S.R. Chubb

SpecTIR™ Remote Sensing Division, Reno, Nevada, United States

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Publications (32)43.17 Total impact

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    ABSTRACT: Previously, we have modeled radar signatures, involving large variations (∼10-15 dB) in radar cross section (RCS), that have been observed at strongly convergent ocean fronts and at an estuarine front. In each of these cases, we obtained quantitative agreement with measurement but only by including wave-breaking (WB) effects in an approximate manner. However, in each case, we used the composite scattering (CS) model at a frequency (9.4 GHz) where this model may be deficient. For this reason, questions remain concerning the importance of WB effects in these simulations. In the present study, we monitor the sensitivity of the simulations with respect to this CS approximation by comparing the results of this model with those from an alternative theory, based on simulations of RCS, derived from a common wave spectrum. The spectrum is calculated using a full-spectral treatment of wave-current interaction. The resulting simulations are used to model the radar signature of the buoyant plume associated with the efflux of fresh water from the Chesapeake Bay that was observed during the Chesapeake Outflow Plume Experiment 2 (COPE-II). In both cases, it is possible to simulate this signature, in quantitative agreement with experiment, but only by including WB effects. We find that the CS model predictions for the behavior of the signature do not agree with the comparable predictions from the remaining model. Additional simulations indicate the difference between the two models, which is rather large because of the large (60°) angle of incidence, occurs because the CS model includes higher order terms that are not included in the Kirchoff Approximation.
    IEEE Transactions on Geoscience and Remote Sensing 04/2003; · 3.47 Impact Factor
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    ABSTRACT: An inversion algorithm for inferring the surface velocity field of buoyant plume frontal features from observed radar imagery has been developed. The inversion technique is based upon an assumption, suggested by Alpers and Hennings' (AH) relaxation model (1984), that near strongly convergent fronts, the radar cross-section should be proportional to the component of the local current gradient that is directed along the radar-look direction. However, at X-band, the technique only works when wave-breaking (WB) effects, which are not included in the AH model, are incorporated. This WB model successfully reproduces the magnitude of the signature in images of the plume front at higher frequencies (X-band), where it is known that the AH model is deficient. WB effects play a dominant roˆle in the backscatter associated with frontal regions with strong surface convergence fields. These results suggest that the enhancements of radar backscatter in the vicinity of strongly-convergent fronts are proportional to the local current-convergence but that the underlying scattering process involves WB in a manner that cannot be understood from the AH model. Results are presented for the estimated velocity field derived from radar imagery of the Chesapeake Bay plume front. Preliminary considerations of the convergence and uniqueness of the inversion technique are extended by means of a controlled numerical experiment involving the inversion of a prescribed input velocity field
    IEEE Transactions on Geoscience and Remote Sensing 12/2001; · 3.47 Impact Factor
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    ABSTRACT: Enhanced radar backscatter occurs in radar imagery of strongly convergent ocean currents, but the origin of this phenomenon is not well-understood. Although the Alpers and Hennings (AH) relaxation model indicates that variations in radar cross-section (RCS) intensity at convergent fronts are proportional to the magnitude of the local current convergence, at higher radar frequencies, this model significantly underpredicts RCS enhancement and exhibits a non-physical look-angle dependence. Full-wave spectral modeling incorporating composite Bragg backscatter does not remedy the situation. In developing a procedure for extracting estimates of the near frontal 2-dimensional surface velocity fields of convergent ocean frontal features from radar imagery, the authors have identified a plausible explanation, empirically, that accounts for the correlation between the presence of bright lines in X-band radar imagery near convergent fronts and the magnitude of the local current convergence. The authors are able to quantitatively infer a 2-dimensional model of the currents, using radar data simulations of wave spectra, and radar cross-section RCS. In the resulting model, enhanced radar intensity occurs where the local convergence increases because of enhanced wave-steepening and wave-breaking. Other effects associated with current structure (for example, shear) are discussed
    Geoscience and Remote Sensing Symposium, 2000. Proceedings. IGARSS 2000. IEEE 2000 International; 02/2000
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    ABSTRACT: The phenomenon of ocean wave-shoaling, and the associated reduction of ocean wave phase speed with decreased water depth, provides useful information for inferring water depth D (bathymetry) in coastal environments. One strategy for relating D to phase speed C and wave-vector k, of long wave length ocean waves, involves using the 1-dimensional, linear (gravity wave) dispersion relationship C=(g*tanh(kD)/k)<sup>1/2</sup>. In principle, this approach has limitations, because the approach is based on a WKB approximation. Thus, it cannot be applied when D varies appreciably over the wavelength of a shoaling-wave. Also, the approach is restricted to waves that have small wave-height. In the present paper, The authors use a set of marine radar image sequences and apply the linear approximation, via a 3D FFT analysis to the sequences. The authors show that for low to moderate wave heights, the approach does retrieve approximately the correct depth. However, an increase in the RMS wave-height from 1 m to 3.5 m produced a much poorer depth estimate, proving the need for an application of a non-linear wave model to the problem, with an associated new retrieval approach. They outline a new procedure for extracting bathymetry that uses the recently developed constituent Boussinesq (CB) equations. The inversion procedure is accomplished using a standard (Levenberg-Marquardt-like), 1-dimensional, cost function minimization procedure
