Local Incidence Angle Research Articles

Three-dimensional spatial compounding of ultrasound scans with weighting by incidence angle.

A three-dimensional (3D) ultrasound imaging system has been used to study spatial compounding of images acquired with different scanhead positions and orientations. A compounding algorithm has been developed that assigns regional weights depending on the local incidence angle of the ultrasound beam. Compound scans were performed of bones in vitro and the shoulder rotator cuff in volunteer subjects. Border measurements (peak value and width) were compiled as a function of ultrasound beam incidence angle and compared for single views and for maximum, mean and weighted mean compounding techniques. The weighted mean produces less variability than that of the maximum and mean for both intensity and border width. The weighted method also demonstrates less blurring of borders than the maximum and mean methods. Surfaces derived from the weighted reconstructions exhibited fewer gaps and fewer spurious connections between surfaces, which could be of particular importance for automated image analysis.

Scattering enhancements for penetrable tilted circular cylinders in water: The computed evolution away from the meridional plane

Significant contributions to the high-frequency backscattering by penetrable bluntly truncated solid circular cylinders include meridional leaky Rayleigh waves [K. Gipson, Ph.D. thesis, Washington State University (1998)] and the caustic merging transition [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 102, 3088 (1997)]. The latter is important for plastic objects (where Rayleigh waves become subsonic relative to water) and is associated with merging transmitted and internally reflected rainbow rays. One way to explore the dependence of these processes on tilt is to compute the exact scattering by infinite circular cylinders away from the meridional plane. The meridional Rayleigh-wave feature for infinite metallic cylinders is a dip in the total scattering and a peak in the background-subtracted scattering [P. L. Marston, J. Acoust. Soc. Am. 102, 358–369 (1997)]. We find it evolves in a way bounded by the locus of those rays reflected with their local angle of incidence matching the Rayleigh-wave coupling angle. In the caustic merging case, the evolution of the rainbow enhancement is similar to optical observations [Mount et al., Appl. Opt. 37, 1534–1539 (1998)]. [Supported by the Office of Naval Research.]

Comparison between the Results of a Snow Metamorphism Model and Remote Sensing Derived Snow Parameters in the Alps

The numerical snow metamorphism model CROCUS derives a complete description of the snow cover according to its geographical location (range), elevation, slope, and orientation. From the remote sensing data, snow parameters that were comparable to the model results were derived: the lower elevation of the snow cover, the surface grain size, and the surface temperature. The Landsat Thematic Mapper sensor was used because it has a good spatial resolution, a short wave infrared channel which is sensitive to grain size, and a thermal infrared channel. A first Landsat TM (Thematic Mapper) scene was acquired on 24 April 1992 and a second one on 11 December 1992. A DEM (digital elevation model) was used to obtain the local incidence angles and the elevation of each snow pixel. The pixels were then grouped according to the CROCUS classification (range, elevation, slope, orientation), and the mean snow characteristics for each class were compared with the CROCUS results. The lower limit of snow and the surface grain size derived from TM data compared favorably with the model results even if those parameters were not always easy to define. Larger differences were found for the temperature because it varies rapidly and is very sensitive to shadowing by surrounding mountains and also because its remote measurement is dependent on atmospheric conditions.

Angular dependence of the sputtering yield of rough beryllium surfaces

A new approach to study the influence of target surface roughness on sputtering is applied for two differently prepared beryllium surfaces. The topography of the Be surfaces are monitored with a scanning tunnelling microscope (STM). Mathematical methods are used to determine a distribution of local angles of incidence for a given nominal angle of incidence; this distribution is taken as input for the Monte Carlo program TRIM.SP for the calculation of the sputter yield of rough surfaces. Additionally, the redeposited fraction of emitted atoms on the rough surface is taken into account. The calculated results are compared with the calculation for an atomically smooth surface and experimental sputter yields. Results are given for deuterium and helium, and selfbombardment at 0.3 and 3 keV and several angles of incidence for rough Be surfaces.

