Local Incidence Angle Research Articles

Validation of digital elevation models from SRTM X-SAR for applications in hydrologic modeling

This study examines the quality of SRTM X-SAR DEMs for applicability in hydrologic modeling. Elevation profiling, image subtraction and ground control points are used to assess the general quality of the data. The relationship between the SRTM elevation error and the local incidence angle of the radar beam and the errors' dependence from land-cover distribution is demonstrated. Test and reference DEM are exposed to comparative digital terrain analysis techniques and are applied to the rainfall–runoff model TOPMODEL to investigate the impact of different topographies on soil moisture distribution and hydrograph simulation. Model results are related to deviations in the frequency distribution of the topographic index, leading to slightly modified runoff patterns and to marginally increased flood peaks under wet watershed conditions. While SRTM X-SAR DEMs partially show low accuracy in mountainous terrain primarily due to radar shadow effects, it can be concluded that the overall quality of the data sets is sufficient for hydrologic model applications.

Stand age retrieval in production forest stands in New Zealand using C- and L-band polarimetric Radar

Regressions of single-, dual-, quad-, and full-polarization L- and C-band synthetic aperture radar (SAR) against stand age from 403 radiata pine stands in Kaingaroa Forest, New Zealand have been carried out, using the National Aeronautics and Space Administration's Airborne SAR instrument. The regressions attempted to find products suitable for the separation of young (two years or less) from old stands (25 years or older), and for the estimation of stand age. Local incidence angle had no significant effect for C-band, but was always significant for L-band, giving a standard error reduction of 13% to 32% for log stand age. Stand density was highly significant for both bands, giving standard error reductions of 7% to 47%. Single- and dual-polarization products were severely biased, and it was impossible to separate young and old stands, except L-band horizontal-(HH)-plus-horizontal-vertical (HV). C-band quad-polarization gave less bias and lower error than for that L-band, when local incidence angle and stand density were excluded. C-band full-polarization using covariance magnitudes gave no improvement over C-band quad-polarization, but L-band did give a significant improvement. The C- and L-band full-polarization products with six polarimetric indices gave significant improvements in the standard error. The results show that regressions of SAR data with stand age are possible with full- and quad-polarization L- and C-band datasets, although the prediction limits increase rapidly with stand age. The smallest error in estimated stand age, with an RMS of 3.22 years, was for L-band full-polarization with six polarimetric indices, calculated from a validation dataset. Separation of young and old forest stands was only possible for full-, quad-polarization, and the L-band HH-plus-HV products.

Resistive suppression of edge effects in MLFMA scattering from finite conductivity surfaces

The use of resistive loading to suppress edge-diffraction effects in three-dimensional numerical multilevel fast multipole algorithm calculation of scattering from arbitrary rough surfaces has been considered. The effectiveness of the loading is similar to that obtained in earlier work using a two-dimensional moment method applied to surfaces that are infinitely uniform in the azimuthal direction. The loading dramatically reduces the direct back-diffraction from the edges. However, current perturbations induced by the loading can introduce significant errors if the local angle of incidence on the leading-edge loaded area exceeds approximately 70/spl deg/. The loading is effective with finite conductivity surfaces modeled using impedance boundary conditions. The method is particularly suitable for use at high incidence (low grazing) with surfaces where the leading and trailing edges on which the loading is applied may be naturally angled downward from horizontal without affecting the scattering.

The effect of microrelief evolution on the angular dependence of polycrystalline Cu sputtering yield

The dynamically steady-state surface relief of polycrystalline copper sputtered by high-dose (1018–1019ion/cm2) 30keV Ar+ ion bombardment at room temperature has been traced as a function of the ion incidence angle θi in a wide (0–80°) angular range. For each incidence angle the distribution of local slope angles of relief facets f(β) in ion incidence plane and the corresponding distribution of the local angles of incidence w(θ) have been measured using the laser goniophotometry (LGP) technique. It has been found that the distribution f(β) has a narrow peak at grazing ion incidence, two wide peaks at oblique, and three peaks at normal ion incidence. The obtained results are discussed in terms of the theory of surface erosion under ion bombardment. A correction to the angular dependence of the sputtering yield was produced using the OKSANA computer code simulation for a smooth copper surface and the measured distributions w(θ). The experimental data and the calculation results agree well in the angular range θi=0−60°. At θi>60° the experimental points are close to the simulated curve for a smooth surface.