    Geoscience and Remote Sensing Symposium, 2000. Proceedings. IGARSS 2000. IEEE 2000 International; 02/2000
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    ABSTRACT: Using simulations of radar cross section (RCS) based on wavecurrent interaction calculations, we investigate the origin of a prominent enhancement in L-band from signals that were transmitted and received, respectively, with horizontal (H) and vertical (V) polarization radar return. This was observed in imagery of the northern boundary of the Gulf Stream (GS) during the first Shuttle Radar Laboratory (SRL-1) mission. The calculations of surface roughness are based on a one-dimensional (1-D) surface current model that closely resembles a current shear that was observed in in situ current measurements, taken at both sides of the GS at the time SRL-1 imaged the GS boundary. In agreement with trends observed in the imagery, significant enhancements in L-band HV polarization cross section occur in the neighborhood of the GS thermal boundary, relative to comparable vertical polarization (VV) cross section signatures at X-, C-, and L-band.We also find reasonably good agreement between the simulated and observed magnitudes of the GS signatures (based on calculations of wave action) using two different radar imaging models, and we provide an overview of a number of additional submesoscale features associated with the GS that were present in the image of the GS boundary.
    IEEE Transactions on Geoscience and Remote Sensing 09/1999; 37(5):2495-2507. · 3.47 Impact Factor
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    ABSTRACT: Bright linear features have been observed in radar imagery taken near the Gulf Stream (GS) boundary on two separate occasions. In each case, these have been observed directly over strong current convergences. Progress has been made in understanding the origin of these signatures through simulations that incorporate environmental forcing from the winds and currents. These simulations significantly underestimate the backscatter unless wave-breaking (WB) effects are included at least approximately. Using a new, quasistatistical procedure that generalizes and quantifies earlier procedures for including WB effects, the authors have been able to successfully simulate the magnitude and behavior of these signatures. The approach combines the statistically based, composite model of radar backscatter with a deterministic feature model that relates backscatter from breaking waves to a particular geometrical model of a spilling breaker. This is accomplished using localized criteria, defined by local wave crest acceleration, to determine the probability of breaking, and by extending the feature model so that its unknown parameters may be evaluated directly from wave-current interaction calculations. The new approach provides an estimate of the critical crest acceleration of a potentially breaking wave, as a function of wind speed, that agrees with independent measurements
    IEEE Transactions on Geoscience and Remote Sensing 08/1999; · 3.47 Impact Factor
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    ABSTRACT: Previously, the authors have modeled radar signatures, involving large (~10-15 dB) variations in radar cross-section (RCS), that have been observed at strongly convergent ocean fronts and at an estuarine front. In each of these cases, they obtained quantitative agreement with measurement but only by including wave-breaking (WB) effects in an approximate manner. However, in each case, they used the composite scattering (CS) model at a frequency, where this model may be deficient. For this reason, questions remain concerning the importance of WB effects in these simulations. In the present study, they monitor the sensitivity of the simulations with respect to this CS model approximation by comparing the results from three different radar model simulations of RCS, derived from a common wave spectrum. The spectrum is calculated using a full-spectral treatment of wave-current interaction. The resulting simulations are used to model the radar signature of the buoyant plume associated with the efflux of fresh water from the Chesapeake Bay that was observed during the COPE-2 experiment. In each case, it is possible to simulate this signature, in quantitative agreement with experiment, but only by including WB effects. However, CS model predictions for the behavior of the signature do not agree with the comparable predictions from the two remaining models. Additional simulations indicate these differences result from higher order terms that are not included in the Kirchoff approximation
    Geoscience and Remote Sensing Symposium, 1999. IGARSS '99 Proceedings. IEEE 1999 International; 02/1999
  • G. M. Nedlin, S. R. Chubb, A. L. Cooper
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    ABSTRACT: A version of the integral equation method is developed, which applies to scattering from a surface of a substance with a large dielectric constant ε. An ``impedance'' boundary condition for the tangential components of electric and magnetic fields on the scattering surface is used, and the integral equation for the tangential components of the total magnetic field on a surface is formulated. This equation is applied to the problem of electromagnetic scattering from a slightly rough surface. It is demonstrated that the commonly used perfect conductor approximation (PCA) (ε=∞) can adequately describe scattering in the case of extremely large ε only. The greatest sensitivity in scattering occurs when the incident and/or scattered waves are vertically polarized. For such cases the PCA does not work until ε>104. In the particular case of scattering from the ocean (|ε|~65), the PCA fails to provide an adequate description of the phenomena for either horizontally or vertically polarized waves, for practically all incidence and scattering angles.
