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Ocean wave periods are estimated using measurements of S-band signals-of-opportunity reflections from the sea surface. The measurements were collected on an oil platform and show a time evolving Doppler centroid frequency and spectral width due to the vertical motion of ocean waves within the glistening zone observed. Previous results for estimating the significant wave height are also re-examined using the Doppler spectral width. For the one month dataset considered, the period of oscillations in the Doppler centroid frequency are found highly correlated ($\mathbf {R \approx 0.78}$ and $\mathbf {Bias \approx 1.02 (s)}$) to ocean wave period information from a nearby buoy.

An analysis of a Level-2 (L2) soil moisture record extending from 1 May 2021 to 1 January 2024 derived from Spire, Inc.’s Global Navigation Satellite System Reflectometry (GNSS-R) observatories is presented. The product's sensitivity to large scale soil moisture variability is demonstrated using an example of a 2022 flood in Pakistan. Product consistency among the constellation's multiple satellites is also investigated; no clear evidence of intersatellite biases is observed. Further comparisons are performed with soil moisture datasets from the Soil Moisture Active Passive (SMAP) and Cyclone Global Navigation Satellite System (CYGNSS) missions, from the European Center for Medium-Range Weather Forecasts Reanalysis v5 (ERA5), and from in situ International Soil Moisture Network (ISMN) sites. Although an overall product correlation with SMAP soil moisture of approximately 85$\%$ is determined, per-pixel correlations vary significantly and per-pixel root-mean-square errors (RMSE) can range from 0.02 to 0.09 (cm$^{3}$/cm$^{3}$) depending on land class. The importance of applying the product's quality flags is also demonstrated. The influence of other calibration effects and inland water body contamination on these results is also discussed.

Terahertz (THz) communications systems have been considered as one of the enabling technologies for beyond 5G mobile networks, with the ability to offer higher capacity and lower latency compared to the current systems. In this paper, we develop a new THz channel model that captures rough surface scattering while considering attenuation due to molecular absorption of the atmosphere. The main advantage of the proposed model is that it conserves the energy of the channel impulse response, regardless of the number of launched rays in the ray-tracing simulator, or the size of the tiles for the scattering surface. Using the proposed model, a simple propagation scenario for the indoor environment has been analyzed, and the improvement over other models discussed in detail.

Mueller matrix polarimetry (MMP) is useful for the characterization of complex samples, which is still challenging for multiple scattering of rough surface materials. The Mueller matrix represents the transfer function of an optical system when it interacts with polarized laser light, and its elements are related to specific properties and surface structures of samples. In this work a Mueller matrix decomposition method is proposed for rough surface materials based on He-Ne laser light scattering and the optimal method. The measured Mueller matrices of rough surface samples are respectively decomposed into Mueller-Jones and depolarization matrices related to nonuniform multiple scattering. The experimental results show that the method improves the Mueller matrix decomposition accuracy of the samples, with the decomposition accuracy of the soil is improved up to 67% compared with the traditional method. The method shows great potential for applications in polarization measurement, laser-matter interactions, target decomposition and polarization scattering of rough surface materials. [ABSTRACT FROM AUTHOR]

An assessment of the ocean surface Level-1 normalized bistatic radar cross Section (BRCS) products provided by the two batch 1 “LEMUR-2” Global Navigation Satellite Systems Reflectometry (GNSS-R) receivers of Spire, Inc. is reported. The analysis uses datasets extending from DOY 345, 2020 to DOY 329, 2021. Initial assessments indicate a highly consistent overall interchannel response with mean normalized bistatic radar cross Section (NBRCS) differences estimated to be at the 1.00% level over a 7–12-m/s European Centre for Medium-Range Weather Forecasts (ECMWF) reference wind speed range. Efforts to validate the observation systems’ aggregate response relative to $\approx 3$ million colocated Cyclone Global Navigation Satellite System (CYGNSS) measurements suggest a highly complementary behavior with an overall NBRCS correlation of 79.03% highlighting the potential utility of “LEMUR-2” measurements for ocean surface wind sensing and related applications. Nonphysical NBRCS dependencies on various Level-1 calibration variables, also observed with GNSS-R previous systems, are nonetheless noted and are explored in detail. [ABSTRACT FROM AUTHOR]

