The Experts below are selected from a list of 333 Experts worldwide ranked by ideXlab platform
Bruce A Campbell - One of the best experts on this subject based on the ideXlab platform.
-
high circular polarization ratios in Radar Scattering from geologic targets
Journal of Geophysical Research, 2012Co-Authors: Bruce A CampbellAbstract:[1] We examine and model the occurrence of circular polarization ratio (CPR or μc) values greater than unity in terrestrial and planetary Radar observations as a guide to the range of associated surface morphology. Lunar crater deposits exhibit maximum μc values at 12.6 and 70-cm wavelength of 2 to 3. CPR values for Maxwell Montes on Venus range up to about 1.5 at 12.6-cm wavelength. Echoes from SP Flow in Arizona exhibit μc up to 2 at 24-cm wavelength. Scattering from rock edges and cracks (dipole-like) produces μc of unity for single Scattering and up to about 2 for multiple reflections. Scattering from natural corner reflectors (dihedrals) formed by pairs of rock facets can yield an average μc of 3–4, but likely requires non-random or scale-limited surface roughness properties in order to dominate the observed echo. The dihedral mechanism is required to satisfy the highest lunar μc observations, while echoes from SP Flow and Maxwell Montes could be explained by just the dipole mechanism. The dihedral scenario requires rocky facets on scales many times the Radar wavelength, which the blocks at SP Flow (and the unknown surface texture of Maxwell) may not provide. The random dipole model is less demanding in terms of structures and scales, and likely increases the CPR of lunar or asteroid regoliths through Scattering from and between rocks.
-
no evidence for thick deposits of ice at the lunar south pole
Nature, 2006Co-Authors: Donald B Campbell, Bruce A Campbell, L M Carter, Jeanluc Margot, Nicholas J S StacyAbstract:The rim of the Shackleton crater at the lunar south pole is a candidate crash site for NASA's LCROSS probe (Lunar CRater Observation and Sensing Satellite), due to launch in 2008. The plume of debris kicked up by the crash is to be analysed in the hope that it will reveal the water thought to be there. Suggestions of lunar ice date from 1996 when data from the Clementine spacecraft gave some indications of the presence of ice on crater walls at the south pole. Now using high-resolution Radar imagery, the Radar Scattering parameter thought indicative of thick ice deposits has been found also to match Radar echoes from the rock-strewn walls and ejecta of young impact craters at all lunar latitudes. There is no evidence for thick ice, though there could be grains of water ice spread more thinly through the lunar soil. Shackleton crater at the Moon’s south pole has been suggested as a possible site of concentrated deposits of water ice, on the basis of modelling of bi-static Radar polarization properties and interpretations of earlier Earth-based Radar images1,2. This suggestion, and parallel assumptions about other topographic cold traps, is a significant element in planning for future lunar landings. Hydrogen enhancements have been identified in the polar regions3, but these data do not identify the host species or its local distribution. The earlier Earth-based Radar data lack the resolution and coverage for detailed studies of the relationship between Radar Scattering properties, cold traps in permanently shadowed areas, and local terrain features such as the walls and ejecta of small craters. Here we present new 20-m resolution, 13-cm-wavelength Radar images that show no evidence for concentrated deposits of water ice in Shackleton crater or elsewhere at the south pole. The polarization properties normally associated with reflections from icy surfaces in the Solar System4,5,6 were found at all the observed latitudes and are strongly correlated with the rock-strewn walls and ejecta of young craters, including the inner wall of Shackleton. There is no correlation between the polarization properties and the degree of solar illumination. If the hydrogen enhancement observed by the Lunar Prospector orbiter3 indicates the presence of water ice, then our data are consistent with the ice being present only as disseminated grains in the lunar regolith.
