Extraction of Ocean Wave Spectra From Simulated Noisy Bistatic High-Frequency Radar Data

An algorithm is developed for the inversion of bistatic high-frequency (HF) radar sea echo to give the nondirectional wave spectrum. The bistatic HF radar second-order cross section of patch scattering, consisting of a combination of four Fredholm-type integral equations, contains a nonlinear product of ocean wave directional spectrum factors. The energy inside the first-order cross section is used to normalize this integrand. The unknown ocean wave spectrum is represented by a truncated Fourier series. The integral equation is then converted to a matrix equation and a singular value decomposition (SVD) method is invoked to pseudoinvert the kernel matrix. The new algorithm is verified with simulated radar Doppler spectrum for varying water depths, wind velocities, and radar operating frequencies. To make the simulation more realistic, zero-mean Gaussian noise from external sources is also taken into account     

非线性随机表面Kirchhoff散射模式

自从Longuet-Higgins(1963)根据非线性作用导出较正态分布为准确的波面高度分布以来,高阶矩在军事、高科技等很多方面得到了应用。Huang等(1980)曾检验了高阶非线性分布,他们发现当波高概率分布中包含直至四阶项时与观测结果相符,但如果考虑更高阶不但效果不好,反而更差。尽管非线性随机过程在海浪理论中获得了广泛的应用,但对海面雷达散射研究仅讨论到三阶矩的影响(Fung et al.,1991;Chen et al.,1992)。电磁随机表面散射理论有适应大尺度随机起伏的粗糙面的Kirchhoff散射模式、轻度粗糙表面的微扰散射模式、大小尺度独立叠加的双尺度散射模式、全波散射模式(Bahar,1987)和积分方程散射模式(Chen et al.,1992)等。对随机粗糙Kirchhoff表面电磁波散射问题,尽管几十年来许多科学家已经进行了大量研究(Fung et al.,1991; Ulaby et al.,1982;Wu et al.,1988),Eom等(1983)曾对Gaussian面和非 Gaussian面的散射特性进行了比较研究,Fung等(1991)将Kirchhoff散射模式推广应用到三阶粗糙随机表面,然而更高阶矩对散射截面的影响还未见报道。本文在Fung等(1991)的基础上将Kirchhoff散射模式推广应用到四阶项,并对模式的应用进行了分析和讨论。     

Target Classification and Remote Sensing of Ocean Current Shear Using a Dual-Use Multifrequency HF Radar

In this paper, we describe a high-frequency (HF) radar capable of multifrequency operation over the HF band for dual-use application to ship classification and mapping ocean current shear and vector winds. The radar is based on a digital transceiver peripheral component interconnect (PCI) card family that supports antenna arrays of four to 32 elements with a single computer, with larger arrays possible using multiple computers and receiver cards. The radar makes use of broadband loop antennas for receive elements, and a number of different possibilities for transmit antennas, depending on the operating bandwidth desired. An option exists in the choice of monostatic or multistatic operation, the latter providing the ability to use several transmit sites, with all radar echo signal reception and processing conducted at a single master receiver site. As applications for such a multifrequency radar capability, we show measurement and modeling examples of multiple frequency HF radar cross section (RCS) of ships as an approach to ship target classification. Results of using 32 radar frequencies to measure the fine structure in ocean current vertical shear are also shown, providing evidence of one edge of a 1-3-m deep uniform flow masked at the surface by wind-driven current shear in a different direction. Other applications of current-shear measurements, such as vector wind mapping and volumetric current estimation in coastal waters, are also discussed     

An analysis of the second-order Doppler return from the ocean surface

A theoretical analysis of the scattering of high-frequency (HF) electromagnetic waves from rough surfaces is proposed. The analysis, when applied to a model for the ocean surface, leads to new predictions of its second-order backscattered radar cross section in addition to those provided by existing theories. Some of the predictions of the theory have already been verified experimentally.     