Recently, Mueller matrix polarimetry (MMP) has been widely applied in many aspects, such as radar target decomposition, monitoring the glucose level, tissue diagnostics, biological samples, etc., but it is still challenging for the complex light–matter interactions of rough surfaces and non-uniform structures such as 3D composite materials. In this work, a unitary matrix-based Mueller matrix decomposition (UMMMD) is proposed for non-destructive testing (NDT) of unmanned aerial vehicles (UAVs) skin. The decomposition model is constructed by the unitary matrix transformation of coherency matrices. In the model, the non-uniform depolarization caused by multiple scattering is quantified with the depolarization matrix and the entropy. From this model, the Mueller matrix of multiple scattering media can be completely decomposed. The proposed method can provide more polarization information than some traditional methods for multiple scattering under different polarization states. The contrast of the obtained polarization image can be improved by about 13 times compared to that of the original image. In addition, the key features of UAV skin such as deformation, shear angles, and density are obtained. The shear angles vary from 17° to 90°, and the average density is about 20/cm2. The provided experimental results show that this method is effective for the NDT of UAVs skin. The method also shows great potential for applications in target decomposition, NDT of 3D composite materials, 3D polarization imaging, light–matter interactions of non-uniform complex structures, etc. [ABSTRACT FROM AUTHOR]

In this paper, we show that speckle averaging helps to reduce the measurement error associated with a Shack–Hartmann wavefront sensor (SHWFS); however, this reduction is rendered ineffective with increasing beacon anisoplanatism. We do so operating in a weak-scintillation regime, where the SHWFS offers robust performance, and using in-plane translation of the illuminated rough surface to accomplish frame-to-frame speckle diversity. Understanding these trade-space limitations is critical when performing wavefront sensing with noncooperative, extended-source beacons. • We derive analytic estimates for RMS wavefront error/aberration in vacuum/turbulence. • We present a new modeling framework to simulate partially correlated speckle frames. • We report simulation results in wavefront error/aberration and irradiance skewness. • We demonstrate the benefits of speckle averaging under isoplanatic conditions. • We identify potential drawbacks of speckle averaging in anisoplanatic conditions. [ABSTRACT FROM AUTHOR]

By the use of Green's second integral identity we determine the field scattered from a two-dimensional randomly rough isotropic or anisotropic Dirichlet or Neumann surface when it is illuminated by a scalar Gaussian beam. The integral equations for the scattering amplitudes are solved nonperturbatively by a rigorous computer simulation approach. The results of these calculations are used to calculate the full angular distribution of the mean differential reflection coefficient. For isotropic surfaces, the results of the present calculations for in-plane scattering are compared with those of earlier studies of this problem. The reflectivities of Dirichlet and Neumann surfaces are calculated as functions of the polar angle of incidence, and the reflectivities for the two kinds of surfaces of similar roughness parameters are found to be different. For an increasing level of surface anisotropy, we study how the angular intensity distributions of the scattered waves are affected by this level. We find that even small to moderate levels of surface anisotropy can significantly alter the symmetry, shape, and amplitude of the scattered intensity distributions when Gaussian beams are incident on the anisotropic surfaces from different azimuthal angles of incidence.

By the use of Green's second integral identity we determine the field scattered from a two-dimensional randomly rough isotropic or anisotropic Dirichlet or Neumann surface when it is illuminated by a scalar Gaussian beam. The integral equations for the scattering amplitudes are solved nonperturbatively by a rigorous computer simulation approach. The results of these calculations are used to calculate the full angular distribution of the mean differential reflection coefficient. For isotropic surfaces, the results of the present calculations for in-plane scattering are compared with those of earlier studies of this problem. The reflectivities of Dirichlet and Neumann surfaces are calculated as functions of the polar angle of incidence, and the reflectivities for the two kinds of surfaces of similar roughness parameters are found to be different. For an increasing level of surface anisotropy, we study how the angular intensity distributions of the scattered waves are affected by this level. We find that even small to moderate levels of surface anisotropy can significantly alter the symmetry, shape, and amplitude of the scattered intensity distributions when Gaussian beams are incident on the anisotropic surfaces from different azimuthal angles of incidence.

The Half-Space Matching (HSM) method has recently been developed as a new method for the solution of two-dimensional scattering problems with complex backgrounds, providing an alternative to Perfectly Matched Layers (PML) or other artificial boundary conditions. Based on half-plane representations for the solution, the scattering problem is rewritten as a system coupling (1) a standard finite element discretization localized around the scatterer and (2) integral equations whose unknowns are traces of the solution on the boundaries of a finite number of overlapping half-planes contained in the domain. While satisfactory numerical results have been obtained for real wavenumbers, well-posedness and equivalence of this HSM formulation to the original scattering problem have been established for complex wavenumbers only. In the present paper we show, in the case of a homogeneous background, that the HSM formulation is equivalent to the original scattering problem also for real wavenumbers, and so is well-posed, provided the traces satisfy radiation conditions at infinity analogous to the standard Sommerfeld radiation condition. As a key component of our argument we show that if the trace on the boundary of a half-plane satisfies our new radiation condition, then the corresponding solution to the half-plane Dirichlet problem satisfies the Sommerfeld radiation condition in a slightly smaller half-plane. We expect that this last result will be of independent interest, in particular in studies of rough surface scattering. [ABSTRACT FROM AUTHOR]

