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。     

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。     

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.     

Single-ping target speed and course estimation using a bistatic sonar

This paper focuses on estimating the two-dimensional (2-D) target-speed vector (course and speed) using a multistatic sonar that consists of one monostatic sonar and one bistatic receiver. The speed and course estimates are obtained after a single transmission. The theory on bistatic Doppler and 2-D target-speed vector estimation is first considered and then applied to simulated and real data. The results can be used to improve classification algorithms or to feed speed information to tracking algorithms, for example.     

On the Development of a Second-Order Bistatic Radar Cross Section of the Ocean Surface: A High-Frequency Result for a Finite Scattering Patch

The development of a model for the second-order bistatic high-frequency (HF) radar cross section on an ocean surface patch remote from the transmitter and receiver is addressed. A new approach is taken that allows a direct comparison with existing monostatic cross sections for finite regions of the ocean surface. The derivation starts with a general expression for the bistatically received second-order electric field in which the scattering surface is assumed to be of small height and slope. The source field is taken to be that of a vertically polarized dipole, and it is assumed that the ocean surface can be described, as is usually done, by a Fourier series in which the coefficients are zero-mean Gaussian random variables. Subsequently, a bistatic cross section of the surface, normalized to patch area, is derived. The result is verified by the following two means: 1) the complete form of the bistatic HF radar cross section in backscattering case is shown to contain an earlier monostatic result that has, itself, been used extensively in radio oceanography applications; and 2) the bistatic electromagnetic coupling coefficient is shown to reduce exactly to the monostatic result when backscattering geometry is imposed. The model is also depicted and discussed based on simulated data     

Bistatic synthetic aperture imaging of proud and buried targets from an AUV

The use of autonomous underwater vehicles (AUVs) for the detection of buried mines is an area of current interest to the Mine CounterMeasures (MCM) community. AUVs offer the advantages of lower cost, stealth, reduced operator risk, and potentially improved coverage rates over more traditional mine hunters. However, AUVs also come with their own set of difficulties, including significant error in navigation and low communication rates with the mother platform and each other. In the case of bistatic detection scenarios, AUVs will therefore have difficulty knowing where exactly in space they are and the trigger time of sources on other platforms, be they ships or other AUVs. However, the potential improvement in detection and coverage rates offered by bistatic sonar concepts makes resolution of these issues a high priority. In this paper, the problems of inaccurate navigation and source timing information are addressed for the Generic Oceanographic Array Technology data set. In this experiment, conducted off Marciana Marina during June 1998, a MIT AUV with a SACLANTCEN acoustic array and acquisition system was used together with a TOPAS parametric sonar to explore issues of buried target detection using AUVs. In this paper, solutions to the navigation and timing problems are proposed which enable the effective use of bistatic synthetic aperture sonar (SAS) concepts for the detection of buried objects in the mid-frequency regime of 2-20 kHz.     

High-frequency bistatic sea-floor scattering from sandy ripple bottom

To obtain the bistatic scattering function on the sandy ripple bottom, high-frequency bistatic sea-floor scattering measurements were made in the shallow waters off the east coast of Korea. A sand ripple field was present at the site, with wavelength generally in the 10-20-cm range. The mean ripple orientation relative to the direction of wave propagation was estimated to be roughly 20/spl deg/-30/spl deg/. Field experiments were made to measure forward (in-plane) and out-of-plane scattering from the ripple bottom. The measured scattering strengths were compared to the predictions of the APL-UW bistatic scattering model. Overall, forward-scattering strength measurements showed favorable comparison with the model predictions. The global scattering characteristics for the ripple bottom gave an augmented out-of-plane scattering.     

