Theory Of Ground-wave Propagation Research Articles

Significant wave height extraction using a low‐frequency HFSWR system under low sea state

High-frequency surface wave radar (HFSWR) systems are usually located on the coast and measure electromagnetic backscatter from the sea in the 3-30 MHz band. Based on the theory of ground wave propagation, the detection distance of a low-frequency HFSWR system could be longer that of a high-frequency one. Therefore, a low-frequency HFSWR system is more suitable to extract the significant wave height at a distance. The Barrick's algorithm-based significant wave height extraction method is widely used, and the accuracy of the method has been validated by a large number of experiments. However, it is important to note that under the low sea state, the wave conditions of the data recorded by a low-frequency HFSWR system may not satisfy the wave height condition required in Barrick's algorithm. Hence, the algorithm may produce unreliable results. To address this issue, a method based on Barrick's algorithm, a new non-linear model, and the radial basis function neural network is proposed. Simulations on real-world data collected on the coast of the Tianjin from 1 May 2015 to 25 May 2015 support the method developed.

Effects of noise and system~s parameters on the tomographic image reconstruction of ground surfaces

In a recent paper by the author (for original paper see ibid., vol. 48, p. 1384-1392 (2000)) it has been shown that using the measurements of the scattered fields produced by an isolated ground surface from transmitting antennas in different azimuthal directions, the distribution of the dielectric properties of the isolated ground surface can be reconstructed using well-established ground wave propagation theory. In practice, the operation of a system using this technique is subject to measurement errors and noise, and it also depends on a number of parameters including the operating frequency, the number of antennas used, and their locations. These effects are, thus, studied and the results of the reconstruction errors are presented.

Application of radio ground-wave propagation theory to the tomographic imaging of ground surfaces

Radiowave propagation over ground has been historically studied for predicting the radiated fields when the ground properties are known and the field has been well covered over the past hundred years. The theory of ground-wave propagation is applied to the inverse problem of the tomographic imaging of ground surfaces. After the inverse problem is formulated, an iterative technique for solving it is illustrated. This is based on the minimization of the cost function of the measured and estimated scattered fields produced by the surface under investigation in isolation. Numerical simulation results are presented for the reconstruction of four different ground features surrounded by sea water namely: (1) dry ground; (2) wet ground; (3) dry-wet mixed ground; and (4) dry ground with a central water pool. It has been demonstrated that the technique is able to reconstruct the distributions of normalized surface impedance of the isolated surfaces and, hence, their images. The iterative process can converge with a small number of iterations using the normalized surface impedance of sea water as the initial guessed values. However, better images can be produced using a priori information. This study thus illustrates a new application of ground wave propagation theory with possible applications in ground surface mapping, remote sensing, target positioning and monitoring, and navigation.

Ground-wave attenuation function for a Gaussian modulated carrier signal

The time-harmonic theory of ground-wave propagation is generalised to the time domain for a Gaussian modulated carrier signal. The model is a vertical electric dipole located on a flat imperfectly conducting earth. It is shown that the radiated field is characterised by a modified Sommerfeld-type attenuation function with a time-dependent ‘numerical distance’.

Electric Field Propagation in the Proximal Region

A model for electric field propagation above planeearth in the region from 1 meter to 10 km from the source, specifically tailored to permit rapid quantitative solutions of EMI (electromagnetic interference) propagation problems, is obtained from classical ground-wave propagation theory extended to include induction fields. The solution is presented in the form of attenuation and height-gain curves for both vertical and horizontal polarizations and covers the spectrum from dc to 1 GHz. The quantitative effects of soil conditions are investigated by comparing propagation over good earth with propagation over the extremes of dry sand and sea water.

A comparison of Millington's method and the equivalent numerical distance method with the theory of ground-wave propagation over an inhomogeneous earth

The paper presents a comparison of Millington's method and the equivalent numerical distance method with theory. It is shown that Millington's method may be used in most practical problems of groundwave propagation. For overland paths the errors of this method are very small; for land-sea paths they may be larger, up to about 2.7 dB for 2-section paths, up to about 5.5 dB for 3-section paths, and up to about 2.5 dB for each land-sea boundary for very long paths. In most cases the equivalent numerical distance method shows considerable errors. This method may be used for paths representing small numerical distances, especially for overland paths. It may also be applied to longer paths when the differences between the electrical parameters of the sections are insignificant.

The use of equivalent secondary sources in the theory of ground-wave propagation over an inhomogeneous earth

The paper deals with the propagation of a vertically polarized ground-wave over an inhomogeneous spherical earth. On the basis of approximate boundary conditions and a specially chosen auxiliary function in Green's theorem, the integral equation for the attenuation function is derived. A general method of solving this integral equation by numerical techniques for paths composed of a number of homogeneous sections is discussed. It is proved that the result is in agreement with the reciprocity theorem.The actual field is shown to be the superposition of the fields generated by the primary source and by secondary sources distributed along the path. It is demonstrated that the distributed secondary sources may be approximately replaced by a number of suitably chosen equivalent secondary sources. An approximate method of calculation based on this property is introduced. The method is discussed in some detail in two cases: when the paths may be regarded plane and when they extend into the diffraction zone. The proposed method makes possible a simple and rapid calculation of field strength in many practical cases. It may be used for any phase characteristic of the complex permittivity of the soil.The paper confirms the important conclusion that placing the transmitting or receiving antenna over a well-conducting soil may considerably improve the reception. It is shown further that the electrical properties of the earth in the neighbourhood of the transmitter also have an influence on the phase of the field.