A method for mapping the dynamics of the South Asian monsoon season is proposed based on the use of the coherence of CYGNSS Level 1 delay-Doppler Map measurements. Because the apparent coherence of the specular forward scattering observed by CYGNSS provides information on the presence of inland water, it can be related to different phases of the monsoon season. Results reported from 2017 to 2020 illustrate inter-annual variations in the seasonal monsoon, and show high degrees of correlation with SMAP soil moisture maps over the Indian subcontinent. Comparisons with precipitation information from the global precipitation measurement mission also show relationships indicative of the monsoon's dry and wet phases. The analysis presented in this work highlights the potential of using spaceborne GNSS-R systems for the operational mapping of monsoon dynamics.

We present the development and implementation of a novel wavelet shrinkage technique for the retrieval of obscured characteristic resonant signatures in the scattered terahertz (THz) reflectivity of molecular crystals. In this implementation, the wavelet basis functions associated with the absorption features were identified using the second-order total variation of the wavelet coefficients. Additionally, wavelet coefficients at certain scales were modified using the phase function corrections and wavelet hard thresholding. Reconstruction of the original spectra using these modified wavelet coefficients yielded the exact resonant frequencies of the chemicals, which were otherwise unrecognizable in the spectral artifacts of the rough surface scattering. We examined the robustness of this method over controlled levels of rough surface scattering, validated using the Kirchhoff approximation, in spectroscopic targets made from α-lactose monohydrate and 4-aminobenzoic acid (PABA), which have close spectral lines. We successfully retrieved the spectral absorption fingerprints in both specular and off-specular reflection geometries. This technique can be utilized for stand-off material characterization using the THz reflection spectroscopy in uncontrolled environments and potentially can be adopted for other broadband spectroscopic modalities.

This work further investigates the use of ocean wind speed retrievals from NASA's Cyclone Global Navigation Satellite System (CYGNSS) mission that are closely spaced in space and in time to detect regions in the atmosphere undergoing “rapid” change. As in a previous investigation, CYGNSS measurement “clumps” (i.e., groups of wind speed measurements satisfying specified time/space separation criteria) are used to create a wind speed change detector, and the results are compared to a set of atmospheric properties derived from Modern-Era Retrospective Analysis for Research and Applications 2 model datasets. The analysis uses a four-year CYGNSS Level-2 wind speed dataset developed by the National Oceanic and Atmospheric Administration and highlights the potential for using CYGNSS's wind speed measurements to locate various dynamic activities occurring over the ocean's surface. The presented results also highlight some of the challenges of the method, including the inherent dependence of detection performance on the quality of CYGNSS's wind speed estimates as well as the validity of comparisons to “truth” datasets having temporal resolutions much coarser than those provided by CYGNSS.

Currently, sea-ice thickness is often estimated using microwave radar altimetry. It is well known that radar measurements are highly dependent on the roughness characteristics of the observed scene and can have a profound effect on altimetry-based thickness estimates. This simulation-based study highlights the importance of sea-ice roughness parameterization in the context of radar altimetry. This work provides a new sea-ice roughness parameterization in the form of an elevation spectrum developed from field campaign data. The formulation of this elevation spectrum is partly based on the generalized power law. Simulations have been developed to predict electromagnetic scattering from surfaces described by the new spectrum, with a focus on the X-band (10 GHz) scattering with nadir incidence. In this work, both the analytical small slope approximation and numerically exact Method of Moments are applied to various sea-ice surface spectra, representing a wide range of surface roughness conditions. The results show that the normalized radar cross section of the sea-ice interface varies significantly depending on the surface elevation spectrum and the statistics of the rough interface. Comparisons with numerically exact methods show that analytical approximations to rough surface scattering work reasonably well for the cases considered here, except at large scattering angles. [ABSTRACT FROM AUTHOR]

An adaptive version of the multirank Capon algorithm is constructed for acoustic-source localization by a vertical array in the case of incomplete information on the random propagation channel. Under the assumption that sound scattering is mainly caused by wind seas, statistical modeling is performed to determine the accuracy of source coordinate estimation and the probability of correct source localization. The proposed method is tested by experimental data to demonstrate its performance in the Barents Sea. [ABSTRACT FROM AUTHOR]

First, an acceleration to compute the impedance matrix obtained from the electric field integral equation (EFIE) discretized by the Galerkin method of moments (MoM) with Rao–Wilton–Glisson basis functions is addressed. It is based on a far-field approximation and makes it possible to avoid looping on the source and observation triangles. Next, the impedance matrix is split into strong and weak interactions; the latter is compressed by expressing it from Toeplitz submatrices. Then, the linear system is efficiently solved by a bi-iterative scheme. For a given order, the least-squares QR (LSQR) algorithm is applied to solve the sparse linear system related to the strong interactions, while the matrix–vector products, related to the weak interactions, are accelerated by using FFTs. Numerical results of the field scattered by perfectly conducting paraboloid-shape object and Gaussian rough surface are shown. [ABSTRACT FROM AUTHOR]