Reflectance Modeling of Snow-Covered Forests in Hilly Terrain

Seasonal snow covers large land areas of the Earth. Information about the snow extent in these regions is important for climate studies and water resource management. A linear spectral mixture model for snow-covered forests (the SnowFor model) has previously been developed for flat terrain. The SnowFor model includes reflectance components for snow, trees and snow-free ground. In this paper, the model is extended to handle radiometric effects caused by topography on mixed pixels of snow and trees through subpixel topographic reflectance modeling. Empirical reflectance models for snow and trees, based on the local solar incidence angle, are proposed (TopoSnow and TopoTree models), and integrated into the SnowFor model. Experiments with two Landsat Thematic Mapper (TM) images are carried out in hilly, forested terrain in Alptal, Switzerland with full snow cover. Results show that the calibrated TopoSnow and TopoTree models enhance the modeling of reflectance variability from snow-covered forests for visible and near-infrared wavelengths. The performance of four other topographic correction methods is evaluated for snow-covered forests.

Simultaneous Effects on Topography, Composition and Texture in Ion Assisted Deposition of Thin Films

ABSTRACTIon bombardment during deposition may simultaneously affect thin film topography, composition and crystallographic texture. Ion etching can produce periodic ripples that depend on the angle of ion incidence and surface temperature. When applied during deposition, ion bombardment can produce in-plane crystallographic orientation in polycrystalline materials for specific angles of incidence. In addition, ion bombardment changes the composition of multicomponent thin films according to the local angles of ion incidence and ion/atom ratios. Therefore, these three mechanisms may be linked under certain deposition conditions to generate novel topographically patterned materials with locally controlled composition and texture. Examples include metal alloys, oxides and nitrides, and recommendations for specific nanoscale structures are given.

Blade-Wake Interaction Noise for Turbines With Downwind Rotors

The interaction between the rotor and the tower wake is an important source of noise for wind turbines with downwind rotors. The tower wake modifies the dynamic pressure and the local flow incidence angle as seen by the blades and, hence, modifies the aerodynamic loading of the blade during blade passage. The resulting n per revolution fluctuation in the blade loading (where n is the number of blades) is the source of low frequency but potentially high amplitude sound levels. The Wind Turbine Company (WTC) Proof of Concept 250 kW (POC) wind turbine has been observed by field personnel to produce low-frequency emissions at the National Wind Technology Center (NWTC) site during specific atmospheric conditions. Consequently, WTC is conducting a three-phase program to characterize the low frequency emissions of its two-bladed wind turbines and to develop noise mitigation techniques if needed. This paper summarizes the first phase of this program including recent low-frequency noise measurements conducted on the WTC POC250 kW wind turbine, a review of the wake characteristics of circular towers as they pertain to the blade-wake interaction problem, and techniques to attenuate the sound pressure levels caused by the blade-wake interaction.

Three-dimensional effects on regular reflection in steady supersonic flows

The reflection of shock waves between two symmetrical wedges is investigated for the case of three-dimensional flows. Oblique shadowgraphs at various optical angles of yaw and pitch were used to examine the nature of fully three-dimensional flows, with wedge aspect ratios as low as 0.25 being considered. These images were used to construct surface models of the overall flow field for various reflection patterns and aspect ratios, which provides a visual indication of the flow field shape. For a Mach number of 3.1, and suitable wedge angles, the flow field with regular reflection on the tunnel centreline and Mach reflection further out is examined. The point of transition from regular reflection to the peripheral Mach surfaces is identified for various wedge angles and aspect ratios. It is shown that the transition points move outwards from the central plane as the aspect ratio decreases. This shows that three-dimensional flows favor regular reflection, because of the increasing curvature of the incident shock as the wedge becomes narrower, causing a decrease in the local angle of incidence. The height of the Mach stem is shown to be highly dependent on the geometry of the test wedge models. The Mach stem height decreases with aspect ratio due to the three-dimensional relieving effect, where the increase in lateral flow relieves the pressure over the surfaces of the wedges. Experimental evidence of the existence of the strong oblique shock solution in steady flows is presented.