各向异性海面全极化微波双站散射的仿真与分析

为了解各向异性随机粗糙海面的微波双站散射机制及其特性,本文利用解析近似的积分方程模型以及一种改进的半经验海浪谱模型实现了对各向异性随机粗糙海面的全极化微波散射仿真模拟,并与卫星观测数据、经验的地球物理模式函数及已有的解析近似散射模型仿真结果进行了对比,验证了仿真结果的可行性和准确性。利用该模型分析了入射波频率、入射角、极化方式、海面风速及风向等参数对各向异性海面双站散射的影响。模拟结果表明,在不同的入射角、散射角及方位角等观测几何条件下,海面不同波段的双站散射表现出不同的空间散射特性,且对风速、风向等海面动力学参数表现出不同的敏感性,以L波段为例,海面向后半球双站散射在各个极化方式下都对风速较为敏感,而在同极化方式下,其对风向的响应在中低风速和高风速条件下相反,整体而言,低风速下海面双站散射对风向更为敏感。这表明对于海面动力参数的反演,双站散射可以提供比传统单站雷达后向散射更丰富的物理信息。本文探讨了各向异性海面微波双站散射特性,为基于主动式及分布式微波传感器的海洋动力参数遥感反演提供了理论分析基础。     

Measurement of ocean wave spectra using narrow-beam HE radar

A data interpretation algorithm is developed to extract ocean wave information from HF radar backscatter observed by a narrow-beam antenna system. The basis of this measurement is the inversion of the integral equation representing the second-order radar cross section of the ocean surface. This equation is numerically inverted by approximating it as a matrix equation and pseudoinverting the kernel matrix using a singular value decomposition. As a test of this algorithm, comparisons are made between wave spectrum estimates obtained from a WAVEC buoy and a pair of 25.4-MHz ground wave radars, using data collected during the 1986 Canadian Atlantic Storms Program (CASP). Overall, the results of this experiment have been positive and have demonstrated both the basic feasibility of the inversion algorithm and the wave sensing capability of HF radar. For example, significant wave height estimates deduced by two radars differed from the buoy, in an absolute value sense, by only 0.12 m on average. When using only one radar, the mean difference of this important parameter from the buoy was a reasonable 0.33 m     

An analytical model for the HF backscattered Doppler spectrum for the ocean surface

An analysis previously developed for rough surface scattering with narrow-beam reception is extended to wide-beam or omnidirectional reception. The source is considered to be a pulsed electric dipole. The analysis includes multipathing effects on the received signal. The results are used to develop a model for the backscattered radar cross section of the ocean surface, up to a second-order approximation.     

Radar cross section analysis of marine targets using a combining method of physical optics/geometric optics and a Monte-Carlo simulation

In this paper, the radar cross section of flat plates on ocean surfaces is statistically investigated. A combining method of physical optics and geometric optics is applied to establish an effective backscattering analysis procedure. This method is a high-frequency analysis method originally derived from a simplified Stratton-Chu integral equation, assuming that the radar is far away from the target so that Kirchhoff approximation is valid. A Monte-Carlo simulation method is adopted to statistically analyze the effects of undulated ocean surfaces. The ocean surfaces are randomly generated by Pierson-Moskowitz ocean wave spectrum and a directional distribution function. Numerical investigations are carried out for flat plates, with the same height and width but with different inclined angles, on ocean surfaces of various significant wave heights.     

Joint perturbation scattering characterization of a littoral oceanbottom reverberation: theory, scattering strength predictions, and datacomparisons

A joint surface roughness/volumetric perturbation scattering theory is utilized to characterize the reverberation from a littoral ocean bottom. The result is a reflected field spectrum that consists of specular and off-specular components. The predicted scattering strength from the off-specular component is shown to be comprised of interface roughness scattering, sediment inhomogeneity volumetric scattering, and interface roughness/sediment inhomogeneity correlation scattering. The sediment inhomogeneity volumetric scattering is shown to contain two contributions that are due to fractional variations in sediment densities and sound velocities. Both contributions are shown to be affected by the interface effect by a round-trip transmission coefficient factor. These two fractional variations are shown to contribute differently to scattering strength but similarly to backscattering strength. Inversely predicted roughness spectra from various sets of backscattering strength data are shown to be consistent with a generally known roughness spectrum. Both inversely predicted roughness and volumetric scattering physical property spectra are found to be self-consistent. However, the use of only ocean bottom backscattering strength data is found to be insufficient to judge whether the roughness or the volumetric scattering dominates. Reverberation characterizations using bistatic scattering strength data and signal spread data are planned for future studies     