An algorithm for detecting coherence in Cyclone Global Navigation Satellite System (CYGNSS) mission delay-Doppler maps (DDMs) is presented. Because CYGNSS DDMs report only the observed power without phase information, the algorithm uses estimates of power “spread” within the DDM to flag coherency. Since the estimate used is a ratio of the powers in differing portions of the DDM, it is less sensitive to absolute power calibration and to the GPS C/A code type observed, and is applied to CYGNSS Level-1 uncalibrated DDMs. The basic detector formulation is described along with modifications to improve performance in lower signal-to-noise ratio (SNR) situations. The required detection thresholds are determined using matchups with CYGNSS “Raw I/F” mode measurements for which the DDM phase can be computed and used to identify coherence more precisely. Application of the final detector over a large CYGNSS data set suggests that approximately 8.9% of all inland returns are coherent. Inland regions persistently identified as coherent were found largely to be associated with the presence of water bodies. A smaller set of desert locations apparently having very low surface roughness were also found to be associated with persistent coherence. The detector was also applied to a set of ocean measurements, with the results showing that persistent coherence is limited to areas with sheltered waters. Ocean tests avoiding such regions indicate that the detector’s false-alarm rate is approximately 0.0012% for the detection threshold used. [ABSTRACT FROM AUTHOR]

In this article, an efficient algorithm for the reconstruction of a 1-D random rough surface profile separating two lossy dielectric half-spaces is presented. First, the general scattering problem is formulated by the use of surface integral equations (SIEs). Then, the synthetic scattering field data are obtained through the use of these conventional SIEs. In the inverse problem, the same SIEs together with the data equation are solved in an iterative fashion to reconstruct the surface variation. In the numerical implementation, the so-called ill-posed inverse problem is regularized in the sense of Tikhonov, and a least squares solution is obtained by the use of appropriate basis functions. A very detailed numerical assessment of the presented approach is provided which shows that the method is very effective and promising. [ABSTRACT FROM AUTHOR]

In order to study the effect of laser speckle on target detection, the theoretical analysis of the statistical characteristics of the scattering field when the plane wave laser irradiates the rough sphere and cone target was carried out by using the physical optics approximation method, and the second order statistical moment of the scattering field quantity of rough targets was derived. The variation of incoherent scattering component ratio of rough sphere and cone with roughness, scattering angle, radius, and target material is calculated numerically. The results show that the change of scattering angle has an effect on the incoherent component ratio of rough sphere scattering. The larger the roughness, the larger the proportion of incoherent component of target to the total scattering component. As the radius of the rough sphere becomes smaller, the surface of the sphere becomes rougher. The peak position of incoherent component ratio of cone target scattering varies with roughness, but its peaks are all located in the direction of specular reflection. The incoherent component ratio of metallic materials is smaller than that of non-metallic polished aluminum materials, and the radius change is proportional to the incoherent component ratio. The research results provide some reference value for the study of laser scattering characteristics of more complex targets and laser speckle detection and identification. [ABSTRACT FROM AUTHOR]

A method for mapping the dynamics of the South Asian monsoon season is proposed based on the use of the coherence of CYGNSS Level 1 delay-Doppler Map measurements. Because the apparent coherence of the specular forward scattering observed by CYGNSS provides information on the presence of inland water, it can be related to different phases of the monsoon season. Results reported from 2017 to 2020 illustrate inter-annual variations in the seasonal monsoon, and show high degrees of correlation with SMAP soil moisture maps over the Indian subcontinent. Comparisons with precipitation information from the global precipitation measurement mission also show relationships indicative of the monsoon's dry and wet phases. The analysis presented in this work highlights the potential of using spaceborne GNSS-R systems for the operational mapping of monsoon dynamics. [ABSTRACT FROM AUTHOR]

It is demonstrated that acoustic transmission through a phononic crystal with anisotropic solid scatterers becomes non-reciprocal if the background fluid is viscous. In an ideal (inviscid) fluid, the transmission along the direction of broken P symmetry is asymmetric. This asymmetry is compatible with reciprocity since time-reversal symmetry (T symmetry) holds. Viscous losses break T symmetry, adding a non-reciprocal contribution to the transmission coefficient. The non-reciprocal transmission spectra for a phononic crystal of metallic circular cylinders in water are experimentally obtained and analysed. The surfaces of the cylinders were specially processed in order to weakly break P symmetry and increase viscous losses through manipulation of surface features. Subsequently, the non-reciprocal part of transmission is separated from its asymmetric reciprocal part in numerically simulated transmission spectra. The level of non-reciprocity is in agreement with the measure of broken P symmetry. The reported study contradicts commonly accepted opinion that linear dissipation cannot be a reason leading to non-reciprocity. It also opens a way for engineering passive acoustic diodes exploring the natural viscosity of any fluid as a factor leading to non-reciprocity. [ABSTRACT FROM AUTHOR]