    Radio Science 01/1999; 34:27-50. · 1.00 Impact Factor
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    ABSTRACT: Using airborne synthetic aperture radar data from the 1990 Gulf Stream Experiment, this paper investigates the polarization and wavelength dependence of radar signatures for narrow fronts with converging flows occurring within the Gulf Stream. The signal-to-background ratios of the cross-polarization backscatter return from a convergent front were found much higher than those of copolarization returns, when the flight path is crossing the front. However, a second convergent front, imaged at 45°, showed that the signal-to-background ratios are nearly equal for co- and cross-polarizations. A polarimetric procedure, which has been successfully used to measure terrain slopes and to generate elevation maps, is applied to the convergent front to explain the polarization and imaging geometry dependence of these radar responses. A theoretical modeling of radar modulation using an ocean wave model and a composite-Bragg scattering model, which incorporates the effect of breaking waves, was developed. Calculations with the model agree reasonably well with the radar measurements at various polarizations for three radar frequencies: P-band (68 cm in wavelength), L-band (24 cm), and C-band (5.7 cm)
    IEEE Journal of Oceanic Engineering 11/1998; · 1.16 Impact Factor
  • G.M. Nedlin, S.R. Chubb, A.L. Cooper
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    ABSTRACT: The integral equation method (IEM) is an important tool for a theoretical study of electromagnetic scattering from the ocean. However, in its current form, the method can be used only for a perfect conductor approximation (PCA), that considers the relative dielectric constant of sea-water to be ∞. This paper extends the IEM so that it can be used in the cases of finite but large dielectric constant, and accounts automatically for a near-grazing anomaly. Radar cross-sections are determined
    Antennas and Propagation Society International Symposium, 1998. IEEE; 07/1998
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    ABSTRACT: In this paper we investigate the underlying dynamics associated with a strong, line-shaped submesoscale feature that was observed in radar imagery at the boundary between Gulf Stream (GS) and shelf water near Cape Hatteras during the first Naval Research Laboratory High-Resolution Remote Sensing Experiment (HIRES 1). The line-shaped feature, which appears as a pronounced (~10 dB) increase in radar cross section, extends several kilometers in the east-west direction. In situ current measurements have shown that this feature coincides with the boundary of a sharp current convergence front. These measurements also indicate that the frontal dynamics is associated with the subduction of denser GS water under lighter shelf water. Using the observation that the convergence can be attributed to a hydrodynamic instability at the water interface, we have modeled the resulting subsurface hydrodynamics on the basis of a rigid-lid, two-dimensional solution of the Navier Stokes equation. The calculations of subsurface current flow were used as input to a spectral (wave action) model of wave-current interaction to obtain the surface wave field, which in turn was used to provide input for modeling of radar backscatter. The resulting description also includes the effects of surfactant-induced wave damping on electromagnetic backscatter. Our predictions are compared with real aperture radar imagery and in situ measurements from the HIRES 1 experiment.