Quantitative roughness characterization of geological surfaces and implications for radar signature analysis

Stochastic surface models are useful for analyzing in situ roughness profiles and synthetic aperture radar (SAR) images of geological terrain. In this paper, two different surface models are discussed: surfaces with a stationary random roughness (conventional model) and surfaces with a power-law roughness spectrum (fractal model). In the former case, it must be considered that for short profiles (L<200l/sub 0/), the measured values of rms-height s and correlation length l may be significantly smaller than the intrinsic values s/sub 0/ and l/sub 0/. In the latter case, rms-height and correlation length depend on the profile length L, and the surface is better characterized by slope and offset of the roughness spectrum (which are independent of L), The sensitivity of the SAR signature to variations in surface roughness parameters is evaluated by means of theoretical scattering models. For smoother geological surfaces such as most arid terrain types, single scattering is dominant, which means that the roughness parameters can be determined from SAR data using comparatively simple algorithms. Multiple scattering processes on rough surfaces such as a'a lava and variations of the local incidence angle due to large-scale terrain undulations make the retrieval of roughness parameters by means of inverse modeling much more complex. Field data of surface roughness indicate that rougher geological surfaces may be in the diffractal regime at higher radar frequencies, in which the scattering characteristics deviate significantly from the patterns observed for stationary surfaces. On the basis of surface and scattering models, recently published observations of roughness data and radar signatures from volcanic, alluvial, and arid surfaces are examined.

The influence of surface roughness on the angular dependence of the sputter yield

A new approach to study the influence of target surface roughness on sputter yields is explored. The method is based on the experimental determination of the surface topography and the subsequent evaluation of a distribution of local angles of incidence for the incident ions. This distribution is used as input to the Monte Carlo program TRIM.SP for the calculation of the sputter yield of the target with the rough surface. Additionally, the redeposition of sputtered target atoms on the rough surface is calculated. The resulting net sputter yields determined with this procedure are in satisfactory agreement with experimental data.

Estimation of Mars radar backscatter from measured surface rock populations

Reanalysis of rock population data at the Mars Viking Lander sites has yielded updated values of rock fractional surface coverage (about 0.16 at both sites, including outcrops) and new estimates of rock burial depths and axial ratios. These data are combined with a finite difference time domain (FDTD) numerical scattering model to estimate diffuse backscatter due to rocks at both the Lander 1 (VL1) and Lander 2 (VL2) sites. We consider single scattering from both surface and subsurface objects of various shapes, ranging from an ideal sphere to an accurate digitized model of a terrestrial rock. The FDTD cross‐section calculations explicitly account for the size, shape, composition, orientation, and burial state of the scattering object, the incident wave angle and polarization, and the composition of the surface. We calculate depolarized specific cross sections at 12.6 cm wavelength due to lossless rock‐like scatterers of about 0.014 at VL1 and 0.023 at VL2, which are comparable to the measured ranges of 0.019–0.032 and 0.012–0.018, respectively. We also discuss the variation of the diffuse cross section as the local angle of incidence, θi, changes. Numerical calculations for a limited set of rock shapes indicate a marked difference between the angular backscattering behavior of wavelength‐scale surface and subsurface rocks: while subsurface rocks scatter approximately as a cosine power law, surface rocks display a complex variation, often with peak backscattering at high incidence angles (θi = 70°–75°).

Extracting soil moisture information from ERS-1 imagery

Active microwave remote sensing techniques have been shown to be effective in measuring moisture levels at the soil surface. As synthetic aperture radar (SAR) data from satellites increasingly becomes available, the potential exists to repetitively monitor soil moisture conditions over large areas. Here, multitemporal ERS-1 SAR imagery were integrated with digital terrain data, vegetation measurements, and precipitation records to: (1) quantify and reduce the effect of variable terrain on SAR image digital number (DN) values, (2) isolate the amount of soil-contributed backscatter (σSOIL0) from the total backscatter coefficient (σTOT0), and (3) qualitatively examine the relationship between σSOIL0 and surface soil moisture conditions over tallgrass prairie subjected to varying treatments of prescribed burning. Local incidence angle (LIA) was found to have the strongest influence on SAR image backscatter values (r = 0.341). An empirically derived correction function based on the least squares estimate of σTOT0 and LIA data pairs improved image quality by more than 11%. A simple cloud model is then used to calculate the amount of radar energy backscattered by the vegetation canopy (σVEG0) in both burned and unburned watersheds. Subtracting σVEG0 from σTOT0 provides the estimate of σSOIL0. Unburned watershed σSOIL0 was significantly lower than that of burned watersheds, due to the higher density of scattering particles causing elevated σVEG0 values in these areas. At watershed and larger scales, changes in σSOIL0 reflected temporal changes in surface soil moisture levels inferred by local precipitation measurements and water budget consideration. © 1998 John Wiley & Sons, Inc.

Correction of SAR imagery for variation in pixel scattering area caused by topography

The radiometric correction of synthetic aperture radar (SAR) imagery for topographically induced changes in the pixel scattering area is examined. The relative significance of ground slope in the range and azimuth directions is compared, and a widely used correction factor, based on the sin of the local radar incidence angle, is shown to be sufficiently accurate. The importance of performing this correction in slant range, rather than ground range, is demonstrated.

