Backscattering Enhancement Research Articles

Study on Bragg and Non-Bragg Scattering Mechanism and Frequency Shifts From Time-Varying Periodic Water Wave

In this paper, Bragg and non-Bragg scattering mechanisms and frequency shifts from time-varying periodic water wave are studied from three aspects: wave tank measurements with an ultrahigh frequency radar, numerical simulation using the method of moments, and theoretical derivation applying the small perturbation method. The scattering field, radar cross section (RCS), and frequency shifts are discussed in both horizontal and vertical polarizations. The wave tank observations show that backscattering enhancement occurs when water wavelength is an integer multiple of Bragg wavelength, and there are several Doppler harmonics with frequency shifts of water-wave frequency and its integer multiples. Numerical simulations indicate that these Doppler harmonics except the one associated with the water-wave phase velocity are caused by the water surface edge effect. Moreover, theoretical analyses, numerical simulations, and wave tank experiments all show a clear exponential relationship between backscattering RCS and wave height. In addition, we further analyze the bistatic scattering and find that the scattering field is composed of plane waves propagating in the directions determined by water wavelength and radio wavelength, the bistatic frequency shift is the harmonic frequency of water wave, and the bistatic RCS also has an exponential relation with water-wave height.

Ionospheric Scintillation Observation Using Space‐Borne Synthetic Aperture Radar Data

AbstractNetworks of ground‐based global navigation satellite system (GNSS) receivers have been widely used to monitor scintillation caused by irregularities in the disturbed ionosphere. Due to the relative sparseness of such networks, however, scintillation measurements are lacking in many regions of the globe, and even in well‐instrumented geographic areas, the spacing between receivers is often too large to study the systematic spatial changes in scintillation characteristics, for example, across the equatorial anomaly region. This paper discusses the potential of studying ionospheric scintillations using low‐frequency synthetic aperture radar (SAR). It compares standard metrics of scintillation including the amplitude scintillation index S4 and vertically integrated strength of turbulence CkL, from GNSS and SAR, on two different dates with varying ionospheric conditions. For this study, polarimetric L‐band SAR images acquired from the Phased Array‐type L‐band Synthetic Aperture Radar sensor onboard the Advanced Land Observational Satellite‐2 have been used. A number of GNSS satellites also observed the particular scintillation event that was encountered by SAR on the night of 23 March 2015 over the southern and mid‐central India. The S4 index derived from SAR are computed using previously published techniques in terms of radar backscatter (σ°) enhancement and the image contrast. The results show a favorable correlation with the GNSS observations. Along with accurate information about satellite geometry and operating frequency, few spectral properties of ionospheric irregularities, such as spectral index, anisotropy, and outer scale, have been assumed from historically available low‐latitude scintillation observations to calculate the turbulence strength parameter. The results are well corroborated by measurements from four GNSS stations in India, thus demonstrating the utility of the SAR measurements in augmenting and complementing the ionospheric scintillation diagnostics available from GNSS.

Wideband planar retro-reflective metasurfaces for backscattering enhancement under oblique incidence

In this paper, we propose the design of wideband planar retro-reflective metasurface (WRRM) that can enhance backscattering under oblique incidence. By a delicate combination of narrow-band RRMs (NRRMs), the bandwidth with backscattering enhancement can be expanded significantly. As an example, a prototype operating in the X band is demonstrated. The modified metallic square loop structure is used as the sub-cell of four NRRMs. The operating bands of the four RRMs are tailored to act as quarter structures of the WRRM. A prototype was fabricated and measured. Both the simulation and experiment results verify that the WRRM can realize good backscattering enhancement performance in 8.0–12.0 GHz under incidence of angle θ = 20.0°.