Scattering of C-band RADARSAT signal on sloping Earth's surface affected by forest fire areas

This study investigates the backscattering properties of C-band RADARSAT (SAR) signal over sloping mountainous areas, and evaluates the utility of RADARSAT in natural disaster areas (e.g., forest fires), geological mapping, monitoring of natural hazards, natural and associated Earths environmental changes. Conventional change detection techniques in remote sensing are adequate for most applications. However, the changes in backscattering coefficients (σ0) before and after, for example, forest fires are not apparent in certain cases, mainly because of the effects of steeply sloping target scattering surface areas. The slopes of a mountainous test site can be divided into two cases with respect to the incident SAR signal look direction: the front and back slopes, where local incidence angles can vary significantly. Depending on the slope of the scattering surface, the backscattering coefficient increases to the maximum of 3.3. dB in the front-slope, but decreases 4.5 dB in back-slope after forest fires. Forest fire burnt areas provide a good example of a natural sloping scattering surface, which exhibits subtle changes in backscattering coefficients similar to the lithological changes in nature. The April 2000 forest fire in Kangwon province, Korea, burned for several weeks and was one of the worst ones in recent years. Among several firedamaged sites, an area near Samchuk city was investigated in detail with a set of multi-temporal RADARSAT Synthetic Aperture Radar (SAR) data to estimate the fire damages as compared to the unburnt background.

A stochastic, multiscale model of microwave backscatter from the ocean

The conventional view of microwave backscatter from the ocean is based on composite surface and quasi‐specular theories. In this view, backscatter at intermediate incidence angles is due to Bragg scattering from freely propagating short surface waves that are advected and modulated by longer waves. At small incidence angles the scattering process becomes quasi‐specular, coming from small facets aligned normal to the incident waves. The transition between these two processes is said to occur at incidence angles of about 10° to 20°. In this paper we demonstrate that advances in scattering theory and in computing speed make it possible to improve this view. We show that recent scattering theories agree on the form of the backscatter for incidence angles below that where multiple scattering must be considered, i.e., below about 80°. This form involves the Kirchhoff integral multiplied by a coefficient dependent on dielectric constant and incidence angle. We avoid the higher‐order calculations necessary in these theories to include the variable local incidence angle caused by surface wave slopes by applying them over restricted regions of the surface. We successively break the surface into regions from which the scatter comes from small‐, intermediate‐, and large‐scale waves. We show that in this picture, scattering from small‐scale waves is classic Bragg scattering and is very common while from large‐scale waves it is classic quasi‐specular scattering and is rarely important. For intermediate‐scale waves we evaluate the Kirchhoff integral numerically; this type of scattering becomes increasingly important with increasing wind speed. For all scales but the large one we correct the incidence angle for the slopes of all longer waves as required by composite surface theory. On this picture the transition from Bragg scattering to Kirchhoff scattering occurs gradually in a manner that is dependent on incidence angle, azimuth angle, wind speed, and the surface wave spectrum. The model indicates that Bragg scattering is often viable to surprisingly low incidence angles at low wind speeds. The model is sensitive to the wave height variance spectrum over a wide range of wave numbers. We use two recently published forms of this spectrum to compare the predictions of the model to various data that have been collected over the incidence angles range from 0° to 50°. At 0° this model produces a better fit to Ku band data from the TOPEX altimeter than does quasi‐specular theory and does so with no artificial “effective reflection coefficient.” As the incidence angle increases, the model continues to show good agreement with data without an artificial division into “quasi‐specular” and “Bragg” scattering. The advantage of this formulation over a quasi‐specular one is demonstrated by comparing the two models with data on received power taken at 36 GHz for incidence angles between nadir and 30°.

Energy Partition and Attenuation of Regional Phases by Random Free Surface

The earth's topography is generally rough at various scales. Numerical simulation techniques are used in this study to investigate the energy attenuation of regional phases across a randomly rough topography. We demonstrate a clear statistical correlation of the distance-dependent energy distribution with path topographic properties parameterized by the surface correlation length a and the surface root-mean-square height σ . Numerical experiments show that interference of randomly scattered waves by topography can cause regional wave amplitude undergoing strong variations. The topographic-scattering-driven energy distribution over a long distance is usually characteristic of an attenuation trend on the long distance scale, accompanied by amplitude fluctuations on the smaller distance scale. Total energy attenuation can be divided into large-scale and small-scale components that are correlated, respectively, in quite different manners, with along-path topographic statistics. On the one hand, the small-scale energy component is strongly related to the near-receiver topographic geometry. It has a striking similarity to the corresponding topographic curve. The spatial fluctuation of the small-scale component depends on a , whereas its amplitude amplification/deamplification is mainly related to σ , wavelength, and local incident angles. On the other hand, the large-scale component of energy curve, described by a scattering Q , demonstrates a scale-dependent relation with topographic statistics. A two-step analysis method is presented to evaluate the mantle leakage loss due to topographic scattering. The resultant topographic scattering Q is comparable with some observations with Q measured as a mean value in the crust. In summary, the study suggests the concept that topographic scattering might be a powerful mechanism to attenuate regional waves. Manuscript received 15 December 2000.