-
Radar Scattering from a self affine fractal surface near nadir regime
Icarus, 1999Co-Authors: Michael K. Shepard, Bruce A CampbellAbstract:Abstract Planetary surfaces are better described by nonstationary fractal statistics than those more commonly assumed in current Radar Scattering models. Therefore, we have developed a Radar Scattering model applicable to self-affine fractal surfaces when observed near-nadir. The model predicts a family of angular Scattering functions that smoothly transition between forms similar to the commonly utilized Hagfors, Gaussian, and exponential surface models. The model predicts that the near-nadir Scattering behavior is determined by the wavelength-scaled roughness, i.e., the roughness that would be measured by a field worker using a ruler one wavelength in size, and the surface scaling behavior described by the fractal dimension or Hurst exponent. Additionally, this model predicts that the Scattering behavior should scale with wavelength in a self-affine manner, i.e., the Scattering behavior at long wavelengths should look “smoother” than that at short wavelengths. The Scattering behavior predicted by the model is consistent with that observed for Venus by the Magellan altimeter experiment.
-
surface processes in the venus highlands results from analysis of magellan and arecibo data
Journal of Geophysical Research, 1999Co-Authors: Bruce A Campbell, Donald B Campbell, Christopher H DevriesAbstract:The highlands of Venus are characterized by an altitude-dependent change in Radar backscatter and microwave emissivity, likely produced by surface-atmosphere weathering reactions. We analyzed Magellan and Arecibo data for these regions to study the roughness of the surface, lower Radar-backscatter areas at the highest elevations, and possible causes for areas of anomalous behavior in Maxwell Montes. Arecibo data show that circular and linear Radar polarization ratios rise with decreasing emissivity and increasing Fresnel reflectivity, supporting the hypothesis that surface Scattering dominates the return from the highlands. The maximum values of these polarization ratios are consistent with a significant component of multiple-bounce Scattering. We calibrated the Arecibo backscatter values using areas of overlap with Magellan coverage, and found that the echo at high incidence angles (up to 70°) from the highlands is lower than expected for a predominantly diffuse Scattering regime. This behavior may be due to geometric effects in multiple Scattering from surface rocks, but further modeling is required. Areas of lower Radar backscatter above an upper critical elevation are found to be generally consistent across the equatorial highlands, with the shift in microwave properties occurring over as little as 500 m of elevation. These surfaces are not simply characterized, however, by the absence of a highly reflective component. Surface morphology and Radar-Scattering properties suggest that a mantling deposit forms at the highest elevations, most likely by in situ erosion of the original rock. In Ovda Regio, this process mantles or has removed surface festoon structure at the 1- to 10-m scale, implying a significant depth for the weathered layer. Similar Radar-dark areas occur in Maxwell Montes but are apparently unrelated to the current topography of the region. Possible reasons for these observations include mass wasting from areas of steep slopes, compositional or age differences within the montes, vertical tectonic shifts of relict contacts, local topographic effects on surface temperature, or errors in the Magellan topography data in the rugged terrain. While there is evidence for some of these effects in the existing data, no single model at present appears to satisfy all occurrences of high-altitude, Radar-dark terrain. New measurements of the surface and lower atmosphere chemistry of Venus are needed to further refine these conclusions.
-
regolith composition and structure in the lunar maria results of long wavelength Radar studies
Journal of Geophysical Research, 1997Co-Authors: Bruce A Campbell, Ray B Hawke, T W ThompsonAbstract:Radar measurements at 70-cm and 7.5-m wavelengths provide insight into the structure and composition of the upper 5–100 m of the lunar regolith and crust. We combine high-resolution (3–5 km) 70-cm Radar data for the nearside with earlier calibrated full-disk observations at the same wavelength to provide a reasonable estimate of the lunar backscatter coefficient. These data are tested against models for echoes from a buried substrate and Mie Scattering from surface and buried rocks. These mechanisms are expected to dominate the 70-cm Radar echo, with their relative importance determined by the rock population, regolith depth, substrate roughness, and the loss tangent of the soil. Results indicate that the 70-cm Radar echo for the maria comes largely from Mie Scattering by rocks buried within the fine soil. Radar Scattering from a buried substrate is not likely to greatly affect the observed return. We also compared the 70-cm and 7.5-m Radar images to infrared eclipse temperature maps, crater-population age estimates for the maria, and to TiO2 and FeO abundances inferred from Earth-based telescopic and Clementine multispectral observations. These data imply that (1) the TiO2 (ilmenite) content of the regolith controls variations in 70-cm depolarized echo strength among mare units, with higher titanium abundance leading to lower echoes; (2) changes in the average 70-cm return for a given TiO2 abundance between maria of different ages do occur, but uncertainties in the current Radar data do not allow us to uniquely distinguish between variations in rock population with age and calibration effects; (3) the 7.5-m Radar echoes are controlled by the age of the mare basalt flows, with older deposits having a greater degree of fracturing and higher backscatter. Future mapping at 12.6-cm and 70-cm wavelengths will help to resolve some of the issues raised here.