Tunneling Aharonov-Bohm interferometer on helical edge states

We discuss transport through interferometer formed by helical edge states tunnel-coupled to metallic leads. We focus on the experimentally relevant case of relatively high temperature as compared to the level spacing and discuss a response of the setup to the external magnetic flux $\phi$ (measured in units of flux quantum) piercing the area encompassed by edge states. We demonstrate that tunneling conductance of the interferometer is structureless in ballistic case but shows a sharp antiresonances, as a function of magnetic flux $\phi$ - with the period 1/2 - in the presence of magnetic impurity. We interpret the resonance behavior as a coherent enhancement of backward scattering off magnetic impurity at integer and half-integer values of flux, which is accompanied by suppression of the effective scattering at other values of flux. Both enhancement and suppression are due to the interference of processes with multiple returns to magnetic impurity after a number of clockwise and counterclockwise revolutions around setup. This phenomenon is similar to the well-known weak-localization-induced enhancement of backscattering in disordered systems. The quantum correction to the tunneling conductance is shown to be proportional to flux-dependent "ballistic Cooperon". The obtained results can be used for flux-tunable control of the magnetic disorder in Aharonov-Bohm interferometers built on helical edge states.

High-efficiency broadband polarization-independent superscatterer using conformal metasurfaces

Safe detection of an arbitrarily shaped platform is critical for survivability, rescue, or navigation safety in a remote region. Metasurfaces afford great potential due to their strong electromagnetic (EM) wave control. However, studies have mainly focused on the physics and design of metasurfaces on planar plates, which does not satisfy the current requirements of aerodynamics and aesthetics. Herein, we propose a sophisticated strategy to design a metasurface that can wrap over arbitrarily shaped objects with moderate curvature on which optical aberrations are commonly introduced. By designing each meta-atom on the basis of the required position and phase compensation, exact EM wavefronts are restored. For verification, several conformal metasurfaces were designed and numerically studied on metallic cylinders at the microwave spectrum. A proof-of-concept device is fabricated and is experimentally characterized. The results demonstrate the availability of the desirable dual-beam superscatterer with strong backscattering enhancement toward two directions, thus indicating that the distortions induced by an arbitrary platform can be efficiently corrected. Our method affords an efficient alternative for designing high-performance multifunctional optoelectronic devices equipped on a moderately curved platform.

Coherent backscatter enhancement in single scattering.

Solution of Maxwell's equations to the problem of single scattering can be expanded into iterative series in an order-of-scattering form, where the interference between conjugate terms representing reversible sequences of elementary scatterers is constructive at the backscattering direction, resulting in a coherent backscatter enhancement (CBE). The backscattering phase function of randomly oriented particles is amplified by CBE with an amplification factor between 1 and 2 depending on particle habit and refractive index. The angular width of the CBE-induced backscattering peak line for a specific particle habit is inversely proportional to the particle size parameter. The CBE-induced backscattering peak has been identified in the scattering phase function of a wide range of randomly oriented particles, including non-absorptive spheres, spheroids, and hexagonal particles.

Quantitative ray methods for scattering by tilted cylinders

After a review of ray methods and phenomena associated with high frequency scattering by spheres and cylinders in water viewed broadside, generalizations to tilted cylinders relevant to spectral and imaging domains will be summarized. These extensions were found to be useful for meridional as well as helical ray backscattering enhancements associated with leaky (or supersonic) waves on shells [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 112, 528–536 (2002)]. For such enhancements, Fermat’s principle is useful for identifying ray paths of interest. In the case of helical waves (and in the broadside special case) the scattering amplitude can be expressed in terms of a Fresnel patch area where the guided wave is excited on the shell. Fresnel patches also give insight into the relatively large magnitude of meridional ray contributions. The coupling coefficient is proportional to the radiation damping of the leaky wave. For some shells it is necessary to take into account the anisotropy of the phase velocity. Computational benchmarks include scattering into the meridional plane by tilted infinite cylinders. Related phenomena include time-frequency domain features and enhancements from waves crossing truncations and from subsonic and negative group velocity guided waves. [Research supported by ONR.]

