Influence of ionospheric irregularities on the form of radio signal scattering spectrum in over-the-horizon radar sounding of the rough sea surface

This paper is concerned with the backscattering of HF radio waves from the rough sea surface, which have propagated through the ionosphere with random large-scale irregularities.

For the sake of simplicity, it is assumed in calculations that the rough sea surface is a perfectly conducting surface with the known Philips power spectrum of irregularities. Ionospheric irregularities of a random medium that are isotropic and single-scale ones, with a Gaussian spectrum, are considered within the limits of the hypothesis of frozen-in irregularities.

Within the first approximation of perturbation theory, using, as the incident wave and the Green function, their geometrical-optics approximations, we obtained the expression for the backscattering spectrum of the ionospheric chirp radio signal with a Gaussian envelope. The expression involves the parameters of the receive-transmit antenna, the signal, the propagation medium, and of the scattering surface. Numerical simulation was used to investigate the influence of all the above-mentioned parameters on the backscattering spectrum. It is shown that travel of ionospheric irregularities has the largest influence on the scattering spectrum, the signal parameters mainly determine the size of the scattering area in the range, and the form of the coherent integration window determines the form of the received signal and can distort it.     

Lidar signal fluctuations in sounding the sea through a rough surface

We developed a rigorous calculation procedure for the characteristics of lidar signal fluctuations in sounding the upper sea layer through a surface disturbed by one-dimensional wind waves. We study the dependence of the mean value, dispersion and variation coefficient of the signal from a monostationary monoaxial lidar with identical source and receiver parameters on the sounding depth, wind velocity, and lidar beamwidth. The dependence of the correlation coefficient of the lidar signal on the sounding depth is analyzed. A physical interpretation of the results is given. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 10, pp. 992–1005, October, 1999.     

Theory of radio wave scattering at a rough sea surface

The problem of the reflection of a plane single-frequency electromagnetic wave from a statistically rough dielectric boundary with arbitrary is solved in the perturbation approximation. The statistical characteristics (scattering cross section, change of polarization, and frequency spectrum) of a radar signal reflected from a rough sea surface are investigated. The model used for the surface—a small ripple superimposed on large waves—enables the perturbation theory approach to be extended to the decimeter and centimeter wave band.Izvestiya VUZ. Radiofizika, Vol. 9, No. 5, pp. 876–887, 1966     

Spatial spectrum of a radar signal in glancing propagation above the sea surface

Pacific Oceanographic Institute, Far-East Branch of the Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 3, pp. 325–327, March, 1991.     

Influence of ionospheric irregularities on decameter radio wave propagation: Mathematic modeling

Based on numerical simulation and using the Monte Carlo method, an investigation is carried out of the influence of random irregularities in the ionospheric F-region on short-wave propagation along one-hop radio paths.Irkutsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 11, pp. 1439–1446, November, 1994.     

Investigation on global positioning system signal scattering and propagation over the rough sea surface

This paper is devoted to the study of polarization properties, scattering properties and propagation properties of global positioning system (GPS) scattering signal over the rough sea surface. To investigate the polarization and the scattering properties, the scattering field and the bistatic scattering coefficient of modified Kirchhoff approximation with using the tapered incident wave is derived in detail. In modeling the propagation properties of the GPS scattering signal in the evaporation duct, the initial field of parabolic equation traditionally computed by the antenna pattern with using fast Fourier transform (FFT) is replaced by the GPS scattering field. And the propagation properties of GPS scattering signal in the evaporation duct with different evaporation duct heights and elevation angles of GPS are discussed by the improved discrete mixed Fourier transform with taking into account the sea surface roughness.     

Preliminary results of the ultralong-range sounding of ionospheric irregularities using the ducted mode

We present preliminary experimental results concerning transequatorial propagation (TEP) of HF waves upon chirp sounding over the 11950-km path alice Springs (Australia)-Yoshkar-Ola (Russia). The measurements were made in August, 1998. Two anomalous signals with delays of 3.0 and 4.5 ms with respect to the main mode were observed during night time (21:30–23:00 UT). The maximum observed frequencies (MOF) of these signals were 2–3 MHz greater than the main-mode MOF. Simulations allowed us to identify these signals as the ducted signals trapped in theFE interlayer duct due to radiowave refraction on a negative gradient of the electron density and that escaped from the duct due to the scattering by small-scale field-aligned irregularities of the subpolar ionosphere. We discuss radiophysical and geophysical aspects concerning localization of the irregularities responsible for scattering and perspectives of using the ducted mode for over-the-horizon diagnostics of the inhomogeneous structure of the ionosphere with a global network of chirp sounders and HF radars. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 12, pp. 1136–1142, December 1999.     