M Samara - One of the best experts on this subject based on the ideXlab platform.
-
interferometric meteor head echo observations using the southern argentina agile meteor Radar
Journal of Geophysical Research, 2014Co-Authors: Diego Janches, W K Hocking, Steven Pifko, Jose Luis Hormaechea, D C Fritts, C Brunini, R Michell, M SamaraAbstract:A Radar meteor echo is the Radar Scattering signature from the free electrons generated by the entry of extraterrestrial particles into the atmosphere. Three categories of Scattering mechanisms exist: specular, nonspecular trails, and head echoes. Generally, there are two types of Radars utilized to detect meteors. Traditional VHF all-sky meteor Radars primarily detect the specular trails, while high-power, large-aperture (HPLA) Radars efficiently detect meteor head echoes and, in some cases, nonspecular trails. The fact that head echo measurements can be performed only with HPLA Radars limits these studies in several ways. HPLA Radars are sensitive instruments constraining the studies to the lower masses, and these observations cannot be performed continuously because they take place at national observatories with limited allocated observing time. These drawbacks can be addressed by developing head echo observing techniques with modified all-sky meteor Radars. Such systems would also permit simultaneous detection of all different Scattering mechanisms using the same instrument, rather than requiring assorted different classes of Radars, which can help clarify observed differences between the different methodologies. In this study, we demonstrate that such concurrent observations are now possible, enabled by the enhanced design of the Southern Argentina Agile Meteor Radar (SAAMER). The results presented here are derived from observations performed over a period of 12 days in August 2011 and include meteoroid dynamical parameter distributions, radiants, and estimated masses. Overall, the SAAMER's head echo detections appear to be produced by larger particles than those which have been studied thus far using this technique.
Leung Tsang - One of the best experts on this subject based on the ideXlab platform.
-
sea surface Radar Scattering at l band based on numerical solution of maxwell s equations in 3 d nmm3d
IEEE Transactions on Geoscience and Remote Sensing, 2018Co-Authors: Tai Qiao, Leung Tsang, D Vandemark, Simon Yueh, Tienhao Liao, Frederic Nouguier, Bertrand ChapronAbstract:Radar Scattering from ocean surfaces is investigated by 3-D numerical solution of Maxwell’s equations [numerical Maxwell’s model in 3-D (NMM3D)] using the ocean surface profiles stochastically generated from a 3-D Durden–Vesecky ocean spectrum. The surface integral equations (SIEs) are formulated for dielectric surfaces using Green’s functions of the air and the ocean permittivities with the surface tangential electric and magnetic fields as the unknowns. In solving the SIEs using the method of moment, a fast matrix solver of the sparse matrix canonical grid is used in conjunction with Rao–Wilton–Glisson basis functions. The computation has been implemented on a high-performance parallel computing cluster for problems with up to six million surface unknowns. Unlike the two-scale model (TSM) approximation, NMM3D does not require division of the surface spectrum into large- and small-scale ocean waves. The results of backScattering simulations are compared to Aquarius satellite Radar measurements for wind speeds of 5, 8, and 10 m/s and for incidence angles of 29°, 39°, and 46°. The results show that NMM3D ocean backScattering solutions at L-band are in good agreement with Aquarius satellite Radar data for co-polarized VV, HH, and cross-polarized VH returns as well as for the VV/HH ratio. The azimuthal dependence of L-band backscatter is also assessed. Finally, NMM3D results are compared to TSM solutions and are shown to lie close to Aquarius data in observed VV/HH ratio, and their azimuthal dependencies.
