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Roland Romeiser - One of the best experts on this subject based on the ideXlab platform.
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an improved composite surface model for the radar Backscattering Cross Section of the ocean surface 2 model response to surface roughness variations and the radar imaging of underwater bottom topography
Journal of Geophysical Research, 1997Co-Authors: Roland Romeiser, Werner AlpersAbstract:In the companion paper we have presented an improved composite surface model for the calculation of normalized radar Backscattering Cross Sections (NRCS) of the ocean surface. The proposed model accounts for the impact of the full two-dimensional ocean wave spectrum on the radar backscatter and was shown to reproduce measured absolute NRCS values for a variety of radar configurations and wind speeds satisfactorily after some reasonable tuning of the input ocean wave spectrum. This paper focuses on the modulation of the NRCS in the presence of spatially varying surface currents. First, the sensitivity of the NRCS to intensity variations of different ocean wave spectral components is investigated. Then the hydrodynamic modulation of the wave spectrum over underwater bottom topography in tidal waters is computed in different ways, and the resulting radar signatures are discussed. The composite surface model yields comparable radar signatures at high (10 GHz, X band) and low (1 GHz, L band) radar frequencies, which is in much better agreement with experimental results than the predictions of a first-order Bragg scattering model. On the other hand, measured variations of the NRCS at high radar frequencies appear to be still underestimated in some cases, which may be due to shortcomings of our description of the wave-current interaction by conventional weak hydrodynamic interaction theory. Possible improvements of the theory are discussed, and requirements for future experiments are formulated.
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an improved composite surface model for the radar Backscattering Cross Section of the ocean surface 1 theory of the model and optimization validation by scatterometer data
Journal of Geophysical Research, 1997Co-Authors: Roland Romeiser, Werner Alpers, V WismannAbstract:An improved composite surface model for the calculation of the normalized radar Backscattering Cross Section (NRCS) of the ocean surface at moderate incidence angles is presented. The model is based on Bragg scattering theory. A Taylor expansion of the NRCS in the two-dimensional surface slope yields nonzero second-order terms which represent a first approximation for the effect of the geometric and hydrodynamic modulation of the Bragg scattering facets by all waves that are long compared to these facets. The corresponding expectation value of the NRCS varies with the wave height spectral density of all these waves, and it depends in a well-defined way on frequency, polarization, incidence angle, and azimuthal look direction of the radar. We show that measured NRCS values at frequencies ranging from 1 GHz (L band) through 34 GHz (Ka band) and wind speeds between 2 and 20 m/s can be well reproduced by the proposed model after some reasonable tuning of the input ocean wave spectrum. Also, polarization effects and upwind/downwind differences of the NRCS appear to be relatively well represented. The model can thus be considered as an advanced wind scatterometer model which is based on physical principles rather than on empirical relationships. The most promising field of application, however, will be the calculation of NRCS variations associated with local distortions of the wave spectrum by surface current gradients or wind effects.
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optimization of a composite surface model for the radar Backscattering Cross Section of the ocean surface as measured by wind scatterometers
International Geoscience and Remote Sensing Symposium, 1996Co-Authors: Roland RomeiserAbstract:A calibrated composite surface model for the calculation of absolute normalized radar Backscattering Cross Sections (NRCS) of the ocean surface is presented. The model is based on a Taylor expansion of the NRCS, as given by Bragg scattering theory, up to second order in the surface slope. Measured NRCS values for a variety of radar frequencies, polarizations, incidence angles, azimuthal radar look directions, and wind speeds can be well reproduced after some reasonable tuning of the input ocean wave spectrum. The model can thus be considered as an advanced wind scatterometer model based on physical principles. Due to this fact it is also well suited for general applications like the calculation of NRCS variations associated with distortions of the wave spectrum in the presence of surface current gradients.
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on the dependence of radar signatures of surface current variations on the azimuthal radar look direction
International Geoscience and Remote Sensing Symposium, 1994Co-Authors: Roland RomeiserAbstract:An improved two-dimensional composite surface model for the radar imaging of underwater bottom topography in tidal waters is presented. It accounts for the hydrodynamic modulation of the entire wave spectrum by surface current variations, i.e., the effect of waves which are longer than the Bragg waves is included as well as the effect of wave components traveling normal to the radar look direction. These waves affect the radar backscatter by geometric modulation and by hydrodynamic modulation of the Bragg waves, yielding non-zero second-order contributions to the mean normalized radar Backscattering Cross Section (NRCS) which depend on the mean square surface slopes parallel and normal to the radar look direction. The authors discuss the dependence of the resulting radar signatures on radar frequency, polarization, incidence angle, and the azimuthal radar look direction. An interesting result is that the contribution of the normal traveling wave components to the radar backscatter is negative for all incidence angles at vertical (VV) and for steep incidence angles at horizontal (HH) polarization, while the contribution of parallel traveling waves is always positive. This leads to an "inversion" of the theoretical radar signatures under certain conditions, i.e., areas of increased surface roughness can appear as dark areas in an image, where one would normally expect an increased image intensity. >
Werner Alpers - One of the best experts on this subject based on the ideXlab platform.
