Study of a Microwave Radar Signal Backscattered at Small Incidence Angles: In-Situ Measurements

We present the results of data processing of the field experiment devoted to a study of a microwave signal backscattered by a rough water surface and compare them with the theoretical estimates. The measurements were conducted sequentially using a radar with parabolic and knife-beam antennas. The radar was mounted on the Molitovsky bridge at a height of 40 m across the Oka river in Nizhny Novgorod, Russia. For the radar with knife-shaped antenna pattern, we examined the reflected-signal power and the shift and width of the Doppler spectrum as functions of the azimuthal angle for the nadir sounding. For the parabolic antenna, we studied the reflected-signal power as a function of the incidence angle. Comparison with the experimental data shows that the developed theoretical model of a Doppler spectrum is in good agreement with the observations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 8, pp. 661–673, August 2005.     

Determination of the Parameters of Sea-Surface Roughness Using the Doppler Spectrum of a Microwave Radar Signal Reflected from a Water Surface

Within the framework of the Kirchhoff approximation, we derive formulas describing the shift and width of the Doppler spectrum of a microwave signal reflected from a rough water surface in the case of a small incidence angle. The formulas take into account the effect of the radar beam pattern. The new model of the spectrum allows for the fact that the distance from the radar to the observed surface changes in the course of measurements. Our theoretical analysis shows that the Doppler-spectrum parameters and the amount of information on the scattering surface, which is contained in the reflected signal, are strongly dependent on the antenna beamwidth. The results of our study allowed us to develop a new algorithm for measuring the water-surface parameters from a moving carrier.     

Measuring the characteristics of backscattering of sound on a rough surface in the near-field zone of a phased array

The paper presents the results of an experiment on measuring the characteristics of backscattering of sound waves by a rough surface. Measurements were conducted in the near zone of a vertical array, which made it possible with a high degree of accuracy to control the propagation conditions and the angles of incidence of an acoustic wave onto the surface. Tonal signals were emitted in a constant mode at frequencies of 0.5–3.5 kHz. The Doppler spectrum of the scattered signal was analyzed in detail, the spectral component corresponding to Bragg scattering was isolated, and the width of this component was determined. The possibilities of calculating the scattered field using two forms of the sea state spectrum are studied and compared: the two-dimensional spatiotemporal sea state spectrum, which was measured optically, and the conventional one-dimensional time spectrum, measured with a single surveying mark.     

极化合成孔径雷达海面成像模拟及分析

海浪极化SAR成像模拟对海面散射的研究有重要的意义.基于海浪谱理论,在考虑大、小尺度海浪影响的基础上,构建了双尺度的随机粗糙海面.根据该海面模型,利用Bragg散射模型,小扰动模型与速度聚束理论,模拟了不同状况下海浪的极化SAR图像.并针对海浪参数对极化图像的影响进行了分析,结果符合成像统计特性,得出了一些有益于海浪信...     

Doppler spectra of laboratory wind waves at low grazing angle

Eight radar frequencies over a range from 3 GHz to 94 GHz have been used to measure Doppler spectra of wind waves in a laboratory wind-wave tank, at low grazing angle. The VV spectra show relatively little change in shape at different frequencies. The HH spectra show significant trends in the lower radar frequencies, some of which are probably artefacts of the experiment. A new model, the 'fish model' is developed empirically from measurements of the peak Doppler spectra for both vertical and horizontal polarization. This model indicates that the H-pol, not just the V-pol, peak Doppler shift is dependent on the radar frequency.     

Estimation of sea clutter inherent Doppler spectrum from shipborne S-band radar sea echo

Measurement of shipborne radar sea echo is instrumental in collecting the sea clutter data in open sea areas. However,the ship movement would introduce an extra Doppler component into the spectrum of the sea clutter, so the sea clutter inherent spectrum must be estimated prior to investigating the sea clutter Doppler characteristics from the shipborne radar sea echo. In this paper we show some results about a shipborne sea clutter measurement experiment that was conducted in the South China Sea in a period between 2017 and 2018; abundant clutter data have been collected by using a shipborne S-band clutter measurement radar. To obtain the sea clutter inherent Doppler spectrum from these data, an estimation method, based on the mapping relationship between the shipborne clutter spectrum and the inherent clutter spectrum, is proposed. This method is validated by shipborne clutter data sets under the same measuring conditions except for the ship speed. Using this method, the characteristics of the Doppler spectrum lineshapes in the South China Sea are calculated and analyzed according to different sea states, wave directions, and radar resolutions, which can be instrumental in designing the radar target detection algorithms.     

