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Antennas and Propagation, IEEE Transactions on

Issue 1 • Date Jan. 1998

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Displaying Results 1 - 18 of 18
  • Guest Editorial - Special Issue On Low-grazing-angle Backscatter From Rough Surfaces

    Publication Year: 1998 , Page(s): 1 - 2
    Cited by:  Papers (22)
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    Freely Available from IEEE
  • 95-GHz scattering by terrain at near-grazing incidence

    Publication Year: 1998 , Page(s): 3 - 13
    Cited by:  Papers (19)
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    This study, consisting of three complimentary topics, examines the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90°, corresponding to grazing angles of 20° to 0°. The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area σA. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that σA is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient σ0, particularly near grazing incidence. We provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for σ0 is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle θ, the surface relative dielectric constant ε, and the surface roughness ks, where k=2π/λ and s is the surface root mean square (RMS) height View full abstract»

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  • Green's function refinement as an approach to radar backscatter: general theory and applications to LGA scattering from the ocean

    Publication Year: 1998 , Page(s): 57 - 66
    Cited by:  Papers (8)
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    We present a new approach to the computation of radar returns from dielectric bodies whose boundary is the deformation of a plane surface. The method relies on combining a systematic improvement in the Green's function with a good approximate choice of local boundary condition. In this paper, the general theory is presented together with a simple application where the Green's function is that for a lossy dielectric half-space. We derive the root radar cross section (RCS) for a statistical surface and the mean backscatter RCS for a rough surface. We explore the applications to low-grazing-angle (LGA) scattering from statistical surfaces with an ocean-like spectrum and demonstrate that such a model explains some of the previously unexplained LGA phenomena, such as the absolute and relative levels of the vertical (VV) and horizontal (HH) channel RCS measurements View full abstract»

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  • Numerical simulation of low-grazing-angle ocean microwave backscatter and its relation to sea spikes

    Publication Year: 1998 , Page(s): 133 - 141
    Cited by:  Papers (7)
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    This paper presents the results of numerically simulating microwave backscatter from a deep-water breaking wave profile. Enhanced microwave backscatter from the crests of breaking waves has been hypothesized as the source of bright short-lived microwave radar echoes that are observed at low-grazing angles (LGAs). The characteristics of these “sea spikes” are distinctly different from the Bragg-scatter echoes that dominate measurements made at moderate grazing angles. Of particular interest is the high contrast that sea spikes present against ocean background backscatter when observed with horizontally polarized transmit/receive configurations [horizontal (HH) versus vertical (VV)]. This HH/VV contrast disparity has been attributed to polarization-selective cancellation of the direct reflection from the wave crest by the surface reflection. This hypothesis is reinforced first by showing evidence that VV polarization is suppressed in the intensity range that would normally be populated by the brightest scatterers. Histograms of unaveraged Doppler-centroid measurements show further that the depleted VV backscatter population is responding to scatterers that are moving much more slowly than the HH scatterers. The Doppler-centroid histograms provide a sharper delination between the two scattering populations than do the unconditionally averaged Doppler spectra that are more commonly reported. Finally, our numerical simulations show evidence of an interference mechanism that selectively suppresses VV backscatter. In our simulations, the polarization selectivity comes from the phase dependence of the backscatter from the wave crest. A Brewster phenomenon at the surface reflection point is not necessary View full abstract»

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  • Scattering from breaking gravity waves without wind