Research of a wide-angle backscattering enhancement metasurface

To enhance backscattering, corner reflector and Luneburg lens are usually used. They can operate effectively in a broad angle range and also in a quite wide band. However, corner reflector as a typical structure of backscattering enhancement device, has obvious disadvantages in practical application. For example, it is usually made of metal material, which causes it to be too heavy and bulky. Luneburg lens is generally made of dielectric with strong loss and high cost, which is unfavorable for applications. Thus, it is necessary to explore a new way to realize wide-angle backscattering enhancement. In this paper, a phase gradient metasurface with wide-angle radar cross section (RCS) enhancement property is proposed and demonstrated, which consists of two phase gradients with equal magnitude but in opposite directions. Through designing a reflective phase profile along the surface, an equivalent wave vector can be generated, with doubled magnitude but in an opposite direction to the parallel component of the wave vector of the incident wave. At the incidence angles =-45 and 45, electromagnetic (EM) waves are reflected to the directions just opposite to the directions of incident waves. And at incidence angle =0, the incident EM wave is coupled into spoof surface wave and then guided to another region to decouple into a free space wave. These guarantee RCS enhancement property in a related angular domain. The polarization independent Jerusalem cross unit is used to design the phase gradient, and a wide-angle RCS enhancement metasurface is designed. The simulated results indicate that at the designed incidence angles, directions of the reflected waves are all opposite to the directions of incidence waves for both x and y polarized wave. In order to evaluate the RCS enhancement performances, the mono-static RCS of the designed wide-angle RCS enhancement metasurface is measured. Both the simulations and experiments are in good agreement with each other, and show that the designed metasurface obtains tremendous RCS enhancement performances in a wide-angle domain (-45-45) for both x and y polarized wave with frequencies ranging from 9 GHz to 12 GHz.

Microwave Signatures of Snow Cover Using Numerical Maxwell Equations Based on Discrete Dipole Approximation in Bicontinuous Media and Half-Space Dyadic Green's Function

A three-dimensional snowpack scattering and emission model is developed by numerically solving Maxwell's equations (NMM3D) over the entire snowpack on an underlying half-space. The solutions are crucial to microwave remote sensing that requires the preservation of coherent phase information. The heterogeneous snowpack is represented as a bicontinuous medium. Effects of the underlying half-space are included through a half-space Green's function in a volume integral equation formulation. The volume integral equation is then solved using the discrete dipole approximation. The fast Fourier transform is effectuated for all three dimensions with half-space Green's function rather than the conventional free space Green's function. To overcome the snow volume truncation in the finite numerical calculations, periodic boundary conditions are applied in the lateral extent. Thus, in NMM3D, the physical microwave scattering and emission problem is solved without using any radiative transfer equations. In this formulation, the scattering matrix of the snowpack accounts for both the magnitude and phase. The NMM3D simulations are demonstrated at Ku -band frequency for a snow cover up to 25-cm thick. The results are applicable to remote sensing of snow over sea ice, and thin layers of terrestrial snow. Quantitative values of backscattering and bistatic scattering coefficients are derived for active microwave remote sensing, and brightness temperatures for passive microwave remote sensing. The full wave simulation results are compared with those of the partially coherent approach of the dense media radiative transfer (DMRT). The NMM3D results capture effects of backscattering enhancement and coherent layering that are missed in DMRT. The full wave solution to Maxwell equations is important to advance radar polarimetry, interferometry, and tomography that require the preservation of the full phase information and all interface interactions for applications to radar remote sensing of snow cover on land and on sea ice.