    Journal of Geophysical Research Atmospheres 01/1998; 1031:18723-18744. · 3.44 Impact Factor
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    ABSTRACT: The authors use simulations of radar cross-section, based on wave-current interaction calculations, to investigate the origin of a prominent enhancement in L-band, HV polarization radar return that was observed in imagery of the northern boundary of the Gulf Stream (GS) during the first Shuttle Radar Laboratory (SRL-1) mission. The calculations of surface roughness are based on a 1-dimensional surface current model that closely resembles a current convergence that was observed in in-situ current measurements, taken at both sides of the Stream at the time SRL-1 imaged the GS boundary. In agreement with trends observed in the imagery, significant enhancements in L-band HV polarization cross-section occur in the neighborhood of the GS boundary, relative to comparable VV polarization cross-section signatures at X-, C- and L-band. This occurs despite the fact that the magnitude of the L-band HV cross-section is significantly reduced relative to the comparable X-, C-, and L-band VV cross-sections. These results indicate that the associated L-band HV enhancement occurs from tilt-induced modulation in the radar backscatter, which preferentially alters the relative modulation in L-band HV backscatter in regions where considerable variation in surface slope takes place. The authors also provide an overview of a number of additional sub-mesoscale features associated with the Gulf Stream that were present in the image of the GS boundary
    Geoscience and Remote Sensing, 1997. IGARSS '97. Remote Sensing - A Scientific Vision for Sustainable Development., 1997 IEEE International; 09/1997
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    ABSTRACT: Full-spectral modelling of a two-dimensional current structure is used to understand the role of wave-current interaction and wave-breaking (WB) effects in the radar signatures of the rip-like sub-mesoscale feature that were observed during the First High Resolution Remote Sensing (HI-RES-1) experiment. It is found that the large variations in radar cross-section (RCS) in the neighborhood of the cusp-like features within the rip can be reproduced with or without incorporation of wave-breaking (WB) effects. However, when WB effects are not included, consistent with previous models that have used 1-dimensional current structures, in regions away from these cusp-like structures, the composite scattering (CB) model significantly underpredicts the magnitude of the signature. As a consequence, somewhat surprisingly, within the rip, the CB model over-predicts the magnitude of the cusp signature relative to the signature from the rip in non-cusp-like regions. By including WB effects, this deficiency is overcome, and good agreement is obtained. The resulting agreement occurs when the WB effect is based on an estimate of the local critical crest acceleration Ω<sub>c</sub>=0.4 g (g=9.8 m/s<sup>2</sup>) that accompanies the onset of wave-breaking. This value for Ω<sub>c </sub> is in good agreement with independently measured values obtained in wave-tank and field experiments. Incorporation of WB effects also eliminates a non-physical dependence on look-angle that occurs when the CB model alone is used
    Geoscience and Remote Sensing, 1997. IGARSS '97. Remote Sensing - A Scientific Vision for Sustainable Development., 1997 IEEE International; 09/1997
  • G.M. Nedlin, S.R. Chubb, A.L. Cooper
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    ABSTRACT: A theory for electromagnetic scattering from a slightly rough surface is developed that is based on application of an effective boundary condition. The theory generalizes previous approaches that apply only to the infinite dielectric constant (&epsi;→∞) limit to cases of finite but large &epsi;. Also, the theory yields the known results by J. Wright. In a particularly important case of scattering from the ocean, the backscatter cross-section is dramatically different from that calculated for &epsi;→∞ over a wide range of incident angles, for both vertical and horizontal polarization
    Geoscience and Remote Sensing, 1997. IGARSS '97. Remote Sensing - A Scientific Vision for Sustainable Development., 1997 IEEE International; 09/1997
  • R. Fusina, A. L. Cooper, S. R. Chubb
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    ABSTRACT: A new nesting technique has been developed for computing solutions of the steady-state form of the wave action equation. The technique is especially useful for investigating the effects of resolution on the accuracy and stability of the computation. This has importance in the problem of determining ocean wave spectra under the influence of ambient wind fields and current distributions. The technique enables extremely high resolution computations to be performed with minimal computer storage requirements. It is especially useful for applications in modelling radar imagery of the ocean surface. Investigations of the convergence, stability, and accuracy of the procedure are made possible by introducing a fixed grid point location which is common to all the nested grids. In order to display the method, we apply it to a particular model of an oceanographic current rip feature that was recently observed during the first High Resolution Remote Sensing Experiment. Limitations of the method are also discussed.