    Publication Year: 1998 , Page(s): 14 - 26
    Cited by:  Papers (18)
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    Scattering experiments from breaking gravity waves conducted at a wave tank facility at small grazing angles in the absence of wind are analyzed. Breaking gravity waves are studied using a fully plane polarimetric horizontal (HH), vertical (VV), vertically transmitted and horizontally received polarization (VH), and horizontally transmitted and vertically received polarization (HV) pulse-chirped X-band (8.5-9.6 GHz) radar in conjunction with optical instruments: the plane polarimetric optical specular event detector (OSED) and side-looking camera (SLC). Spatially and temporally resolved radar backscatter has been measured and temporally correlated to the data obtained from the optical diagnostics. The experiments yield the following results: (1) enhanced scattering compared to Bragg scattering levels occurs throughout the evolutionary process of wave-breaking, i.e., the radar scatters strongly from both the unbroken and broken surfaces; (2) an explanation is found for the observation that the scatterer Doppler frequency is slightly less than the Doppler frequency corresponding to the fundamental wave phase speed; (3) a representative non-Bragg cross section of a breaking wave can be obtained; and (4) a breaking wave surface is found to be an efficient depolarizer View full abstract»

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  • Multipath scattering in ultrawide-band radar sea spikes

    Publication Year: 1998 , Page(s): 45 - 56
    Cited by:  Papers (13)
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    This paper presents sea scatter data collected with an ultrawide-band (UWB) polarimetric radar system that indicates that multipath scattering plays an important role in the generation of sea spikes. The radar system used in this study produces short pulses with a bandwidth of approximately 3 GHz centered at 9 GHz for a range resolution of approximately 4 cm. Pulse-to-pulse switching allows collection of the microwave echoes produced by all four combinations of linear transmit and receive polarizations [vertical-transmit vertical-receive (VV), horizontal-transmit horizontal-receive (HH), horizontal-transmit vertical receive (HV), and vertical-transmit horizontal-receive (VH)] each of which is collected by a sampling oscilloscope utilizing equivalent time sampling. In June 1996, upwind sea scatter data at grazing angles of 10°, 20°, and 30° were collected while the system was deployed on a research pier on the Outer Banks of North Carolina. An analysis of the strongest echoes (sea spikes) from this data set is presented and discussed. First, the cumulative distribution functions are presented. Second, an increase of approximately 5 dB is shown to occur in the polarization ratio (HH/VV) of the strongest echoes as the grazing angle decreases from 30° to 10°. Third, differences in the spatial and spectral characteristics of the VV and HH spikes are described. Through comparisons with laboratory results and a simple scattering model, these observations are explained by the presence of a multibounce scattering mechanism. The use of the model to extract wave height from the sea-spike frequency response is also explored View full abstract»

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  • On the universal behavior of scattering from a rough surface for small grazing angles

    Publication Year: 1998 , Page(s): 67 - 72
    Cited by:  Papers (17)
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    It is shown that for scattering from a plane of an average rough surface, the scattering cross section of the range of small grazing angles of the scattered wave demonstrates a universal behavior. If the angle of incidence is fixed (in general, it should not be small), the diffuse component of the scattering cross section for the Dirichlet problem is proportional to θ2 where θ is the (small) angle of elevation and for the Neumann problem it does not depend on θ. For the backscattering case, these dependencies correspondingly become θ4 and θ0. The result is obtained from the structure of the equations that determine the scattering problem rather than the use of an approximation View full abstract»

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  • On the discretization of the integral equation describing scattering by rough conducting surfaces

    Publication Year: 1998 , Page(s): 150 - 161
    Cited by:  Papers (21)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (412 KB)  

    Numerical simulations of scattering from one-dimensional (1-D) randomly rough surfaces with Pierson-Moskowitz (P-M) spectra show that if the kernel (or propagator) matrix with zeros on its diagonal is used in the discretized magnetic field integral equation (MFIE), the results exhibit an excessive sensitivity to the current sampling interval, especially for backscattering at low-grazing angles (LGAs). Though the numerical results reported in this paper were obtained using the method of ordered multiple interactions (MOMI), a similar sampling interval sensitivity has been observed when a standard method of moments (MoM) technique is used to solve the MFIE. A subsequent analysis shows that the root of the problem lies in the correct discretization of the MFIE kernel. We found that the inclusion of terms proportional to the surface curvature (regarded by some authors as an additional correction) in the diagonal of the kernel matrix virtually eliminates this sampling sensitivity effect. By reviewing the discretization procedure for MFIE we show that these curvature terms indeed must be included in the diagonal in order for the propagator matrix to be represented properly. The recommended current sampling interval for scattering calculations with P-M surfaces is also given View full abstract»