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an improved composite surface model for the radar Backscattering Cross Section of the ocean surface 2 model response to surface roughness variations and the radar imaging of underwater bottom topography
Journal of Geophysical Research, 1997Co-Authors: Roland Romeiser, Werner AlpersAbstract:In the companion paper we have presented an improved composite surface model for the calculation of normalized radar Backscattering Cross Sections (NRCS) of the ocean surface. The proposed model accounts for the impact of the full two-dimensional ocean wave spectrum on the radar backscatter and was shown to reproduce measured absolute NRCS values for a variety of radar configurations and wind speeds satisfactorily after some reasonable tuning of the input ocean wave spectrum. This paper focuses on the modulation of the NRCS in the presence of spatially varying surface currents. First, the sensitivity of the NRCS to intensity variations of different ocean wave spectral components is investigated. Then the hydrodynamic modulation of the wave spectrum over underwater bottom topography in tidal waters is computed in different ways, and the resulting radar signatures are discussed. The composite surface model yields comparable radar signatures at high (10 GHz, X band) and low (1 GHz, L band) radar frequencies, which is in much better agreement with experimental results than the predictions of a first-order Bragg scattering model. On the other hand, measured variations of the NRCS at high radar frequencies appear to be still underestimated in some cases, which may be due to shortcomings of our description of the wave-current interaction by conventional weak hydrodynamic interaction theory. Possible improvements of the theory are discussed, and requirements for future experiments are formulated.
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an improved composite surface model for the radar Backscattering Cross Section of the ocean surface 1 theory of the model and optimization validation by scatterometer data
Journal of Geophysical Research, 1997Co-Authors: Roland Romeiser, Werner Alpers, V WismannAbstract:An improved composite surface model for the calculation of the normalized radar Backscattering Cross Section (NRCS) of the ocean surface at moderate incidence angles is presented. The model is based on Bragg scattering theory. A Taylor expansion of the NRCS in the two-dimensional surface slope yields nonzero second-order terms which represent a first approximation for the effect of the geometric and hydrodynamic modulation of the Bragg scattering facets by all waves that are long compared to these facets. The corresponding expectation value of the NRCS varies with the wave height spectral density of all these waves, and it depends in a well-defined way on frequency, polarization, incidence angle, and azimuthal look direction of the radar. We show that measured NRCS values at frequencies ranging from 1 GHz (L band) through 34 GHz (Ka band) and wind speeds between 2 and 20 m/s can be well reproduced by the proposed model after some reasonable tuning of the input ocean wave spectrum. Also, polarization effects and upwind/downwind differences of the NRCS appear to be relatively well represented. The model can thus be considered as an advanced wind scatterometer model which is based on physical principles rather than on empirical relationships. The most promising field of application, however, will be the calculation of NRCS variations associated with local distortions of the wave spectrum by surface current gradients or wind effects.
W Alpers - One of the best experts on this subject based on the ideXlab platform.
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using ers 2 sar images for routine observation of marine pollution in european coastal waters
Science of The Total Environment, 1999Co-Authors: Martin Gade, W AlpersAbstract:Abstract More than 660 synthetic aperture radar (SAR) images acquired over the southern Baltic Sea, the North Sea, and the Gulf of Lion in the Mediterranean Sea by the Second European Remote Sensing Satellite (ERS-2) have been analyzed since December 1996 with respect to radar signatures of marine pollution and other phenomena causing similar signatures. First results of our analysis reveal that the seas are most polluted along the main shipping routes. The sizes of the detected oil spills vary between 2 and >56 km 2 . SAR images acquired during descending (morning) and ascending (evening) satellite passes show different percentages of oil pollution, because most of this pollution occurs during night time and is still visible on the SAR images acquired in the morning time. Moreover, we found a higher amount of oil spills on SAR images acquired during summer (April–September) than on SAR images acquired during winter (October–March). We attribute this finding to the higher mean wind speed encountered in all three test areas during winter. By using an ERS-2 SAR image of the North Sea test area we show how the reduction of the normalized radar Backscattering Cross Section (NRCS) by an oil spill depends on wind speed.
V Wismann - One of the best experts on this subject based on the ideXlab platform.