-
Radar Scattering of ocean surfaces at l band based on numerical solutions of maxwell equations in three dimensions nmm3d
International Geoscience and Remote Sensing Symposium, 2017Co-Authors: Tai Qiao, Leung Tsang, D Vandemark, Simon YuehAbstract:We applied the Numerical Maxwell Model in 3 Dimensions (NMM3D) to Radar Scattering from ocean surfaces at L band. The formulation is based on the PMCHWT surface integral equation which uses separate Green's functions for air and ocean permittivities. The Sparse Matrix Canonical Grid (SMCG) is used to compute Method of Moments (MoM) solutions in conjunction with Rao-Wilton-Glisson (RWG) basis functions with the surface electric field and surface magnetic field as the unknowns. Surface sizes used are up to 64 wavelengths by 64 wavelengths. Isotropic sea surfaces simulated using the Durden-Vesecky spectrum are studied for wind speeds ranging from 5 m/s to 10 m/s and incidence angles varying from 29° to 46°. BackScattering from NMM3D and composite two-scale model computations are compared with Aquarius satellite scatterometer estimates. Unlike the two-scale model, NMM3D does not impose separation between large scale roughness and small scale roughness elements. Results show that, in spite of using just the isotropic DV spectrum, NMM3D are in measurably better agreement with data than the composite surface model for co-polarized returns as well as the polarization ratio.
-
surface soil moisture retrieval using the l band synthetic aperture Radar onboard the soil moisture active passive satellite and evaluation at core validation sites
IEEE Transactions on Geoscience and Remote Sensing, 2017Co-Authors: Seungbum Kim, Leung Tsang, Jakob Van Zyl, Thomas J Jackson, J T Johnson, Matha Moghaddam, Andreas Colliander, R S Dunbar, Sermsak Jaruwatanadilok, R WestAbstract:This paper evaluates the retrieval of soil moisture in the top 5-cm layer at 3-km spatial resolution using L-band dual-copolarized Soil Moisture Active-Passive (SMAP) synthetic aperture Radar (SAR) data that mapped the globe every three days from mid-April to early July, 2015. Surface soil moisture retrievals using Radar observations have been challenging in the past due to complicating factors of surface roughness and vegetation Scattering. Here, physically based forward models of Radar Scattering for individual vegetation types are inverted using a time-series approach to retrieve soil moisture while correcting for the effects of static roughness and dynamic vegetation. Compared with the past studies in homogeneous field scales, this paper performs a stringent test with the satellite data in the presence of terrain slope, subpixel heterogeneity, and vegetation growth. The retrieval process also addresses any deficiencies in the forward model by removing any time-averaged bias between model and observations and by adjusting the strength of vegetation contributions. The retrievals are assessed at 14 core validation sites representing a wide range of global soil and vegetation conditions over grass, pasture, shrub, woody savanna, corn, wheat, and soybean fields. The predictions of the forward models used agree with SMAP measurements to within 0.5 dB unbiased-root-mean-square error (ubRMSE) and −0.05 dB (bias) for both copolarizations. Soil moisture retrievals have an accuracy of 0.052 m3/m3 ubRMSE, −0.015 m3/m3 bias, and a correlation of 0.50, compared to in situ measurements, thus meeting the accuracy target of 0.06 m3/m3 ubRMSE. The successful retrieval demonstrates the feasibility of a physically based time series retrieval with L-band SAR data for characterizing soil moisture over diverse conditions of soil moisture, surface roughness, and vegetation.