Spectral characteristics of a microwave radar signal backscattered by the sea surface at small incidence angles

We present a theoretical analysis of the Doppler-spectrum properties of a microwave radar signal scattered by the sea surface at small incidence angles. The dependences of Doppler-spectrum width and shift on the wind velocity and wave development stage and their azimuthal dependence are analyzed. The case of mixed sea (wind wave plus swell) is also considered. The JONSWAP spectrum model is used to describe sea waves. The study shows that Doppler-spectrum parameters are sensitive to variation of sea-surface state; for example, for the case of developed sea waves, an increase in wind velocity by 1 m/sec leads to increases in the Doppler-spectrum width and shift by 15 Hz and 3 Hz, respectively. It is shown that for the case of a moving radar the Doppler spectrum remains sensitive to variation of sea- surface state with a sufficiently narrow radar directivity pattern. Estimates show that in the case of a single sea- wave system on the surface, using Doppler-spectrum parameters we can, in principle, determine wave type (developing wind wave, developed wind wave, or swell), dominant wavelength, wave propagation direction, and wave height; wind velocity, direction, and acceleration distance can be determined for wind waves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 517–526, May, 1996.This paper was supported by the Russian Foundation for Fundamental Research (project 93-02-15892).     

Doppler spectra of laboratory wind waves at low grazing angle

Abstract

Eight radar frequencies over a range from 3 GHz to 94 GHz have been used to measure Doppler spectra of wind waves in a laboratory wind-wave tank, at low grazing angle. The VV spectra show relatively little change in shape at different frequencies. The HH spectra show significant trends in the lower radar frequencies, some of which are probably artefacts of the experiment. A new model, the ‘fish model’ is developed empirically from measurements of the peak Doppler spectra for both vertical and horizontal polarization. This model indicates that the H-pol, not just the V-pol, peak Doppler shift is dependent on the radar frequency.     

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.     

Scattering of a Narrow Wave Beam on a Rough,Locally-Specular Surface in the Case of Pulsed Illumination

We consider scattering of a pulsed narrow wave beam on a rough surface with a locally-specular indicatrix. Analytical expressions for the average received power are obtained for normal distributions of heights and slopes of the rough surface in two cases in which the direction to the receiver is close to or strongly different from the direction of specular reflection. It is shown that in these cases, the received echo pulses have drastically different profiles determined by the source and receiver parameters, the scheme of sounding, and the variance of the heights and slopes of the rough surface. The obtained analytical expressions for the average received power agree well with the results of numerical simulations.     

机载C波段全极化SAR海面风矢量反演理论研究及实验验证

机载全极化SAR海面风矢量反演研究对于近海岸复杂气象条件下风矢量探测具有重要意义. 本文从极化散射理论出发,通过分析全极化SAR探测数据与海面风矢量的关系, 设计了全极化SAR海面风矢量反演方案.依据机载SAR高机动性和全极化两个探测特点, 针对VV极化探测数据,提出了基于最大似然估计的海面风矢量反演方法,并设计了飞行实验方案; 针对VH极化探测数据,提出了通过带约束最优拟合的VH极化海面散射模型反演风速, 再利用CMOD5地球物理模型函数计算风向的海面风矢量反演方法. 利用机载全极化SAR探测的台风'海葵'边缘数据,开展了海面风矢量反演实验研究. 研究结果表明,两种风矢量反演方法均可不借助辅助信息,反演复杂气象条件下的海面风矢量. 前者反演风向、风速的均方根误差分别为18.0°, 1.8 m/s, 后者反演风向、风速的均方根误差分别为9.3°, 1.2 m/s,后者的反演精度优于前者. 这是因为VH极化归一化雷达截面与风向和雷达入射角无关,仅与风速密切相关, 更适合复杂气象条件下的海面风矢量反演. 关键词： 机载全极化SAR 海面风矢量 理论研究 实验验证     