Different processing levels of SIR-C/X-SAR radar data for the correction of relief induced distortions in mountainous areas

The third Shuttle Imaging Radar (SIR-C) was the first multifrequency and multipolarization SAR system monitoring the Earth from space. The availability of several frequencies and polarizations offers the opportunity of low level radiometric enhancement of SAR data via principal component or ratio analysis to correct the relief induced distortions of SAR images in mountainous areas. The potential of this method is presented based on the example of the Timna Valley in the south of Israel. Especially in areas where no DEM for the correction is available the method to be presented has a high application potential. For comparison the SIR-C/X-SAR data were also corrected conventionally on the basis of the local incidence angle derived via a DEM. This correction technique has the great advantage of getting real backscatter values for each frequency and polarization.

Focused ion beam sputter yield change as a function of scan speed

In many of the applications of focused ion beams, such as integrated circuit sectioning and TEM sample preparation, considerable volume of materials may need to be removed. Thus optimizing the sputter yield is important. For very rapid scan speeds at normal incidence, each pass of the beam removes a thickness of material which is much smaller than the beam diameter. In this case, this milling yield corresponds to the yield at normal incidence. However, if the scan speed is slowed down so that the thickness removed per pass is comparable to the beam diameter, then locally under the beam the ions are not normally incident even though the beam is normal to the surface. The milling yield of Si and SiO2, for example, increases by a factor of seven to eight in going from normal incidence at 0° to 75°–85°. Thus the material removal rate can be significantly increased by reducing the scan speed. We have measured the milling yield of Si and SiO2 as a function of scan speed in one axis by milling boxes, typically 7 μm×9 μm using 30 keV Ga+ ions. In the other axis, the scan speed is many orders of magnitude faster so that the beam can be thought of as a sheet or “blade.” To measure the dependence of yield on scan speed, we milled the boxes with a single scan in the slow direction, and then measured the depth of the boxes with an atomic force microscope. The beam currents used were 1 and 2.9 nA. At the slowest single-axis speeds, the sputter yield increased by a factor of two. The observed dependence on scan speed agrees with existing models, which assume a single local angle of incidence under the beam. We also measured the redeposition on the bottom of the pit and found that it increased as the scan speed was decreased. In many applications, a small amount of redeposition would be unimportant.

INTEGRATION OF SPOT AND SAR FRACTAL IMAGES FOR TERRAIN COVER CLASSIFICATION

ABSTRACT This paper presents the terrain cover classification from combining SPOT with SAR fractal images. Due to its special geometric characteristics, SAR image needs special geometry ratification before the process of overlaing with other types of imagery is proper and feasible. In this study, we intend to develop and verify a simple model to compensate for the elevation effect in the SAR image. Besides, operations of radiometric calibrations are necessary to perform three-term corrections including elevation antenna gain pattern, local incident angle variations, and range spreading compensation. The important finding is that fractal information significantly improves the discrimination capability of the heterogeneous area such as in urban regions, while it slightly degrades accuracy for homogeneous areas, such as open water. The overall classification performance can make to results that are much superior to those in production by use of SPOT image only. The integration of SAR fractal image and SPOT data for terrain cover classification proves to be effective and efficient.

Mapping seasonal snow with SIR-C/X-SAR in mountainous areas

The objective of this study is to examine the capability of SIR-C/X-SAR to map seasonal snow covers in alpine regions. We develop two classification trees. The first classifier can map snow and discriminate dry from wet snow, but it requires a digital elevation model (DEM) for radiometric terrain correction and reduction of the effect of local incidence angle. The second classifer can map wet snow without using a DEM. We demonstrate some processing techniques that are necessary to discriminate wet snow from smooth, bare surfaces and to discriminate dry snow from short vegetation. For validation, we compare our results with snow maps of both binary and snow-fraction classifications obtained from the Landsat Thematic Mapper (TM) at about the same time. The SAR is about 77% as accurate as the TM binary classification, but it underestimates snow cover when compared with snow-fraction classification, with the major differences occurring for mixed pixels of snow and trees.