-
coherent model of l band Radar Scattering by soybean plants model development evaluation and retrieval
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 2016Co-Authors: Huanting Huang, Leung Tsang, Tienhao Liao, Seungbum Kim, Thomas J Jackson, Simon YuehAbstract:An improved coherent branching model for L-band Radar remote sensing of soybean is proposed by taking into account the correlated Scattering among scatterers. The novel feature of the analytical coherent model consists of conditional probability functions to eliminate the overlapping effects of branches in the former branching models. BackScattering coefficients are considered for a variety of scenarios over the full growth cycle for vegetation water content (VWC) and the complete drydown conditions for soil moisture. The results of the coherent model show that HH Scattering has a significant difference up to 3 dB from that of the independent Scattering when VWC is low, e.g., ${\mathbf{ 0.2}}\;{\mathbf {kg}}/{\mathbf {m}}^{\mathbf {2}}$ . Forward model calculations are performed for the Scattering from the soybean field for the full range of three axes of root-mean-square (RMS) height of bare soil, VWC, and soil moisture using the coherent model. The soybean volume Scattering including the double-bounce term is combined with the back Scattering of bare soil from the numerical Maxwell solutions that incorporates RMS height, soil permittivity, and correlation length, to form the forward model lookup table for the vegetated soil. The results are compared with data from 13 soybean fields collected as part of the soil moisture active passive validation experiment 2012 (SMAPVEX12). Time-series retrieval of soil moisture is also applied to the soybean fields by inverting the forward model lookup table. During the retrieval, the VWC is optimized with physical constraints obtained from ground measurements. The retrieval performances are significantly improved using the proposed coherent model: the root-mean-squared error (RMSE) of the soil moisture retrieval is decreased from ${\mathbf {0.09}}\;\mathbf {to}\;{\mathbf {0.05}}\;{\mathbf {cm}}^\mathbf {3}/{\text{cm}}^{\mathbf {3}}$ and the correlation coefficient is increased from 0.66 to 0.92.
F T Ulaby - One of the best experts on this subject based on the ideXlab platform.
-
microwave Radar and radiometric remote sensing
2013Co-Authors: F T Ulaby, D G Long, William J Blackwell, C Elachi, A K Fung, C S Ruf, Kamal Sarabandi, H A Zebker, Jakob Van ZylAbstract:Introduction Electromagnetic Wave Propagation Remote-Sensing Antennas Microwave Dielectric Properties of Natural Earth Materials Radar Scattering Microwave Radiometry and Radiative Transfer Microwave Radiometric Systems Microwave Interaction with Atmospheric Constituents Radiometric Sounding of the Atmosphere Surface-Scattering Models and Land Observations Volume-Scattering Models and Land Observations Emission Models and Land Observations Radar Measurements and Scatterometers Real- and Synthetic-Aperture Side-Looking Airborne Radar Interferometric SAR Radar Remote Sensing of the Ocean Spaceborne Altimetry Radiometric Remote Sensing of the Ocean.