Late backscattering enhancement from a rubber spherical shell associated with waveguide coupling and propagation

Acoustic scattering from rubber targets has received modest attention in the past due in part to rubber’s subsonic sound speeds. In particular, slow shear waves in rubber get rapidly attenuated, giving rubber the fluid-like property of negligible shear coupling. This makes rubber a material of interest for coating or cloaking underwater objects and vehicles. In this study, backscatter from a rubber spherical shell in water is considered experimentally and through such models as partial wave series, finite elements, and waveguide normal mode analysis. The target is a commercially available American handball of unspecified rubber composition. Experimental and modeled results exhibit the importance of a strong, slightly subsonic late echo, which is demonstrated through path timing models and frequency analysis to be due to waveguide propagation through the shell wall. The various models use wave speeds for the rubber determined experimentally, allowing for determination of the phase and group velocities of the lowest order waveguide mode within the shell. These are found through normal mode analysis for both flat and curved waveguides, and again through Sommerfeld-Watson theory. Frequency domain results show how the waveguide path interferes with reflective paths from the shell in a frequency-dependent manner. [Work supported by ONR.]

Mapping Flooded Vegetation Using COSMO-SkyMed: Comparison With Polarimetric and Optical Data Over Rice Fields

The capability of COSMO-SkyMed (CSK) radar to remotely sense standing water beneath vegetation using an automatic algorithm working on a single image is investigated. The objective is to contribute to tackle the problem of missed detection of inundated vegetation by near real-time flood mapping algorithms using SAR data. The focus is on CSK because its four-satellite constellation is very suitable for rapid mapping. A set of CSK observations of an area in Northern Italy where many rice fields are present and recurrent artificial inundations occur were analyzed. Considering that double-bounce is the key process to detect floodwater under vegetation and that polarimetry is potentially able to discriminate double-bounce among different scattering mechanisms, single polarization CSK observations were compared with ALOS-2 and RADARSAT-2 fully polarimetric data. Such a multifrequency and multiangle dataset helped understanding the multitemporal signature of CSK data. A set of Landsat-8 images collected under cloud free conditions were also used as reference. Satellite acquisitions were gathered in order to ensure both spatial overlap among the images of the various sensors and temporal overlap along most of the rice growing season. The comparison between CSK and polarimetric data showed that at least for a slender leaf plant like rice, CSK can be able to detect the enhancement of double-bounce backscattering involving water and vertical plant stems. For some selected fields, it was found a good agreement between CSK-derived floodwater maps and those produced using the normalized-difference water index derived from Landsat-8 images, as well as double-bounce detection from polarimetric data.

Polarization-Independent Backscattering Enhancement of Cylinders Based on Conformal Gradient Metasurfaces

The electromagnetic backscattering enhancement of both elliptic and circular conducting cylinders is investigated in this paper through the design of conformal and polarization-independent gradient metasurfaces. The presented metasurface designs employ varying phase gradient along the circumferential direction of the involved cylinder so that effective retroreflection can be achieved through redirecting the scattering dispersed by the conducting cylinder back to the direction from which the plane electromagnetic wave is coming. Supported by a grounded thin substrate with a relatively high dielectric constant, a modified loop array with compact unit cell is used to implement the nonuniformly or uniformly sampled phase gradient of the metasurface. It is observed that the metasurface-coated elliptic and circular cylinders can generate backscattering very close to that by corresponding flat conducting plates with their main planes perpendicular to the incident wave vector, for both transverse magnetic (TM) and transverse electric (TE) polarizations. Compared with the conducting cylinders without coating, the backscattering is thus effectively enhanced by the conformal gradient metasurfaces. Good agreement between simulated and measured backscattering results validates the observations.

On the applicability of asymptotic formulas of retrieving “optical” turbulence parameters from pulse lidar sounding data: I–equations

Asymptotic solutions for the problem of retrieving the distribution of the structure characteristic of refractive index fluctuations from measurement data on the backscattering enhancement factor have been found. The solutions are written in terms of fractional derivatives of the enhancement factor in the case of receivers with a small aperture or via usual derivatives in the case of receivers with a large aperture. Properties of the kernel of the integral equation from which the asymptotic formulas follow have been studied in detail. Attention is paid to the fact that the kernel is oscillating in the general case. The kernel oscillations slightly affect the magnitude of the enhancement factor; however, their effect on derivatives of this factor can be significant.

Far-field coherent backscatter enhancement from random aggregations of scatterers and comparisons to backscattering from single isolated spheres.