Backscattering properties of a wet snow cover derived from DEM corrected ERS-1 SAR data

This paper presents the results from a study on the use of Digital Elevation Model (DEM) corrected and absolute calibrated ERS-1 SAR data for snow parameter extraction in mountainous areas. Several ERS-1 SAR data sets, Landsat Thematic Mapper (TM) and in situ measurements from Kvikne area in south Norway have been available. This paper focus on the results obtained from analysis of simultaneously acquired SAR and TM data from 31 May 1992. The SAR derived backscattering coefficient, sigma0, from a wet snow cover versus local incidence angle have been found to correspond well to surface scattering model given by Kirchoff stationary phase approximation. A linear fit between the surface model and the SAR data gave values for the surface slope and the dielectric constant which corresponds to values obtained from field measurements. The SAR data have been slope effect reduced using both modified Muhleman scattering model and surface scattering model. Spatial analysis of the relation between TM derived snow parameters and slope effect reduced SAR data have been performed in areas where exposed rock/sparsely vegetated areas are partly covered with a wet snow cover. sigma0 is found to linearly decrease as the TM derived snow ratio increases. The slope effect reduction process is shown to significantly increase the correlation coefficient between sigma0 and TM derived snow ratio.

Multiscale Surface Roughness and Backscattering - Summary

A new description for multiscale surface roughness containing all frequencies above a minimum value has been developed. Analytic equations for autocorrelation functions are given to surfaces with Gaussian, exponential, isotropic exponential and transformed exponential multiscale surface correlations. A method for applying these autocorrelation functions to surfaces rough only in separate frequency bands is also presented. The obtained multiscale exponential autocorrelation function fits well to measured 1 m-50 m long profiles of Baltic sea ice. The surface description is also combined with the Integral Equation Method for calculating the surface backscattering coefficient and applied to Baltic sea ice. However, the validity of approximating the local incidence angles with the radar incidence angle, typically used in IEM calculations, was seriously limited by the small permittivity of ice. Reliable estimation of the field coefficients requires further investigation.

Influence of O+2 energy, flux, and fluence on the formation and growth of sputtering-induced ripple topography on silicon

The formation of ripples on Si(100) by O+2 sputtering at an angle of incidence of 40° and energies from 1 to 9 keV has been studied using secondary ion mass spectrometry and scanning electron microscopy. At 1 keV no ripples are observed. Between 1.5 and 9 keV ripples are observed oriented perpendicular to the ion direction with average wavelengths that increase, from ∼100 to 400 nm, approximately linearly with O+2 energy. Two-dimensional fast Fourier transforms of secondary electron images are used to investigate the frequency distribution of the ripples. For the conditions studied, the distributions of frequencies appear approximately Gaussian. At 1.5 keV, the wavelength and growth rate with sputtered depth are independent of flux for fluxes from 15 to 150 μA/cm2. Accompanying ripple formation are changes in secondary ion yields. The changes occur abruptly at depths that increase, from ∼0.2 to 5.6 μm, with O+2 energy. In contrast, sputtering with Ar+ at 1.5 and 7 keV to depths 5–10 times those that produce ripples with O+2 produce no observable topography. These results are discussed using several existing theories for ripple formation and growth. Ripple growth and the variations in secondary ion yield are modeled by accounting for the change in local angles of incidence as the ripples grow. This model describes well the variation in secondary ion yield assuming an exponential growth rate. Ripple formation is discussed in terms of a balance between roughening (by sputtering-induced surface stress and by the dependence of the sputtering yield on surface curvature) and smoothing (by both diffusion and ion mixing). Variation in ripple wavelength with energy is not simply explained by these theories. Surface smoothing by cascade ion mixing can, however, make the wavelength, as observed, independent of ion flux. Finally, the possibility of formation of ripples by phase separation within the SiOx surface layer is discussed.

Local Incidence Angle Effects on X- and C-Band Radar Backscatter of Boreal Forest Communities

Les images de trois lignes de vol acquises dans les bandes X (9,25 GHz) et C (5,30 GHz), en polarisation HH, a partir d'un radar a antenne synthetique (RAS) monte a bord du Convair 580 du CCT fournissent une bonne couverture d'un site-temoin de 24 sur 19 km situe dans le Parc provincial Quetico, en Ontario (48° 20' de latitude N, 91° 10' de longitude 0). La topographie accidentee combinee a des angles de depression du faisceau radar variant de 16 a a 90° donnent lieu a une variete d'angles d'incidence dans la scene. Les auteurs ont utilise un modele d'elevation numerique pour calculer l'angle d'incidence pour chaque pixel des images radar. Chaque ligne des images radar a ete normalisee en vue d'obtenir une relation homogene entre la retrodiffusion radar et l'angle d'incidence local. Cette facon de proceder est jugee preferable aux methodes standard de normalisation des colonnes ou des rangees. Des donnees superposees acquises par le capteur thematique de Landsat ont ete converties en indice de foret a feuilles caduques (nPDF), qui est une mesure de la dominance des especes a feuilles caduques dans cette foret de feuillus et de resineux. L'indice de foret a feuilles caduques nPDF, en combinaison avec les angles d'incidence locaux et les trois series d'images radar dans les bandes C et X permettent une caracterisation globale de la retrodiffusion produite par les forets mixtes. Les donnees obtenues dans les bandes C et X sont tres similaires, a l'exception des forets a dominance resineuse, particulierement lorsqu'elles sont observees sous de petits angles d'incidence locaux. L'effet des angles d'incidence etaient plus importants dans la determination de la retrodiffusion que la composition de la foret, sauf dans le cas de faibles angles d'incidence, pour lesquels il y a une association distincte de plus faible retrodiffusion mesuree avec les forets a dominance resineuse. Ces resultats montrent que les donnees radar dans les bandes C et X acquises a de faibles angles de visee permettent une identification optimale de la composition des forets.