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  • Sea-spike backscatter from a steepening wave

    Publication Year: 1998 , Page(s): 108 - 113
    Cited by:  Papers (27)
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    H-pol and V-pol backscatter cross sections at incidence angles of 85°, 60°, and 40° are computed at X-band for two temporal sequences of simulated sea waves, one with a wavelength of 1 m and the other with a 2.3 m wavelength, that are steepening as they undergo breaking. At an incidence angle of 85° (5° grazing) H-pol and V-pol backscatter from these waves are shown to have the characteristics of a sea spike. At a 60° incidence angle only the l m sequence produces a sea spike. No sea spike is seen from either sequence at 40 View full abstract»

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  • Application of iterative moment-method solutions to ocean surface radar scattering

    Publication Year: 1998 , Page(s): 121 - 132
    Cited by:  Papers (37)
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    Numerical methods such as the banded matrix iterative approach (BMIA) represent a major advance in the direct numerical simulation of rough surface-wave scattering. This paper considers the application of iterative methods such as the BMIA to ocean-radar scattering. It is shown that for typical microwave radar frequencies and sea-surface roughness, the BMIA is actually of limited use. A more general iterative solution based on a multigrid decomposition and the generalized conjugate residual (GCR) method, is thus developed. The multigrid approach is ideally suited to the broad-band ocean surface, as it solves the scattering problem on a sequence of grids, each corresponding to a different range of spatial frequencies or length scales. This approach is applied here to several sea scattering problems, including very low grazing angles and both horizontal and vertical polarization. Good agreement is obtained with perturbation theory in the appropriate limits and several qualitative characteristics of radar backscatter data are reproduced View full abstract»

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  • Directional characteristics of sea-wave scattering observed at low-grazing angles

    Publication Year: 1998 , Page(s): 41 - 44
    Cited by:  Papers (1)
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    The mathematical and statistical description of waves on the sea surface has evolved over more than two centuries starting with Laplace (1776), Gerstner (1802), Airy (1845), Stokes (1847), Kelvin (1887), and Helmholtz (1888). Most of the major developments in this century are summarized by Apel (see J. Geophys. Res., vol.99, no.C8, p.16269-91, 1994) who has derived an improved model for the ocean surface wave vector spectrum of equilibrium sea states. Part of the motivation for this paper is Apel's observation that “…in spite of the incorporation of an anisotropic angular distribution of wave energy, the observed azimuthal variation of radar scatter is not captured…” The data set analysis summarized in this paper show that wave group processes are responsible for directionally organized sea surface radar back scattering patterns, which are long lived and consequently extend over large areas. This includes crosswind traveling wave systems. A property of wave group process structure is the steepening of waves as they pass through the center of the group. The mid-group sharpening of the waves, which can include crest spilling or breaking, significantly increases the radar scattering cross section observable with low grazing angle radars, usually most noticeable with horizontal (HH) polarization. Therefore, low grazing angle (LGA) radar provides a vastly improved means of seeing wavegroup phenomena over conventional oceanographic methods View full abstract»

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  • Low-grazing scattering from breaking water waves using an impedance boundary MM/GTD approach