A novel closed-form high-order small perturbation method (HOSPM) for the analysis of scattering from 1-D conducting random rough surfaces under TE incidence is developed. The main theoretical contributions of the HOSPM are as follows: (1) our method yields a general high-order SPM form for scattered fields of arbitrary orders, (2) Faà di Bruno's formula is introduced into computational electromagnetics (CEM) for the first time to expand a tapered incident wave and its partial derivatives in power series form, and (3) the form is simple and easy to program and does not require any mathematical pretreatment. Comparisons are made between the method of moments (MOM) and different-order HOSPMs in terms of several aspects, including accuracy and time efficiency. The order convergence of the HOSPM is discussed, the regions of validity with regard to correlation lengths and root mean square (RMS) heights are demonstrated for the 2nd-order HOSPM, and the robustness of the 2nd-order HOSPM is proven over a broad range of frequencies. [ABSTRACT FROM AUTHOR]

A major problem with forward-looking ground-penetrating radar (FLGPR) detection of buried explosive threats is the scattering from the rough ground surface. The authors have previously developed a real-time 3-D algorithm for single-view emulation of synthetic aperture FLGPR scattering from rough terrain at low grazing angles. This article extends the method to the real-time 3-D simulation of a multiview moving platform transmitter/receiver array moving as fast as 15 km/h. The need to perform the new calculation at every frame for the moving antenna array platform is shown to be unnecessary, with the requirement relaxed to once for only a single frame at the center of 10–20 m forward interrogation range, leading to about 50 times faster computation. The computation of the scattered waves comprising surface clutter is reduced for all moving frames to a mere multiplication of three matrices: a pre-computed impulse response matrix of rough terrain, a pre-computed correction matrix of moving frames, and a matrix characterizing the transmitting input signal. The method is evaluated via 3-D Monte Carlo simulation for various rough surface parameters, and its applicability for subsurface scattering reconstruction to characterize the buried threat objects is shown. For a vehicle-mounted FLGPR detection system, this leads to a significant saving of computation resources. Our developed algorithm provides 3-D modeling of rough terrain scattering for lossy and frequency-dispersive soil and compares well with full-wave finite-difference frequency-domain (FDFD) method computation. [ABSTRACT FROM AUTHOR]

This paper examines the impact of ocean surface swell waves on near-coastal L-band high-resolution synthetic aperture radar (SAR) data collected using the National Aeronautics and Space Administration’s (NASA) Soil Moisture Active/Passive (SMAP) radar at 40° incidence angle. The two-scale model and a more efficient off-nadir approximation of the second-order small-slope-approximation are used for co- and cross-polarized backscatter normalized radar cross-section (NRCS) predictions of the ocean surface, respectively. Backscatter NRCS predictions are modeled using a combined wind and swell model where wind-driven surface roughness is characterized using the Durden–Vesecky directional spectrum, while swell effects are represented through their contribution to the long wave slope variance (mean-square slopes, or MSS). The swell-only MSS is numerically computed based on a model defined using the JONSWAP spectrum with parameters calculated using the National Data Buoy Center and Wave Watch III data. The backscatter NRCS model is further refined to include fetch-limited and low-wind corrections. The results show an improved agreement between modeled and observed HH-polarized backscatter NRCS when swell effects are included and indicate a relatively larger swell impact on L-band compared to higher radar frequencies. Preliminary investigations into the potential swell retrieval capabilities in the form of excess MSS are encouraging, however further refinements are required to make broadly applicable conclusions.

Electromagnetic wave scattering from planar dielectric films deposited on one-dimensional, randomly rough, perfectly conducting substrates is studied by numerical simulations for both p- and s-polarization. The reduced Rayleigh equation, which is the integral equation satisfied by the scattering amplitude after eliminating the fields inside the film, is the starting point for the simulation. This equation is solved numerically by considering a random surface of finite length, and by introducing wave number cut-offs in the evanescent part of the spectrum. Upon discretization, a system of linear equations is obtained, and by solving this matrix system for an ensemble of surface realizations, the contribution to the mean differential reflection coefficient from the incoherently scattered field. 〈∂Rν/∂θ〉incoh (ν = p,s), is obtained nonperturbatively. It is demonstrated that when the scattering geometry supports at least two guided waves, 〈∂ν/∂θ〉incoh, has, in addition to the well-known enhanced backscattering peak, well-defined satellite peaks in agreement with theory, for most of the parameters considered.

