refractive index profile of the atmosphere. In order to calculate exactly the phase velocity, signal strength, etc., it is necessary to solve the electromagnetic wave equation, rigorously incorporating all of the above mentioned parameters. Since this is an impossible mathematical task, several approximate methods with various degrees of complexities had been sought by various authors in the past. Among these approximate methods or propagation models, the following are notable: (1) homogeneous flat earth model of Sommerfeld and Norton, (2) homogeneous spherical earth model of Watson, Van der Pol and Bremmer (the last two authors included also the effect of inhomogeneous atmosphere), (3) Millington's empirical model for an earth with mixed surface conductivities for computing signal strength, (4) Pressey and his associates' empirical model for earth with mixed surface conductivities for computing phase, (5) Wait's multisection spherical earth model, (6) Hufford's integral equation approach to the propagation over an irregular surface, and (7) Jollier's integral equation approach which is a generalization of Hufford's method with the aid of a computer. This paper presents a general overview of these methods and their limitations with a special emphasis directed towards Loran-C.

Hard Limiting and Sequential Detection Applied to Loran-C

The combination of an antenna, a 100 kHz bandpass filter, a hard limiter, and a sequential detector can supply highly accurate Loran-C data to a digital processor, even under low signal-to-noise-ratio conditions. For such a simple, low-cost receiver, calculations are given for the accuracy of the envelope and phase tracking of the Loran-C signal as a function of the signal-to-noise (Gaussian and atmospheric) ratio, averaging time, and radian speed of the observer with respect to the transmitter. Mentioned are the quasi-noise censoring effects of the hard limiter. Besides the Loran-C application, the hard limiter-sequential detector system can in general be applied for low-cost, synchronous signal detection under poor signal-to-noise ratio.

ロランC受信波形と時間差測定法の研究

The Loran-C system is one of the more accurate operational methods of continuous position. But Loran-C position accuracy depends upon the signal-noise ratio and measurement errors. A tracking method of several zero cross points is one of the measurement method of Loran-C signal to improve the signal-noise ratio and to discriminate between ground-wave and sky-wave with intervals of Loran-C cycles. Ship tests have demonstrated the performance of this method and have established to provide accurate continuous position.