Informative properties of the underwater solar path

We study the possibility of determining characteristics of wind-induced waves by using the sea-surface image observed from under water. The statistical model of the solar path resulting from the light refraction by rough sea surface is developed. It is shown that the statistical processing of the solar-path image allows one to determine not only the variance of the surface slopes, but also the spatial-correlation coefficient of the slopes and, therefore, their spatial spectrum. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 1, pp. 36–45, January 2009.     

风浪因素对海洋波导雷达回波作用机理的研究

采用文氏谱模型描述近海海洋动力环境海面的粗糙特征, 并运用海浪谱理论得到风速和均方根高度偏差的关系. 进而应用修正的离散混合傅里叶变换方法和改进的雷达散射系数模型分别计算了电磁波传输损耗和雷达散射系数. 在此基础上, 通过数值计算分析了风浪因素对海洋波导环境雷达回波功率值的影响. 结果表明, 风浪因素对不同观测高度处的传输损耗的影响均不大; 风浪因素对雷达回波功率有显著影响, 并且相对于传输损耗, 雷达散射系数项受风浪因素影响较大因而导致较大的雷达回波功率的变化.     

Doppler Spectra of Centimeter and Millimeter Microwaves Backscattered from a Rough Water Surface at low Grazing Angles

We present the results of processing of X- and Ka-band radar signals backscattered from a rough water surface at low grazing angles. Processing is made by the method of decomposing a Doppler spectrum into three basic line profiles. Each of these profiles corresponds to a certain scattering mechanism. Comparative evaluations of the contribution of the Bragg mechanism described by the Gaussian line profile and two non-Bragg mechanisms described by the Lorentz and Voigt profiles are obtained.     

Scattering of a narrow wave beam by a randomly rough locally diffuse surface subject to pulse illumination

We consider the scattering of a narrow pulse wave beam by a randomly rough surface with a complex local scattering indicatrix. Analytical expressions are found for the mean received power for a normal distribution of heights and slopes of the surface in two cases: where the direction to the receiver is close to the direction of mirror reflection and where the direction to the receiver is very different from the direction of mirror reflection. It is shown that in these two cases the echo pulse is very different in shape and is controlled by the parameters of the source and receiver, the sounding scheme, and the variance of heights of a rough surface. The received power is strongly dependent on the width of the local scattering indicatrix, and the form of this dependence is determined by the angles of illumination and reception. The analytical expressions for the mean received power are in good agreement with the results of numerical calculations. Institute for Radioelectronics and Laser Engineering of the N. é. Bauman State Technical University of Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol.42, No.4, pp. 333–339, April 1999.     

Transformation of Short Waves in a Nonuniform Flow Field on the Ocean Surface. The Effect of Wind Growth Rate Modulation

We develop a model of transformation of the short surface wave spectrum in the presence of a nonuniform flow on a water surface, in which the modulation of wind-wave growth rate is taken into account. The model of a turbulent near-water atmospheric layer is used to calculate the modulated growth rate. In this model, turbulent stresses in the wind are described using a gradient approximation with model eddy viscosity specified with allowance for the known laboratory experiments. The examples of short-wave modulation in the presence of nonuniform flows on a water surface, originating from ripples and intense internal waves, are considered. It is shown that deformations of the wind-velocity profile and its long-wavelength perturbation due to the nonlinear interaction between the wind surface waves and the wind has a significant effects on the short-wave growth rate and its modulation. In the case of ripples, this deformation reduces to an increase in the roughness parameter of the wind-velocity profile and to a velocity-profile modulation with ripple period. The modulated growth rate is calculated within the framework of a quasi-linear model of surface-wave generation by a turbulent wind, in which the hypothesis of random phases of the wind-wave field is used. The amplitude and phase of the hydrodynamical modulation transfer function are calculated within the framework of the relaxation model. The calculation results are in reasonable agreement with the available experimental data. A model described by the combined Korteweg–de Vries equation is used to study a surface flow field generated by intense internal waves. The internal-wave parameters are takes from the results of the COPE experiment. We calculate the wind growth-rate dependences on the wave-train phase for the cases of downwind and upwind propagation of an internal wave. The calculation results agree qualitatively with experimental data.     