An analysis of the coherency properties of specular scattering from ocean and land surfaces as observed in global navigation satellite system-reflectometry (GNSS-R) and signals of opportunity systems is presented. This analysis applies existing approximate models for the coherent and incoherent contributions. Approximate expressions are developed for when one component of the return is likely to dominate as a function of surface and observing system properties. The model developed is then applied for sea surface returns, and the relative contribution of the coherent term is expressed as a function of the receiver height, frequency, incidence angle, and wind speed. For L-band spaceborne measurements, it is shown that coherence is expected only for wind speeds less than 2–3 m/s, while for P-band spaceborne measurements, coherence can dominate returns for wind speeds up to 5–7 m/s. For land surface measurements from space, it is shown that the surface rms height needs to be sufficiently low for coherent components to dominate returns. Coherence dominates for roughness values not exceeding a range of 5–7 cm for the L-band and 15–30 cm for the P-band. For the L-band, these conditions over land are likely to be created primarily by inland water bodies. A model for the specular scattering from a water body, including earth curvature effects, is then developed to highlight the strong dependence of the resulting coherent field on the shape of the water body and any offset in its location from the specular point. These results further clarify the significant variability that should be expected in coherent scattering from inland water bodies. [ABSTRACT FROM AUTHOR]

Ground penetrating radar (GPR) is a viable tool for fast and high fidelity detection of concealed explosive threats. The radar effectiveness is limited by scattering from rough terrain which considerably obscures the buried target response. To calculate the rough ground scattering, a 3-D full-wave algorithm such as finite-difference frequency-domain (FDFD) method is required but is often prohibitive for multiple frames when the GPR antennas are distant from the target region. This paper presents a real-time 3-D modeling of a moving platform forward-looking GPR scattering from rough terrain located at great electrical distances from the GPR antenna. For a synthetic aperture, the computational domain of the focal region is reduced to a very small subset of the entire observed volume, and the surface clutter is computed via a mere multiplication of a precomputed impulse response matrix of the rough ground with the matrix characterizing the GPR transmitting signal. For a vehicle-mounted GPR detection system, this results in a significant reduction of complexity and saving of computation resources. The effectiveness of the algorithm is evaluated through an implementation of 3-D Monte Carlo simulation for various rough surface parameters. Our developed model compares well with the direct FDFD results, and can be used for lossy and frequency-dispersive soils. [ABSTRACT FROM AUTHOR]

A high-fidelity full-wave simulator is presented to perform numerical experiments for rough sea scattering problem by considering different polarizations, frequencies, grazing angles, wind speeds, and sea surface spectra. The simulator is based on a novel finite-element domain decomposition (FEDD) method for solving the problem of 2-D electromagnetic scattering over 1-D sea surface. This noniterative method partitions the computational domain into a number of overlapping subdomains and solves each domain individually by employing the locally conformal perfectly matched layer (LC-PML) at the truncation boundaries. LC-PML has a unique feature such that it can be applied to irregular domains on the contrary to standard PML methods, and hence inspired the birth of FEDD. The FEDD method is used at each Monte Carlo realization corresponding to a sample from random rough surfaces and decreases the computational load, especially for electrically large problems. The accuracy and computational efficiency of the method are investigated through several simulations. Using the FEDD method, the statistical behavior of the bistatic radar cross section (RCS) is obtained for both the horizontal and vertical polarizations. A special emphasis is given to forward-scattered RCS and the mean reflection coefficient for sea surface, especially at low grazing angles, and it is shown that the simulator produces results in agreement with the Ament and Miller–Brown approximations, and experimental data, proving the reliability of the simulation approach. The results are also compared with the standard finite-element method and method of moments. Rough sea surfaces are created by using both the Pierson–Moskowitz and Elfouhaily spectra. [ABSTRACT FROM AUTHOR]

A domain decomposition method for analyzing very large finite-difference time domains, tens of thousands of wavelengths long, is demonstrated by the application to the problem of radar scattering in the maritime environment. Success depends on the elimination of artificial scattering from the “sky” boundary which is ensured by an ultra-high-performance absorbing termination that eliminates this reflection at angles of incidence less than 0.03° off grazing. The results are cross validated against a parabolic equation (PE) method and a surface integral equation method on a 1.7 km sea surface problem, and to a PE method on propagation through an inhomogeneous atmosphere in a 4 km-long space, both at X-band. Further comparisons are made against boundary integral equation and PE methods from the literature in a 3.6 km space containing an inhomogeneous atmosphere above a flat sea at S-band. Details of the method are given for the 2-D problem. Application to 3-D is demonstrated by comparing the 2-D solution with the 3-D solution of a sea corridor 1.3 m wide, 2.25 m tall, by 110 m long. [ABSTRACT FROM AUTHOR]