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an improved composite surface model for the radar Backscattering Cross Section of the ocean surface 1 theory of the model and optimization validation by scatterometer data
Journal of Geophysical Research, 1997Co-Authors: Roland Romeiser, Werner Alpers, V WismannAbstract:An improved composite surface model for the calculation of the normalized radar Backscattering Cross Section (NRCS) of the ocean surface at moderate incidence angles is presented. The model is based on Bragg scattering theory. A Taylor expansion of the NRCS in the two-dimensional surface slope yields nonzero second-order terms which represent a first approximation for the effect of the geometric and hydrodynamic modulation of the Bragg scattering facets by all waves that are long compared to these facets. The corresponding expectation value of the NRCS varies with the wave height spectral density of all these waves, and it depends in a well-defined way on frequency, polarization, incidence angle, and azimuthal look direction of the radar. We show that measured NRCS values at frequencies ranging from 1 GHz (L band) through 34 GHz (Ka band) and wind speeds between 2 and 20 m/s can be well reproduced by the proposed model after some reasonable tuning of the input ocean wave spectrum. Also, polarization effects and upwind/downwind differences of the NRCS appear to be relatively well represented. The model can thus be considered as an advanced wind scatterometer model which is based on physical principles rather than on empirical relationships. The most promising field of application, however, will be the calculation of NRCS variations associated with local distortions of the wave spectrum by surface current gradients or wind effects.
Vladimir Karaev - One of the best experts on this subject based on the ideXlab platform.
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the algorithm for retrieving the surface waves parameters using doppler spectrum measurements at small incident angles
IEEE Transactions on Geoscience and Remote Sensing, 2019Co-Authors: Yuriy Titchenko, Vladimir KaraevAbstract:In this paper, we present an algorithm for retrieving all the second statistical moments of the sea surface that affect the scattering of waves at small angles of incidence. A feature of this algorithm is the analysis of the spectral characteristics of the reflected waves only, without taking into account the Backscattering Cross Section. The analysis of the width and the shift of the Doppler spectrum (DS) allows us to use the obtained algorithm without the preliminary instrumental calibration, which is necessary for the Backscattering Cross Section. We give final expressions for calculating the width and the shift of the DS in the monostatic problem statement for small angles of incidence. We obtain the expressions within the framework of the Kirchhoff approximation, which makes them reliable only in the quasi-specular reflection region. Features of these formulas are the description of the surface by five statistical moments of the second order and taking into account the anisotropic antenna pattern (AP). To solve the inverse problem of retrieving sea surface parameters, we propose a measurement scheme including three motionless transceiver antennas with different APs. For this measurement scheme, we give analytical expressions for all five unknown parameters of surface waves. Also, we give a theoretical analysis of the accuracy of the algorithm obtained for different wind speeds and wind directions. All the results are valid for the scattering of both acoustic and electromagnetic waves.
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the effect of sea surface slicks on the doppler spectrum width of a backscattered microwave signal
Sensors, 2008Co-Authors: Vladimir Karaev, M. B. Kanevsky, E M MeshkovAbstract:The influence of a surface-active substance (SAS) film on the Doppler spectrum width at small incidence angles is theoretically investigated for the first time for microwave radars with narrow-beam and knife-beam antenna patterns. It is shown that the requirements specified for the antenna system depend on the radar motion velocity. A narrow-beam antenna pattern should be used to detect slicks by an immobile radar, whereas radar with a knife-beam antenna pattern is needed for diagnostics from a moving platform. The study has revealed that the slick contrast in the Doppler spectrum width increases as the radar wavelength diminishes, thus it is preferable to utilize wavelengths not larger than 2 cm for solving diagnostic problems. The contrast in the Doppler spectrum width is generally weaker than that in the radar Backscattering Cross Section; however, spatial and temporal fluctuations of the Doppler spectrum width are much weaker than those of the reflected signal power. This enables one to consider the Doppler spectrum as a promising indicator of slicks on water surface.
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Retrieval of the surface wind speed from satellite radio-altimeter data: A review of algorithms
Radiophysics and Quantum Electronics, 2006Co-Authors: Vladimir Karaev, M. B. Kanevsky, E M Meshkov, D. Cotton, C. GommengingerAbstract:We give a review of surface wind-speed retrieval algorithms using satellite radio-altimeter data and analyze the efficiency of these algorithms. The data processing by new radio altimeters (JASON and ENVISAT) showed that the two-parameter algorithm developed for the satellite “Topex A” effectively operates with the data of the new radio altimeters and exceeds in accuracy the conventional algorithm by 5–10%. We discuss two main reasons for the wind-speed retrieval errors: (i) ambiguous relationship between the wind speed and the Backscattering Cross Section and (ii) regional features of the wave climate formation. Within the framework of a two-parameter algorithm, we propose a new approach for the wind-speed retrieval problem, in which two-mode structures (wind waves and a swell) are taken into account. The data processing confirmed that the new approach allows one to reduce the error related to regional features of the wave climate formation by another 5–7%.