基于最优估计理论、联合星载主被动传感器资料的液态云微物理特性反演研究

针对液态云微物理特性精确反演的迫切需求, 综合主被动传感器的探测优势, 联合CloudSat雷达反射率和Aqua光学厚度资料, 提出基于最优估计理论的液态云微物理参数反演算法.通过假设粒子谱服从对数正态分布, 基于前向物理模式建立测量变量与反演变量的函数关系, 借助谱分布参数的先验信息、通过算法迭代得到谱参数的最优解, 进而利用前向物理模式反演液态云微物理参数, 并根据误差传递理论计算反演不确定度.通过设计反演方案, 基于实测个例数据并与CloudSat官方发布产品和经验算法反演结果对比验证.结果表明: 基于最优估计理论、联合主被动传感器资料的液态云微物理参数反演结果与官方发布产品一致性较好, 弥补了经验算法误差大、扩展性差的不足, 对于开展国内星载和机载W波段毫米波雷达液态云微物理参数反演研究具有重要的借鉴意义. 关键词： CloudSat Aqua 液态云 最优估计理论     

Simulation study of phase retrieval for hard X-ray in-line phase contrast imaging

X-ray phase contrast imaging (XPCI) is a novel method that exploits the phase shift for the incident X-ray to form an image. For light elements such as carbon, hydrogen and oxygen, the phase-shift term can be up to 1000 times greater than the absorption term in the hard X-ray energy region. So XPCI has attracted much attention in recent years. Various methods for XPCI have been proposed and demonstrated on synchrotron devices and other X-ray sources[1―13], particularly the in-line metho…     

Modulation of radar signal reflected from rough surface by relief slope and soil electrodynamic-parameter inhomogeneities

The authors study variations of the specific effective scattering surface (SESS) of a radar signal reflected from a rough surface with small irregularities due to changes in the slopes of the small-scale relief and inhomogeneities of the complex permittivity of the soil in the case of a surface resolution element whose linear dimensions are much smaller than the characteristic scale of the relief. In the opposite case, formulas are derived for corrections to the SESS that are proportional to the variance of the slopes of the large-scale relief. It is shown that the correction for horizontal polarization is greater than that for vertical; the effect of the three-dimensionality of the relief roughness is also greater. A nonzero SESS for orthogonal polarization is due to the presence of relief slopes across the plane of incidence.Kharkov Radio-Astronomy Institute, Academy of Sciences of Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 4, pp. 444–457, April, 1994.     

C波段机载合成孔径雷达海面风场反演新方法

针对基于散射计地球物理模型函数的机载合成孔径雷达(SAR)海面风场反演中存在的风向获取依赖于图像风条纹或数值预报、 散射计数据和浮标等背景场资料, 风向与SAR图像时空分辨率不匹配, 进而影响机载SAR海面风场反演精度等问题, 本文根据机载SAR对海探测特点, 研究一种适用于C波段机载SAR的海面风场反演新方法. 利用SAR图像距离向不同入射角的后向散射系数, 依据地球物理模型构造最小代价函数, 通过代价函数的求解直接从机载SAR数据同时反演出海面风速和风向. 利用论文提出的海面风场反演方法分别对仿真SAR数据和实测C波段机载SAR数据进行风向、 风速的反演误差分析及试验验证.研究结果表明, 该方法适用于机载SAR海面风场反演, 可不依赖背景风向直接反演出精度较高的风速和风向; 雷达后向散射系数误差是决定海面风速、风向反演精度的关键因素, 辐射定标精度越高则反演误差越小; 海面风速反演误差随着风速的提高而增大, 当海面风速大于18 m/s时, 风速反演误差显著增加, 而海面风向的反演误差与风速无明显关系. 关键词： 机载合成孔径雷达 海面风场 多入射角