Non-Rayleigh distributed radar clutter is widely reported in studies of radar scattering from sea and land surfaces. Existing models of scattered field amplitude distributions have been developed primarily through empirical fits to the statistics of radar backscatter measurements. In contrast, this paper investigates a physics-based approach to determine the amplitude distributions of fields scattered from rough surfaces using Monte Carlo simulations and analytical methods, for both backscattering and bistatic configurations. The rough surface is represented using a “two-scale” model. An individual surface facet contains “small-scale” roughness, for which scattered fields are evaluated using the second-order small slope approximation. Individual surface facets are tilted by the slopes of the “large-scale” roughness in a given observation. The results show that non-Rayleigh amplitude distributions are obtained when tilting is performed, and that the departure from the Rayleigh distribution becomes more significant as the variance of the tilting slope increases. Further analysis shows that this departure results from variations in the mean scattering amplitude from a facet (the texture) as tilting occurs. The distribution of the texture is studied and compared with existing models. Finally, the distribution of the scattered field amplitude is modeled through the compound Gaussian model, first using the distribution of the texture, and then in terms of the probability density function of tilting slopes (which avoids the requirement of the knowledge of the texture distribution). The results from the above two methods are in good agreement and both agree well with the Monte Carlo simulation. [ABSTRACT FROM PUBLISHER]

The moments of a plane wave scattered at low grazing angles from a one-dimensional perfectly reflecting rough surface are considered. The mean intensity and autocorrelation of the scattered field and the corresponding angular spectrum are obtained to second order in surface height. The derivations are based on an operator expansion of the extended (two-way) parabolic integral equation solution. The resulting operator series describes successively higher-order surface interactions between forward and backward going components. The expressions derived may be regarded as backscatter corrections to those obtained via the standard (one-way) parabolic integral equation method. [ABSTRACT FROM PUBLISHER]

High-spatial-frequency optical fields or sources are often encountered when simulating directed energy, active imaging, or remote sensing systems and scenarios. These spatially broadband fields are a challenge in wave optics simulations because the sampling required to represent and then propagate these fields without aliasing is often impractical. To address this, two spatial filtering techniques are presented. The first, called Fresnel spatial filtering, finds a spatially band-limited source that, after propagation, produces the exact observation plane field as the broadband source over a user-specified region of interest. The second, called statistical or quasihomogeneous spatial filtering, finds a spatially band-limited source that, after propagation and over a specified region of interest, yields an observation plane field that is statistically representative of that produced by the original broadband source. The pros and cons of both approaches are discussed in detail. A wave optics simulation of light transiting a ground glass diffuser and then propagating to an observation plane in the near-zone is performed to validate the two filtering approaches. [ABSTRACT FROM AUTHOR]

Use of a bistatic reflectometry technique which employs signals of the Global Navigation Satellite System (GNSS) scattered off the Earth surface opened new opportunities in the environmental remote sensing. At the same time, similar approach can be implemented with non-GNSS signals. Recently, it was demonstrated that other signals of opportunity can be used, such as produced by communication satellites in K-, X-, S- at VHF bands and even by the Sun. This paper presents a theoretical analysis of a possible use of a noise-like broadband radio signal emitted by a point source for measuring reflectivity properties of Earth’s surface. [ABSTRACT FROM AUTHOR]

Statistical parameters of the beat signal of the frequency-modulated continuous-wave (FMCW) radar altimeter in case of scattering from cylindrical rough surface are determined in the Kirchhoff approximation. A concept of the double dimensionality of the beat signal is formulated in this paper. Results of this paper can be used to construct a simple simulation model to assess signal processing algorithms implemented in an FMCW radar altimeter. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate, by means of experimental measurements and full-wave simulations, the dominant factors for the very high-resolution (VHR) radar image speckles from exponentially correlated rough surfaces. A Ka-band radar system was used to collect the return signal from such a surface sample fabricated by 3D printing and that signal was further processed into images at different resolution scales, where the image samples were obtained by horizontally turning around the surface sample. To cross-validate the results and to further discuss the VHR speckle properties, full wave simulations by full 3D Finite Difference Time Domain (FDTD) method were conducted with 1600 realizations for the speckle analysis. At the considered very high resolution, speckle statistics show divergence from the fully developed Rayleigh distribution. The factors that impact on the high-resolution speckle properties from exponentially correlated rough surface, are analyzed in views of the equivalent number of scatterers theory and scattering scales, respectively. From the data results and extended discussions, it is evident that both of the above factors matter for VHR speckle of backscattering, from the exponentially correlated rough surface as a good representative for the ground surface.