Shallow water reverberation: normal-mode model predictions comparedwith bistatic towed-array measurements

A normal-mode model for calculating reverberation in shallow water is presented. Some illustrative calculations are given for the bistatic case and for vertical and horizontal line-array receivers. Emphasis is on comparison with measurements of bistatic reverberation obtained at a shallow-water area in the Mediterranean. The data are from explosive sources received by a towed array, analyzed in one-tenth-decade frequency bands at subkilohertz frequencies. Model calculations for a flat-bottomed environment indicate a strong dependence on propagation conditions and a weak dependence on beam steering direction. Preliminary comparisons give quite good agreement between measured reverberation and model predictions, but point to the need for extending modeling efforts to handle range-dependent environments     

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     

240-kHz bistatic bottom scattering measurements in shallow water

High-frequency bistatic sediment scattering experiment was conducted in the shallow waters off the east coasts of Korea. Acoustic data were taken as a function of grazing angle (30°, 45°, and 60°), scattered angle (30°, 45°, and 60°), and bistatic (azimuthal) angle (0°, 60°, and 120°). Besides a flat bottom it was artificially raked so as to produce directional ripples. The measured scattering strengths for a flat bottom were compared to model predictions of D.R. Jackson et al. (1986). The surface reverberation component is seen to dominate over the volume scattering part at the frequency of 240 kHz. Compared to the flat bottom case, the scattering strengths for directional ripples showed lower and higher variation depending on the ripple's orientation     

Techniques for sonar target identification

We describe bistatic and mostly monostatic techniques that are useful for target classification by means of active sonar. The echoes returned by any submerged elastic body, exemplified here by a wolfram carbide (WC) sphere, contain features caused by the poles of the scattering amplitude of the problem. We have found these poles and shown that they naturally split into two large sets from which one can separately extract shape or material composition information. The composition information seems easier to determine than details about the shape. Together these sets of poles unambiguously characterize any scatterer.     

Modeling bistatic bottom scattering strength including a forwardscatter lobe

An upgrade to bistatic scattering strength modelling that is based on the authors' current understanding of bottom topographic scattering with an emphasis on modeling the `forward lobe' where Lambert's law fails quite significantly is reported. Low-frequency bottom scatter modeling is reviewed with particular emphasis on the issues of the forward scattered lobe. A specific model (a modified version of BISSM) is proposed, and the model's advantages and limitations are discussed. The requirement for certain high-resolution geomorphic data needed to support the model is discussed. Like the original BISSM, the version does not modify the accepted form for diffuse scattering, but it does modify the form of the forward lobe     

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。     

On Experience in using the remote acoustic method of partial reflections in studies of the lower troposphere

Using the phenomenon of the partial reflection of acoustic waves from anisotropic wind-velocity and temperature inhomogeneities in the lower troposphere is justified in determining the structure of these inhomogeneities. The data (obtained with the method of bistatic acoustic sounding) on signals reflected from stratified inhomogeneities in the lower 600-m layer of the troposphere are given. A detonation-type pulsed acoustic source was used. The methods of isolating a small (in amplitude) reflected signal against the background of noise and determining the reflecting-layer height and the partial-reflection coefficient from the measured parameters (time delay and amplitude) of a reflected signal are presented. The method of estimating the vertical gradients of the effective sound speed and the squared acoustic refractive index from the partial-reflection coefficient previously calculated is described on the basis of an Epstein transition-layer model. The indicated parameters are experimentally estimated for concrete cases of recording reflected signals. A comparison of our estimates with independent analogous data simultaneously obtained for the same parameters with monitoring instruments (a sodar and a temperature profiler) has yielded satisfactory results.     

Reduction of radar cross section by multiple passive impedance loadings

A general formulation to reduce scattering from conducting objects by multiple passive impedance loadings is derived. The method of moments is used to analyze the conducting objects, and biquadratic programming as an iterative minimization method is used to find the optimum passive impedance. Computer programs to reduce the bistatic radar cross section for arbitrarily shaped thin-wire scatterers by multiple passive impedance loadings have been developed. Some calculated results for a straight wire, a circular loop, and a rectangular wire frame are shown. It is found that multiple loadings are much more effective to control scattering than a single loading.     