-
mmw polarimetric Radar bistatic Scattering from a random surface
IEEE Transactions on Geoscience and Remote Sensing, 2007Co-Authors: A Nashashibi, F T UlabyAbstract:This paper explores the nature of bistatic Radar Scattering from terrain by reporting the results of an investigation involving measurements of the hemispherical pattern of the field scattered by a random soil surface. The measurements were performed by a 35-GHz fully polarimetric Radar system with transmitter and receiver modules mounted on separate rotatable arches. The acquired data were analyzed to determine the angular sensitivities of several attributes of the scattered field, including amplitudes and phase differences of the polarized Scattering coefficients, and their copolarized and cross-polarized ratios. Generally speaking, the Scattering pattern exhibits a weak dependence on the Scattering angle thetass (except along the backward direction and forward specular direction), but it exhibits a strong dependence on the azimuth angle phi, particularly for the cross-polarized components. Much of the dependence is attributed to the vectorial definition of polarization in a standard frame of reference. Comparison of the measured data with calculations based on the second-order physical optics model reveals reasonable overall agreement between theory and observations (typically within 4 dB)
-
an empirical model and an inversion technique for Radar Scattering from bare soil surfaces
IEEE Transactions on Geoscience and Remote Sensing, 1992Co-Authors: Yisok Oh, K Sarabandi, F T UlabyAbstract:Polarimetric Radar measurements were conducted for bare soil surfaces under a variety of roughness and moisture conditions at L-, C-, and X-band frequencies at incidence angles ranging from 10 degrees to 70 degrees . Using a laser profiler and dielectric probes, a complete and accurate set of ground truth data was collected for each surface condition, from which accurate measurements were made of the rms height, correlation length, and dielectric constant. Based on knowledge of the Scattering behavior in limiting cases and the experimental observations, an empirical model was developed for sigma degrees /sub hh/, sigma degrees /sub vv/, and sigma degrees /sub hv/ in terms of ks (where k=2 pi / lambda is the wave number and s is the rms height) and the relative dielectric constant of the soil surface. The model, which was found to yield very good agreement with the backScattering measurements of the present study as well as with measurements reported in other investigations, was used to develop an inversion technique for predicting the rms height of the surface and its moisture content from multipolarized Radar observations. >
Bertrand Chapron - One of the best experts on this subject based on the ideXlab platform.
-
sea surface Radar Scattering at l band based on numerical solution of maxwell s equations in 3 d nmm3d
IEEE Transactions on Geoscience and Remote Sensing, 2018Co-Authors: Tai Qiao, Leung Tsang, D Vandemark, Simon Yueh, Tienhao Liao, Frederic Nouguier, Bertrand ChapronAbstract:Radar Scattering from ocean surfaces is investigated by 3-D numerical solution of Maxwell’s equations [numerical Maxwell’s model in 3-D (NMM3D)] using the ocean surface profiles stochastically generated from a 3-D Durden–Vesecky ocean spectrum. The surface integral equations (SIEs) are formulated for dielectric surfaces using Green’s functions of the air and the ocean permittivities with the surface tangential electric and magnetic fields as the unknowns. In solving the SIEs using the method of moment, a fast matrix solver of the sparse matrix canonical grid is used in conjunction with Rao–Wilton–Glisson basis functions. The computation has been implemented on a high-performance parallel computing cluster for problems with up to six million surface unknowns. Unlike the two-scale model (TSM) approximation, NMM3D does not require division of the surface spectrum into large- and small-scale ocean waves. The results of backScattering simulations are compared to Aquarius satellite Radar measurements for wind speeds of 5, 8, and 10 m/s and for incidence angles of 29°, 39°, and 46°. The results show that NMM3D ocean backScattering solutions at L-band are in good agreement with Aquarius satellite Radar data for co-polarized VV, HH, and cross-polarized VH returns as well as for the VV/HH ratio. The azimuthal dependence of L-band backscatter is also assessed. Finally, NMM3D results are compared to TSM solutions and are shown to lie close to Aquarius data in observed VV/HH ratio, and their azimuthal dependencies.
-
directional short wind wave spectra derived from the sea surface photography
Journal of Geophysical Research, 2013Co-Authors: Maria Yurovskaya, V A Dulov, Bertrand Chapron, Vladimir KudryavtsevAbstract:[1] New field measurements of 2-D wave number short wind wave spectra in the wavelength range from few millimeters to few decimeters are reported and discussed. The measurement method is based on stereophotography and image brightness contrast processing. As found, the spectra of decimeter waves are almost isotropic and weakly dependent on the wind speed. Both directional anisotropy and wind sensitivity rapidly increase at wave numbers larger than 100 rad/m. These aspects are consistent with other previously reported optical and Radar data. Following these new in situ measurements, a revision of a semiempirical model of short wind wave spectrum is suggested. This revised model can readily be implemented in other studies (Radar Scattering, air-sea interaction issues) where detailed knowledge of short wind wave spectra is crucial.