In this paper, the well-known Ament model is generalized to multiple rebounds (reflections) from rough sea surfaces in a stratified medium. To solve this propagation-scattering problem in the radar microwave domain, the physical optics (PO) approximation is applied to determine the currents on the rough sea surface. Next, to solve the propagation problem, i.e., to calculate the scattered field in the duct, an appropriate Green function of the duct (which is a stratified medium with linear-square refraction index profile) is used to radiate the currents from Huygens’ principle. Due to refraction phenomenon, this field induces new currents on the surface (second rebound), which are again evaluated by PO. The process is iterated. Then, for each rebound, the coherent components of the surface currents and the scattered field are calculated and compared with those of a flat surface in order to derive the generalized Ament model. In addition, this model is compared with the results computed by the rigorous method of moments and accelerated by the multilevel subdomain decomposition iterative method combined with the adaptive cross approximation. [ABSTRACT FROM AUTHOR]

A computationally efficient near-ground field prediction model is proposed to facilitate realistic wireless sensor network (WSN) simulations. In this model, using the principle of Fresnel zones, path loss is split into three segments. Certain propagation mechanisms dominate in each part. The distances that define the edges of each section are derived theoretically. The model is validated against several experimental datasets obtained in open and forested areas. It is noticed that the proposed model has higher accuracy compared to existing analytical near-ground propagation models. This enhancement is achieved by careful assessment of key features relevant to near-grazing propagation such as diffraction loss due to obstruction of the first Fresnel zone and higher order waves produced by terrain irregularities. Monte Carlo simulations are used to investigate the effects of antenna height, frequency of operation, polarization, and terrain dielectric and roughness properties on WSNs performance. It is realized that antenna height is by far the most influential geometric parameter to low-altitude WSNs connectivity and average number of neighbors. [ABSTRACT FROM PUBLISHER]

Based on the reflection properties of rough surfaces, we propose a statistical multiple-input multiple-output (MIMO) antenna system in the terahertz (THz) band for nanocommunications. Firstly, our analysis of scattering from a rough surface indicates that the reflection from a single surface can be a cluster of rays. Secondly, a new MIMO model for THz communications is proposed. In this model, the number of multipaths is highly dependent on the roughness of the reflecting surfaces. When the surface is ideally smooth, the MIMO channel is sparse and as a result, the capacity is sub-linear with the MIMO scale. On the other hand, when the surface is rough, more degrees of freedom are provided by the scattered rays. Finally, channel capacities with different surface roughness are numerically calculated and compared between different MIMO scales. The results show that in contrast to the GHz range, large scale THz multiple antennas may not provide as much multiplexing gain. Therefore, it is necessary to determine the antenna scale according to the actual propagation environment. [ABSTRACT FROM AUTHOR]

A full-wave theory of plane wave scattering from rough surfaces called the correction current (CC) method was recently developed. This paper extends the CC method to the more general case of beam wave scattering. Gaussian beams are assumed incident upon a deterministic metal rough surface. Scatter patterns based on the CC method are calculated and compared to scatter patterns determined by the method of moments solution of an electric field integral equation formulation of the scatter problem. It was shown that good agreement between these two solution methods is obtained for scatter surfaces for which the CC method is shown to satisfy conservation of power and an accuracy criterion. [ABSTRACT FROM PUBLISHER]

ABSTRACT The coordinate transformation technique (with its current name of transformation electromagnetics) is applied to the finite-element method (FEM) with periodic boundary conditions for efficient Monte Carlo simulation of one-dimensional random rough surface scattering problems. In a unit cell of periodic structure, two coordinate transformations are used, one of which is a real transformation designed to model the rough surface with flat surface, and the other is a complex transformation used to design a perfectly matched layer for mesh truncation. The approach is easily implementable because only the nodal coordinates are replaced by the transformed coordinates in a standard FEM. Since a simple and single mesh with flat surface is used, Monte Carlo simulations are performed in an efficient manner. Numerical results are presented for various rms surface heights, correlation lengths, and angles of incidence. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:322-329, 2016. [ABSTRACT FROM AUTHOR]

In this study, small perturbation method (SPM) and Kirchhoff approximation (KA) are incorporated into ray-tracing (RT) routines to model multiuser multi-input multi-output (MU-MIMO) channels formed on a rough dielectric terrain. The effect of surface roughness and correlation length, solid soil fractions, moisture content, link range, antenna height, polarization, radiation pattern, and carrier frequency are examined on received power, power delay and angular profiles, root mean square (RMS) delay and angular spread, coherence bandwidth, and coherence distance. Quantitative and qualitative analyses reveal that antenna directionality and terrain undulation and textural composition have significant impacts on the received signal power and channel multipath parameters and, hence, the performance of MU-MIMO terrain-based communication systems. [ABSTRACT FROM PUBLISHER]