    Publication Year: 1998 , Page(s): 93 - 100
    Cited by:  Papers (30)
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    The radar backscattering from water waves of various degrees of breaking Is numerically examined. A hybrid moment method geometrical theory of diffraction (MM/GTD) technique previously used for small-grazing scattering from perfectly conducting surfaces is reformulated using impedance boundary conditions, allowing the treatment of large (but finite) conductivity scattering media such as sea water. This hybrid MM/GTD approach avoids the artificial edge effects that limit the standard moment method when applied to rough surfaces, allowing the calculation of the scattering at arbitrarily small grazing angles. Sample surfaces are obtained through the edge-detection of video stills of breaking waves generated in a wave tank. The numerical calculations show that the strength of the backscatter is closely associated with the size of the plume on the breaking wave. Strong interference appears in the both horizontal (HH) and vertical (VV) backscatter when the surfaces are treated as perfectly conducting. The VV interference is dramatically reduced when a sea water surface is used, but the HH interference is unaffected. The interference leads to HH/VV ratios of up to 10 dB. The behavior of the scattering is consistent with the multipath theory of sea-spike scattering View full abstract»

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  • Measurement and classification of low-grazing-angle radar sea spikes

    Publication Year: 1998 , Page(s): 27 - 40
    Cited by:  Papers (28)
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    High-resolution dual-polarization X-band images of the ocean surface were obtained at a grazing angle of about 3°. Area extensive imaging allowed us to study the backscatter properties of sea spikes and to compare radar measurements with visual surface features evident from video recordings. The vertically polarized radar images consist of distributed scatter whose amplitude and Doppler velocity are modulated by larger scale gravity waves consistent with Bragg scattering and composite surface theory (CST). The horizontally polarized radar images are dominated by spatially discrete scattering centers (or sea spikes) moving at velocities comparable to the phase velocities of gravity waves beyond the spectral peak. These sea spikes also exist in the corresponding V-pol radar images, but are less prominent due to the dominant Bragg backscatter. Sea spikes are characterized by polarization ratios H/V that often exceed unity, typically by about 5 dB. Comparison of the larger spikes with simultaneous co-registered video recording of the surface indicates that approximately 30% of observed sea spikes are associated with actively breaking waves (whitecaps) while the remainder are identified with “steep” wave features. By classifying the larger sea spikes according to their corresponding surface features, we find hat the Doppler velocities for sea spikes due to whitecaps are noticeably faster (about 50%) than other sea spikes, though the distributions for both overlap significantly. We also find little measurable difference in the polarization ratios of the two classes of sea spikes as observed on the open ocean View full abstract»

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  • Monte Carlo simulations of large-scale one-dimensional random rough-surface scattering at near-grazing incidence: Penetrable case

    Publication Year: 1998 , Page(s): 142 - 149
    Cited by:  Papers (38)
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    Scattering from dielectric one-dimensional (1-D) random rough surfaces at near grazing incidence is studied for both TE and TM cases. To obtain accurate results at incidence angles of 80°-85°, we use long surface lengths of up to 1000 wavelengths. Numerical results are illustrated for dielectric surfaces corresponding to soil surfaces with various moisture contents. Results indicate that TM backscattering is much larger than that of TE backscattering. The ratio of TM to TE backscattering increases as a function of soil moisture and can be used as an indicator of soil moisture in remote sensing applications. However, the ratio of TM to TE backscattering is much lower than that predicted by the small perturbation method. To facilitate computation of scattering by such long surfaces, the previously developed banded-matrix iteration approach/canonical grid method (BMIA/CG) has been extended to dielectric surfaces. The numerical algorithm consists of translating the nonnear-field interaction to a flat surface and the interaction subsequently calculated by fast Fourier transform (FFT) View full abstract»

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  • Two-scale model and ocean radar Doppler spectra at moderate- and low-grazing angles

    Publication Year: 1998 , Page(s): 84 - 92
    Cited by:  Papers (17)
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    Results of numerical calculations of polarized radar Doppler spectra from the ocean surface at low-grazing angles and at various wind speeds are presented. Calculations are based on the modified two-scale model, which includes Bragg scattering from both free and bound capillary waves. Here, we derive an analytical expression for the Doppler spectrum in the form of a two-dimensional (2-D) integral over large-scale slopes. For the surface description, the model of a directional wave spectrum is used, which takes into account the wave age (a limited fetch). A comparison of computed curves with experimentally measured Doppler spectra shows that the inclusion of “fast” bound capillary waves significantly improves results. Using reasonable parameter values for the model of bound waves we obtained widths of Doppler spectra which are in good agreement with experiments for low-grazing angles θg=10° and for winds up to U 10~10 m/s. However, the remaining discrepancies show that using Bragg scattering from bound waves is not enough to close the problem. Despite limitations the approach can be used for a more thorough analysis of measured Doppler spectra in order to determine the contributions of various scattering mechanisms View full abstract»