A comparison of perturbation theory and the small-slope approximation for acoustic scattering from a rough interface for a Biot medium

In this paper, the lowest order small-slope approximation (SSA) scattering cross section for Biot theory is derived. Numerical results are obtained for both backscattering and bistatic scattering using a modified power law spectrum, and these results are compared with those of lowest order perturbation theory (PT). Frequencies ranging from 100 Hz to 3 kHz are used for surfaces with RMS heights h of 0.1 and 1 m and a correlation length l of 10 m. The angle of incidence for the bistatic results is limited to 45/spl deg/. It is found that for the smaller surface height roughness (h = 0.1 m), the SSA and PT give the same results for frequencies up to almost 1 kHz for both backscattering and bistatic scattering. For h = 1 m, the SSA and PT backscatter results are in good agreement at all frequencies for incident grazing angles up to approximately 45/spl deg/. For the bistatic results, the SSA and PT results agree only at low grazing angles of scatter. In the specular region, the results differ significantly.     

Bistatic radar cross section of ship targets

Data describing simultaneous bistatic (BRCS) and monostatic (MRCS) radar cross sections of freighters atXband near grazing incidence are presented. Details of the measurement system, calibration procedures, and target-radar geometry are discussed. Measured values of BRCS were typically less than the MRCS and decreased with increasing bistatic angle.     

Bistatic quasi-mirror radar of the sea surface

The main features of a bistatic quasi-mirror radar of the sea surface are considered in an approximation of quasi-mirror scattering from isotropic surface waves. Surface-wave scattering plots determining the energetics and width of the view area of airspace radars are constructed under horizontal polarization and surface-radiating ray fall angles that are away from the nadir. We consider the factors determining the form of the “flare track”: projections of the difference-wave vector in the lateral and transversal directions; the rates of tilt of the wind waves; and the finite radius of the Earth’s curvature, which is important for ocean sounding from space. The resolution with respect to an axis of horizontal length is the worst at the mirror point; however, in most of the wide viewing area, this resolution is only a few times lower than the resolution of a conventional (monostatic) radar for the same width of the signal spectrum. We also consider the conditions for synthesizing the directional diagrams of a receiving antenna when there are two viewing areas that are not separated with respect to the slant range, as well as the influence that surface nonstationarity has on the synthetic aperture radar focusing.     

Models of radar imaging of the ocean surface waves

A number of models which would explain ocean wave imagery taken with a synthetic aperture imaging radar are analyzed analytically and numerically. Actual radar imagery is used to support some conclusions. The models considered correspond to three sources of radar backscatter cross section modulation: tilt modulation, roughness variation, and the wave orbital velocity. The effect of the temporal changes of the surface structure, parametric interactions, and the resulting distortions are discussed.     

Comparison of inversion algorithms for HF radar wave measurements

All ocean wave components contribute to the second-order scattering of a high-frequency (HF) radio wave by the sea surface. It is therefore theoretically possible to estimate the ocean wave spectrum from the radar backscatter. To extract the wave information, it is necessary to solve the nonlinear integral equation that describes the relationship between the backscatter spectrum and the ocean wave directional spectrum. Different inversion techniques have been developed for this problem by different researchers, but there is at present no accepted “best” method. This paper gives an assessment of the current status of two methods for deriving sea-state information from HF radar observations of the sea surface. The methods are applied to simulated data and to an experimental data set with sea-truth being provided by a directional wave buoy     

On the accuracy of current measurements by means of HF radar

The accuracy of surface current velocities measured by high-frequency (HF) radar is investigated. Data from the two radar systems of the University of Hamburg, CODAR (Coastal Radar) and WERA (Wellen Radar), are compared with in situ data. In one experiment, CODAR and a near-surface current meter were operated simultaneously over a 19-day period. In addition, WERA was operated for 6 days during that period. In the other experiment, WERA and a bottom-mounted current meter were operated simultaneously over a 35-day period. Both radars use frequencies of about 30 MHz where backscattering is due to ocean waves of 5 m wavelength. The influence of the orbital motion of underlying longer waves on radial velocity errors is investigated. In accordance with theory, the measured standard deviations of HF-measured current velocities depend on the sea state. Depending on the sea state, estimated errors range from 3 to 10 cm·s^-1 and explain only part of the rms difference of 10-20 cm·s^-1 found between HF and in situ current measurements. The rest is assumed to be due the differences of the quantities measured, e.g., the spatial averaging     