Background fluctuations in the GPS-microlab-1 ionospheric radio sounding experiment

We analyze the spectra of the phase and amplitude fluctuations of radio signals recorded in the course of four ionospheric radio-sounding sessions at altitudes from 70 to 120 km. Our study is aimed at determining the sources of these fluctuations. Comparing the statistical properties of fluctuations measured at two wavelengths with theoretical calculations, we conclude that only the low-frequency part of background fluctuations is of ionospheric origin, while the high-frequency fluctuations are caused by noise of the measuring system. The amplitude fluctuations are more informative in the high-frequency region, since they are mainly due to ionospheric irregularities. We discuss the possibility of separation of the components of amplitude fluctuations caused by ionospheric irregularities and the irregularities of the neutral atmosphere in the case of sounding at the altitudes at which those components have comparable values. It is shown that this problem cannot be solved by dispersion and extrapolation methods, which are used for separation of the regular ionospheric and atmospheric components of the amplitude and phase variations of radio signals. A. N. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 6, pp. 511–523, June 1999.     

Spectrum of radar signal reflected from sea surface

The theoretical problem of the spectrum of a microwave radar signal reflected from the sea surface is examined with allowance for the simultaneous contributions of two scattering mechanisms — reflective and Bragg. The characteristics of the scattered-field spectrum are studied as functions of wind velocity, radar-beam angle of incidence, radar velocity, and antenna beam width. The results indicate high sensitivity of the spectrum parameters to changes in the sea-surface state.Institute of Applied Physics, Russian Academy of Science. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 1, pp. 3–15, January, 1993.     

Angular dependence of centimeter-band radar scattering by sea surface

Dependences of radar-scattering intensity on local slope of wind waves on the sea surface are obtained under real-sea conditions at a frequency of 20 GHz. To calculate the dependences, we use the fine structure of Doppler spectrum of backscattering. The results are qualitatively compatible with model calculations within the framework of the theory of perturbations on a two-scale surface and allow us to estimate the inhomogeneity of short gravity waves and gravity-capillary ripple, which are modulated by large-scale waves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 538–546, May, 1996.The author thanks V. G. Makarov and A. Yu. Gurulev of the laboratory of geophysical hydrodynamics at the Pacific Institute for Oceanology for assistance in numerical processing of the data.This work was supported by the Russian Foundation for Fundamental Research (grant No 94-05016630).     

Multiple scattering of HF radio waves propagating across the sea surface

A problem that is of considerable importance to the HF communications and radar communities is the calculation of the spatio-temporal modulations imposed on a radio wave as a result of propagating across a rough sea surface. We derive equations, in the form of a multiple-scattering series, describing the spatio-temporal structure of a radio wave at a point on the rough sea surface after propagation from an antenna at another point on the surface. We present calculations illustrating the sensitivity of the resultant Doppler spectra to ocean surface conditions, as described by well known parametric models. The generality of the approach means that it can easily accommodate user-specified antenna patterns, arbitrary coastline geometries, arbitrary sea-wave spectrum models, wave-field inhomogeneity and a wide range of seawater conductivities.     

Use of phase measurement in ionospheric sounding by FMCW signal

Ionosphere sounding by FMCW signal with additional amplitude modulation is suggested. It is shown that measurement of the phase delay of the envelope allows one to define the virtual heights more precisely.St. Peterburg State University, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 10, pp. 1341–1344, October, 1994.     

基于未充分发展海谱的分形海面模型及其电磁散射研究

重构了未充分发展的完全海谱(NDFSS)，提出了一种更符合海面实际情况的基于NDFSS的分形模型(NDFFM)，使用双尺度法计算了NDFFM海面的后向雷达散射截面(BRCS)，详细讨论了NDFFM海面各形成参数与BRCS的关系.将计算数据与传统的分形模型和实测雷达数据进行比对，证明了该模型的准确性和有效性. 关键词： 未充分发展 完全海谱 分形模型 双尺度法     

The effects of shadowing on modelling forward radar propagation over a rough sea surface

In order to accurately predict the effects of ocean roughness on forward radar propagation, it is important to incorporate the relevant physical scattering mechanisms. Shadowing is one such mechanism that is particularly important for low-grazing incidence yet it is often not taken into account. One broad class of approximations defines a roughness reduction factor in terms of an ensemble average over surface heights of a Rayleigh phase factor. The widely used Ament approximation and Miller–Brown approximation are two examples from this class of approximations that do not incorporate shadowing. In this paper, we use this class of approximations to develop a ‘shadowed’ approximation that implements a probability density function (PDF) for a rough sea surface illuminated at grazing incidence. A rigorous method of moments calculation is used to compare and quantitatively assess the accuracy of the Ament, Miller–Brown, and shadowed approximations at horizontal polarization. It is shown that the shadowed approximation yields significantly more accurate results than either the Ament or Miller–Brown approximation. In addition, these three approximations are compared with experimental data and it is shown that the shadowed approximation is in good agreement with the data. Finally, numerical curve fitting is used to develop a simple analytical expression for a roughness reduction factor that incorporates shadowing.