Coherent backscatter enhancement (CBE), a multiple scattering phenomenon, may cause an enhancement of up to a factor of two in the average intensity backscattered from a random aggregation of scatterers. In the ocean, CBE may occur when a fish school or a bubble cloud is remotely illuminated. The research reported here explored the possibility that CBE might be used to remotely discriminate between an aggregation of many scatterers and a single isolated scattering object. For this investigation, the far-field harmonic acoustic pressure backscattered from aggregations of randomly placed omnidirectional point scatterers was determined from numerical solution of the equations from Foldy [(1945) Phys. Rev. 67(3,4), 107-119], and compared to equivalent results from single spherical scatterers having hard surfaces, pressure-release surfaces, or aggregation-matched effective-medium properties. Interestingly, CBE causes a spherical aggregation to backscatter as much or more sound than a single perfectly reflecting sphere of the same size when (ka)1/2(ks)-4/5(kσs1/2)3/4 ≥ 2.3, where k is the acoustic wave number, a is the aggregation radius, s is the average spacing between scatterers, and σs is a scatterer's cross section. And, backscattered intensity samples (in dB) from all simulated aggregations followed an extreme value distribution, a finding that supports the conventional use of backscatter statistics for remote aggregation-versus-single-object discrimination.

Intensity fluctuations of reflected wave from a diffuse target with a hard edge in atmospheric turbulence

The intensity fluctuation of the reflected field from a diffuse circular plate with a hard edge in turbulence is investigated by combining the Rytov theory and the Extended Huygens-Fresnel principle. The normalized covariance and variance of the reflected intensity are formulated and calculated. The enhancement effect on the normalized variance is discussed around the backscattering direction, which disappears rapidly as the receiving point moves away from the transmitting center. The “averaging effect” of the target aperture is also discussed, and the results show that the normalized variance and the backscattering enhancement effect decreases with increasing target size.

Uniform integral representation for the fluctuating field propagating through the multi-scale media

A uniform integral representation for the wave field propagating through the statistically homogeneous multi-scale medium was obtained. It is shown that the considered expression for the field contains the limiting cases for both the Born’s approximation for the single-scattered field and a phase approximation of geometric optics method. Hybrid method formulae for calculation of the fluctuation field, taking into account the effect of backscattering enhancement can be obtained from the investigated integral representation, provided that the spectrum of inhomogeneities can be represented as a sum of two uncorrelated components: small-scale and large-scale. The paper contains the numerical simulation results of the effect of average intensity enhancement of the backscattered field in the medium containing both large- and small-scale irregularities. Unlike the hybrid method of calculation, the proposed approach does not require explicit separation of the inhomogeneities spectrum into two uncorrelated components and allows investigating the effect of the average intensity enhancement of the backscattered field in a more general situation. In particular, numerical calculations have shown that the effect of enhancement of the backscattered signal may occur in the study of the high-latitude ionosphere by method of incoherent scattering of radio waves.

Enhancement of backscattering by a conducting cylinder coated with gradient metasurface

This paper proposes a highly effective method for enhancing the backscattering by a conducting cylinder that is coated with a gradient metasurface. The employed metasurface exhibits a phase gradient continuously varying along the circumferential direction of the cylinder so that in-phase retroreflection can be produced to enhance the backscattering. It is demonstrated that the cylinder coated with the proposed gradient metasurface can generate backscattering very close to that from a conducting plate with the same dimensions as the cylinder's cross-section perpendicular to the incident plane wave. Compared with a bare conducting cylinder, the backscattering is significantly enhanced by the gradient metasurface made of conducting strips printed on a grounded dielectric substrate. Effects of cell numbers along the cylinder axis, incident angle, and polarization of the incoming electromagnetic wave on the backscattering enhancement are examined and discussed. A good agreement between simulated and measured backscattering results validates the observations.