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  • Forward-backward method for scattering from imperfect conductors

    Publication Year: 1998 , Page(s): 101 - 107
    Cited by:  Papers (26)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (316 KB)  

    The previously developed forward-backward method for calculating scattering from perfectly conducting azimuthally homogeneous surfaces is extended to imperfect conductors, where the dielectric constant has a large imaginary part such as sea water at X-band (10 GHz). An example shows that highly accurate results at X-band are obtained for the case of a steepened sea wave View full abstract»

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  • Grazing behavior of scatter and propagation above any rough surface

    Publication Year: 1998 , Page(s): 73 - 83
    Cited by:  Papers (34)
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    At grazing, propagation and scatter become inextricably connected. For sufficiently low source/observer heights, free-space inverse-distance propagation no longer applies and plane-wave descriptions of scatter give way to surface-wave modes. Concepts like surface radar cross section must be reinterpreted; lack of awareness of these facts in attempts to correlate measurements with grazing-angle laws has led to contradictions. When plane-wave depictions hold, a regime is entered where backscatter follows a grazing angle-to-the-fourth power dependence for surfaces of any roughness scales for both polarizations and for perfectly conducting as well as impedance boundaries above penetrable media. Propagation is described in terms of a roughness-modified effective impedance/admittance that approaches a constant at grazing for all roughness profiles. These facts are first explored with numerical examples, after which we establish universal laws that confirm these suspicions. We derive expressions for the first Taylor-series expansion terms for scatter and impedance/admittance versus grazing angle. Statistics are neither required nor excluded-the laws hold for single arbitrary deterministic profiles as well as averages over ensembles of random surface samples. Proofs of these claims are based on two-dimensional (2-D) fields over one-dimensional (1-D) impedance/admittance boundaries View full abstract»

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  • A numerical study of low-grazing-angle backscatter from ocean-like impedance surfaces with the canonical grid method

    Publication Year: 1998 , Page(s): 114 - 120
    Cited by:  Papers (23)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (148 KB)  

    A numerical study of 14-GHz low-grazing-angle (LGA) backscattering from ocean-like surfaces described by a Pierson-Moskowitz spectrum is presented. Surfaces rough in one dimension are investigated with Monte Carlo simulations performed efficiently through use of the canonical grid expansion in an iterative method of moments. Backscattering cross sections are illustrated at angles from 81° to 89° from normal incidence under the impedance boundary condition (IBC) approximation with the efficiency of the numerical model enabling sufficiently large profiles (8192 λ) to be considered so that angular resolution problems can be avoided. Variations with surface spectrum low-frequency cutoff (ranging over spatial lengths from 175.5 m to 4.29 cm) at 3 m/s wind speed are investigated and initial assessments of the small perturbation method (SPM), composite surface theory, operator expansion method (OEM), small slope approximation (SSA), and curvature corrected SPM predictions are performed. Numerical results show an increase in horizontal (HH) backscatter returns as surface low-frequency content is increased while vertical (VV) returns remain relatively constant, as expected, but none of the approximate models considered are found to produce accurate predictions for the entire range of grazing angles. For the cases considered, HH scattering is always observed to be below VV, further demonstrating the importance of improved hydrodynamical models if “super-event” phenomena are to be modeled View full abstract»

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IEEE Transactions on Antennas and Propagation includes theoretical and experimental advances in antennas.

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Editor-in-Chief                                                 Kwok W. Leung