Small perturbation method of high-frequency bistatic volumescattering from marine sediments

The first-order small perturbation method (Born approximation), as frequently applied to high-frequency scattering from marine sediments, is critically reviewed, tested for accuracy and extended. The sediment is modeled as an acoustic-fluid half space with random fluctuations in density and compressibility. Several cases of volume scattering from typical marine sand and mud sediments are presented to illustrate the effects of two important assumptions: (1) the effects of assuming the density and compressibility fluctuations are proportional, and (2) half-space effects. By relaxing the assumption that the sediment density and compressibility are proportional, the bistatic scattering cross section is significantly altered. The effects of properly modeling the sediment as a random half space (as opposed to an infinite continuum) are also discussed. In the context of first-order perturbation theory, half-space effects manifest themselves as a “modified” spectra for density and compressibility fluctuations. It is shown that, for lossy sediments and for scattering near the specular direction, half-space effects are significant and cannot be neglected. This result is significant because current models of sediment volume scattering do not include half-space effects. In addition to the theoretical model, exact numerical simulations are used to evaluate the accuracy of the perturbation model for a limited number of cases     

Multistatic Sonar Localization

This paper develops bistatic contact-localization statistics as a function of source-target-receiver geometry, source and receiver location errors, sound-speed errors, time and bearing errors, and array-heading errors. We verify the analysis through Monte Carlo simulation and illustrate localization accuracy for some representative choices of source-target-receiver geometry and measurement error assumptions. We examine the improved localization accuracy of multistatic contacts that result from the fusion of multiple sonar contacts     

Acoustic scattering from composite wind-wave surfaces in`bubble-free' regimes

The problem of underwater acoustic scattering from truly composite wind-wave surfaces under zero-gradient conditions (Δc=0) is examined. Here the dominant small-scale component is postulated to be a soliton surface ensemble, produced by the nonlinear wind-wave interactions and associated with the wind-drift surface layer riding on the underlying, mostly large-scale gravity-capillary component of the composite surface. A general bistatic analysis, based on the Kirchhoff approximation, is presented, which includes arbitrary geometries, beam patterns, and general signals. Both low-frequency O(0.2-1 kHz) and high-frequency O(≳3 kHz) signals are considered, and far-field (Fraunhofer) geometries are assumed. Surface Doppler, including Doppler spread and the modulation effects of the large-scale component, are examined. Both forward-scatter and backscatter regimes are considered in the determination of the scattered field and received wave intensities, scattering cross-sections, and coherency measures of surface scatter. Particular attention is given to the high-frequency cases, with small grazing angles, moderate-to-strong mean surface winds, and essentially bubble-free regimes. Recent empirical data appropriate to these conditions are included, which support the soliton conjecture and illustrate the general results. Both coherent and incoherent scattering are examined, along with relevant surface Doppler data     

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.     

Data-model comparisons of reverberation at three shallow-watersites

Reverberation measurements made by the SACLANT Undersea Research Centre at three shallow-water sites (130-190-m depth) are compared with each other and with estimates from the DREA normal-mode reverberation model OGOPOGO. The experiments over silt-clay and sand seabeds were conducted at slightly bistatic geometries (0.7-6.0-km source-receiver separation), using explosive sources detonated at mid-water depths. The signals were received on hydrophones of either a vertical or horizontal array and analyzed in one-tenth-decade frequency bands from 25 to 1000 Hz. The data are compared with each other to investigate the site differences and frequency dependencies, and with the estimates from the reverberation model OGOPOGO to interpret the data and to obtain a qualitative measure of the scattering. For modeling purposes, geoacoustic models of the seabed were assumed, and the reverberation data were fitted by adjusting the Lambert bottom scattering coefficients. Good model agreement was obtained with both individual hydrophone and data. Though somewhat sensitive to the geoacoustic the Lambert coefficients give a measure of the frequency dependence of the scattering. For the silt-clay bottom, the scattering is weak but is independent of frequency; for the sand bottoms, the scattering is stronger and increases with frequency. These results are compared with estimates from other experiments