DEM corrected ERS-1 SAR data for snow monitoring

Abstract This paper presents the results from a study on the use of ERS-1 Synthetic Aperture Radar (SAR) data for snow parameter extraction in mountainous areas. Eight ERS-1 SAR data set and data from four field measurements from the Kvikne area in south Norway have been available. The data have been acquired during the snow melt period from April until June 1992. The SAR data have been calibrated and geocoded using Digital Elevation Model (DEM) data, and analysed in terms of evaluating the relationship between the backscattering coefficient σ0, and snow conditions. A decrease in σ0 of ∼3 dB between dry and wet snow has been observed, which indicates that ERS-1 SAR data can be used for monitoring the extent of wet snow cover. Significant differences between σ0 as a function of local incidence angle for ascending and descending passes have been observed. These differences are likely to be explained by differences in snow properties. For the dry snow cover we have an indication of the volume scattering effects.

Evaluation of the AIRSAR system for soil moisture analysis

In many disciplines of science, soil moisture content (SMC) is an important parameter for modeling of dynamic water cycles. In this study, the sensitivity of the backscattering coefficient was investigated in conjunction with the soil moisture content of vegetation-covered (corn) fields. As part of the MAC-Europe campaign in July 1991, the NASA/JPL three frequency polarimetric AIRSAR system collected data over a test site at Oberpfaffenhofen in Germany. The AIRSAR overflight at Oberpfaffenhofen was complemented by intensive ground truth measurements. The study was carried out with a monotemporal data set. The sampled cornfields are nearly in the same SAR incidence angle range (≈30°) to avoid effects caused by differences in the local incidence angle. The fields include three different soil types to get more variation in SMC. In summary, it can be stated that at L-band, and especially at C-band, frequencies and an incidence angle of approximately 30°, the attenuation coefficient of the vegetation canopy is too high for monitoring soil moisture with only a monotemporal data set. Other parameters like phenology, plant density and geometry, and row direction mask differences in the SMC The results show that the backscattering, measuring using the P-band, can be well described with just two parameters. SMC is the dominant parameter influencing the backscattering. The secondary parameter is the row direction of the corn plants relative to look direction. With these two parameters, about 90% of the backscattering can be described. The best polarization combinations for SMC analysis are HH polarization and the difference in power between HH and HV polarizations. The study results suggest that the P-band is very suitable for soil moisture analysis from SAR imagery.

Inferring snow wetness using C-band data from SIR-C's polarimetric synthetic aperture radar

In hydrological investigations, modeling and forecasting of snow melt runoff require timely information about spatial variability of snow properties, among them the liquid water content-snow wetness-in the top layer of a snow pack. The authors' polarimetric model shows that scattering mechanisms control the relationship between snow wetness and the copolarization signals in data from a multi-parameter synthetic aperture radar. Along with snow wetness, the surface roughness and local incidence angle also affect the copolarization signals, making them either larger or smaller depending on the snow parameters, surface roughness, and incidence angle. The authors base their algorithm for retrieving snow wetness from SIR-C/X-SAR on a first-order scattering model that includes both surface and volume scattering. It is applicable for incidence angles from 25/spl deg/-70/spl deg/ and for surface roughness with rms height /spl les/7 mm and correlation length /spl les/25 cm. Comparison with ground measurements showed that the absolute error in snow wetness inferred from the imagery was within 2.5% at 95% confidence interval. Typically the free liquid water content of snow ranges from 0% to 15% by volume. The authors conclude that a C-band polarimetric SAR can provide useful estimates of the wetness of the top layers of seasonal snow packs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>