GNSS-R观测下的海面飓风风速反演

利用全球导航卫星系统在地球表面的反射信号（GNSS-R）进行海面风速反演已经被广泛研究并作为一种重要的遥感手段。目前,该L波段微波信号的相关功率已可以在多普勒频率和时延码片空间进行多普勒时延图像的成像。由于该图像的图像特征与海面粗糙度有较高的相关性,因此能够用来进行海面风场反演。然而,对于该遥感手段而言,其双基雷达前向散射截面（BRCS）理论上与海面粗糙度有更高的相关性,如同目前合成孔径雷达使用后向雷达散射截面而非相关功率。所以,本文通过改进已有的GNSS-R的双基雷达散射截面方程,代替相关功率在多普勒时延空间进行成像,得出了与海面粗糙度相关的双基雷达散射截面图像（BRCS map）。基于该图像,本文提出了三种与其形状特征相关的观测量,通过2005年Dennis飓风GNSS-R机载数据生成的16000多幅图像进行地球物理模式函数建模并与经典的一维时延波形匹配方法得出结果进行对比分析,得出更为精确的风速反演结果。     

A Method of Swell-Wave Parameter Extraction From HF Ocean Surface Radar Spectra

A new method for the extraction of swell-wave parameters from high-frequency (HF) radar spectra is presented. The method of extraction of the parameters, period, direction, and height, relies on a frequency-modulation approach that describes the hydrodynamic interaction of the swell waves with the resonant, shorter, Bragg waves. The analysis process minimizes the electromagnetic second-order interaction and a simulation model was used to validate the approach. This simplified method provides a fast means of examining swell conditions over large areas of the ocean surface. Data are acquired using a pair of coastal ocean surface radar (COSRAD) systems deployed at Tweed Heads, Qld., Australia. The radar covers a sweep (approximately 60deg) every 30 min with spatial resolution of the order of 3 km. A sample set of data from this deployment is used in a case study to show the extraction of swell direction and amplitude using these methods. The results support the use of the COSRAD HF radar for mapping swell in the near-shore zone     

Bragg scattering of centimeter electromagnetic radiation from the sea surface: The effect of waves longer than Bragg components

The effect of sea surface waves longer than Bragg components on the backscattering of centimeter electromagnetic radiation is studied on the basis of data on direct field measurements of sea-surface slopes. A situation in which waves with wavelengths greater than 10 cm are longer than the Bragg components is considered. The increase in the backscattering cross section that is due to the presence of long waves is numerically estimated for sounding at horizontal and vertical polarization. Nonlinear effects in the field of surface waves result in the departure of the distribution of sea-surface slopes from a Gaussian distribution and lead to a change in the backscattering cross section. At a sounding angle of 35°, this change may reach 15% with respect to the cross section calculated for a Gaussian surface.     

弱流场调制下的海面微波散射模拟研究

根据海面微波散射的多尺度模型以及波流相互作用理论,对一维弱流场调制下的海面微波散射截面进行了数值模拟。结果表明,利用数值方法直接求解波作用量方程获得的海浪调制谱并结合多尺度模型可以较好地模拟弱流场引起的雷达散射截面的变化。内波等海洋现象调制了海浪谱,使得雷达散射截面反映出这些海洋特征,整个调制过程的模拟对于分析这些海洋现象并更好地利用其雷达数据具有重要意义。     

Vandemark-Chapron算法与Young算法联合反演Jason-1雷达高度计海面风速方法研究

针对现行业务运行的宙达高度计风速反演算法从考虑0～20 m/s的缺陷,提出了vc算法(Vandemark-Chapron Algorithm)与Young算法联合反演高度计海曲风速的方法,通过对Jason-1资料的统计试验,确定了后向散射截面临界点,当高度计后向散射截面大于等于该临界点时采用VC算法反演风速,反之采用Y...