Comprehensive analysis of photonic nanojets in 3D dielectric cuboids excited by surface plasmons

In this paper we study the excitation of photonic nanojets (PNJ) in 3D dielectric cuboids by surface plasmons at telecommunication wavelengths. The analysis is done using the effective refractive index approach. It is shown that the refractive index contrast between the regions with and without cuboid should be roughly less than 2 in order to generate jets at the output of the cuboid. The best performance at λ0 = 1550 nm is obtained when the height of the cuboid is 160 nm producing a jet just at the output interface with a subwavelength resolution of 0.68λ0 and a high intensity enhancement (×5) at the focus. The multi‐wavelength response is also studied demonstrating that it is possible to use the proposed structure at different wavelengths. Finally, the backscattering enhancement is numerically evaluated by inserting a metal particle within the PNJ region, demonstrating a maximum value of ∼2.44 dB for a gold sphere of radius 0.1λ0. image

Circular synthetic aperture sonar images of backscattering enhancements on solid elastic cubes due to Rayleigh waves and other mechanisms

Aspect dependent backscattering enhancement mechanisms aid in detecting and identifying underwater objects. This work examines enhancement mechanisms on a solid metal cube observed in acoustic images, emphasizing Rayleigh type surface waves that undergo retroreflection at the corners of the face of the cube [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 105, 700–710 (1998)]. Backscattering from a steel cube insonified in water was measured using a circular synthetic aperture sonar system. Image reconstruction was performed using Fourier based algorithms. Rayleigh waves are launched on a face of the cube when the local incident angle is equal to the Rayleigh coupling angle. The presence of four right-angle retroreflectors on each face means that multiple reverberations of this wave are observed. In reconstructed images of the cube, the Rayleigh wave mechanism is responsible for some of the brightest features. If the relative orientation of the circular aperture and the cube is such that specular glints are suppressed, edge and corner diffraction features are clearly observed. This relative orientation may be deduced from the reconstructed image. Discussions of both the spectral response and temporal response of the cube as a function of viewing angle will also be included. [Work supported by ONR.]

Multiple Scattering Effects With Cyclical Correction in Active Remote Sensing of Vegetated Surface Using Vector Radiative Transfer Theory

The energy transport in a vegetated (corn) surface layer is examined by solving the vector radiative transfer equation using a numerical iterative approach. This approach allows a higher order that includes the multiple scattering effects. Multiple scattering effects are important when the optical thickness and scattering albedo of the vegetation layer are large. When both the albedo and the optical thickness exceed 0.4, higher orders contribute significantly (e.g., vertical polarization at L-band). The approach is applied to vegetated surfaces using typical crop structure for backscattering from L-band to Ku-band. For corn fields at L-band, multiple scattering effects are more important for vertical scattered wave with vertical incidence (VV). For example, when vegetation water content (VWC) is $3 \text{kg}/{\text{m}}^2$ , the deviation between first order and multiple scattering for corn field for VV could be 3.5 dB while 0.7 dB for horizontal scattered wave with horizontal incidence (HH). The iterative approach also allows the separation of the contribution to backscattering from each scattering order and scattering mechanism. Each scattering mechanism is associated with a unique scattering path. By examining the duality of the paths, we are able to identify the cyclical terms with existence of a reflective boundary. The cyclical correction to the backscattering accounts for backscattering enhancement effects on the copolarization by a factor of two. The approach is validated against the SMAPVEX12 L-band corn dataset over the entire crop growth and large soil moisture variations. The model prediction matches the observation with 1.93 and 1.46 dB root-mean-square error (RMSE) for VV and HH, respectively, while correlations are 0.67 and 0.88, respectively. Time-series retrieval is also applied successfully for both soil moisture and VWC with 0.06 $\text{cm}^{3}/\text{cm}^{3}$ and $0.44 \text{kg}/\text{m}^{2}$ RMSE, respectively, while correlations are 0.7 and 0.92, respectively. For large VWC, this approach corrects the underestimated backscatters in the single scattering caused by large attenuation.