    Journal of Computational Physics 01/1997; 132:215-225. · 2.14 Impact Factor
  • GM Nedlin, SR Chubb, AL Cooper
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    ABSTRACT: It is shown that a resonant nonlinear interaction of waves may result in a variety of collective multiwave modes of coherent resonantly coupled waves. The equations of motion for the modes have a universal set of integrals ({open_quote}{open_quote}collision constants{close_quote}{close_quote}) reflecting a {open_quote}{open_quote}particle balance{close_quote}{close_quote} (in quantum-mechanical terms) in wave collisions. The existence of these integrals ensures that the collective modes are completely integrable. Any multiwave mode can be described by an effective single degree of freedom conservative Hamiltonian, similar to the one known for three-wave coherent modes (triads). {copyright} {ital 1996 The American Physical Society.}
    Physical Review Letters 11/1996; 77(16):3267-3271. · 7.73 Impact Factor
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    ABSTRACT: Examines the origin of the strong radar signatures from the rip-like feature observed during the First High Resolution Remote Sensing (HIRES-1) experiment, based on an “end-to-end” strategy in which radar signatures are modelled from a full-spectrum wave approach, using surface and subsurface currents, derived from the underlying hydrodynamics. The authors find that it is necessary to include wave-breaking (WB) effects to obtain satisfactory agreement with experiment for the magnitude of the observed radar signature. They accomplish this using an earlier WB model and by introducing an improved WB model. The new model combines the statistically-based, composite model of radar backscatter with a deterministic, feature model that relates the predominant backscatter from breaking waves to a particular geometrical model of a spilling breaker. This is accomplished by using localized WB criteria, based on critical crest acceleration Λ<sub>c</sub> information, to determine the probability of breaking, and by extending the feature model so that its assumed geometry may be determined statistically. This allows the authors to eliminate all of the unknown parameters of the feature model using calculations of wave-height spectra
    Geoscience and Remote Sensing Symposium, 1996. IGARSS '96. 'Remote Sensing for a Sustainable Future.', International; 06/1996
  • A.L. Cooper, S.R. Chubb, J.A.C. Kaiser
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    ABSTRACT: X-band radar signatures of a current rip convergence, resulting from denser Gulf Stream fluid interacting with fresh coastal shelf water near Cape Hatteras, were observed during the First High Resolution (Hi-Res I) experiment. These signatures, which appeared as intense (~10 dB) enhancements in radar cross section (RCS) in the form of meandering linear segments, were accompanied by secondary parallel meandering segments of reduced (~5-10 dB) RCS on the shelf water side. The effects of wave-current interaction scale with the surface velocity (u) while the effects of surfactants scale as the ratio of surface current to phase velocity (u/c). Unlike internal waves which `graze' upon the ambient surface film material, current rips `herd' the ambient surface film material to a convergence point where u/c=1. The latter features can induce singular behaviour for a monomolecular surface film. A number of fundamental issues need to be resolved including: continuity of the surface film in regions of wave breaking; buckling of the monomolecular film; and subduction of the surfactant at the frontal boundary. These issues are highlighted through application to the Hi-Res I rip feature using a simplified one-dimensional model of the feature as well as the surface manifestation of currents derived from the associated depth-dependent structure
    Geoscience and Remote Sensing Symposium, 1996. IGARSS '96. 'Remote Sensing for a Sustainable Future.', International; 06/1996
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    ABSTRACT: The authors demonstrate the ability of interferometric radar imagery to determine both relative and absolute surface velocities in the open ocean. Absolute phase calibration is accomplished by noting the azimuthal displacement of range-travelling targets-demonstrating for the first time that under favourable circumstances phase calibration can be achieved in open-ocean in the absence of ground truth. The high resolution of radar imagery permits observation of sharp velocity discontinuities, e.g. the Gulf Stream boundary and the wave field. The recent SIR-C/X-SAR shuttle missions dramatically emphasize the experimental and observational aspects of space-based radar. The combination of absolute velocities, high spatial resolution, and wide-area coverage suggest that interferometric radar imagery can provide a unique and powerful aid both for studies of global circulation patterns and detailed analysis of slope/shelf water interactions with ocean currents. In particular, the authors employ this measurement of the surface currents and wave field near a velocity front to help refine and bound results of their modeling of calculated radar images of the front. The results of this paper are compared with available ground truth
    IEEE Transactions on Geoscience and Remote Sensing 10/1995; · 3.47 Impact Factor
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    ABSTRACT: The Shuttle Imaging Radar, (SIR)-C/X-SAR, first and second Shuttle Radar Laboratory (SRL) missions, SRL-1 and SRL-2, took place April 9-20 and Sept. 30-Oct. 10, 1994. The authors report on a major, multi organizational series of experiments designed to investigate oceanographic phenomena at the Gulf Stream (GS) Supersite off the east coast of the US during these two missions. The investigations emphasized current-wave and air-sea interactions with extensive ground/sea/air truthing. The authors summarize a number of detailed findings associated with SRL-1 and provide a preliminary description of SRL-2 results
    Geoscience and Remote Sensing Symposium, 1995. IGARSS '95. 'Quantitative Remote Sensing for Science and Applications', International; 08/1995