Scalar Kirchhoff's model for light scattering from dielectric random rough surfaces

In this paper, the Kirchhoff scalar approach is investigated for scattering from rough dielectric surfaces. Our aim is to derive expressions simpler than in the vector formalism in order to introduce surface scattering in scalar problems or to simplify some complicated problems, for applications. In transmission, expressions similar to those for reflection are derived with a first-order development of the local normal component of the transmitted wavevector versus the slope. The variations of reflection and transmission coefficients with local angle of incidence are also introduced in our calculations. The domain of validity of the model is discussed. The predictions of our treatment are in good agreement with published data. In particular we give a correct account of interesting effects such as the attenuation of the refractive index contrast already observed by Nieto-Vesperinas et al.

Surface roughness and slope measurements using polarimetric SAR data

In this paper, the circular polarization coherence, /spl rho//sub RRLL/, is investigated as a potential estimator of terrain surface roughness and small-scale slopes. The studies utilize microwave backscatter collected from 1) dielectric surfaces in an anechoic chamber and 2) a desert test site using P-, L-, and C-band NASA/JPL AIRSAR data. These experimental studies and supporting theory, indicate a sensitive decrease of |/spl rho//sub RRLL/| with increasing surface roughness ks over a range 0 /spl les/ ks /spl les/ 1. For the present studies this decrease is caused largely by the depolarizing effects of small-scale surface slopes in the azimuth direction rather than by volume, or multiple scatter. For cases when the scatter is reflection symmetric, the value of |/spl rho//sub RRLL/| depends on the surface roughness and on the local incidence angle. The dependence of |/spl rho//sub PRRLL/| on the local incidence angle is supported by theory and experimental results. For these same scattering cases, however, |/spl rho//sub PRRLL/| is independent of the surface dielectric constant. Estimation of the functional dependency of |/spl rho//sub PRRLL/| versus ks, for a mid-range incidence angle, has been carried out using roughness estimates derived from an empirical model.

433 Eros Global Basemap from NEAR Shoemaker MSI Images

During its year-long orbital survey of Asteroid 433 Eros (February 14, 2000 to February 12, 2001) the NEAR Shoemaker spacecraft returned over 160,000 images of the surface, obtained under a wide variety of viewing conditions and resolutions. To handle this large volume of images of Eros, we have utilized specialized techniques for projecting, mosaicking, and photometrically analyzing images of irregular objects. Local incidence and emission angles are calculated on a pixel-by-pixel basis from the global shape model to produce a normalized reflectance map of the entire asteroid. This map provides not only a medium resolution (∼10 m/pixel) scientific product in its own right but also a regional context for high-resolution (<1 m/pixel) image sequences.

The use of microwave SAR images for forest decline monitoring in mountainous area

In this paper, an attempt to use microwave images for the monitoring of forest deterioration in the Sudety Mountains has been made. For Synthetic Aperture Radar (SAR) images acquired over mountainous area the removal of geometric and radiometric distortions should precede a thematic interpretation. In order to control the influence of digital terrain model on the accuracy of geocoding, a method of fast and precise rectification of SAR images to Polish topographic maps was elaborated. The dependence of the corrected and calibrated signal on the local incidence angle for different land cover forms was investigated. The classification aimed on a discrimination of forest from non forest area has been performed. The achieved accuracy depends on the choice of multi-temporal microwave data according to the varying moisture conditions.

Complex roughening of Si under oblique bombardment by low-energy oxygen ions

Surface roughening of Si under bombardment with oblique O2+ beams at energies between 0.5 and 2 keV was studied with atomic force microscopy and secondary ion mass spectrometry. At beam energies of 1 keV and below, the general features of the topography and the magnitude of the roughness depended critically on the incidence angle. In most cases there were two angular ranges where surface roughening was strong, in-between and at grazing incidence roughening was minimal. Apart from the known topographical features—ripples and irregular bumps—triangular elevations were observed. In many cases, the local angle of incidence at the beam-facing slopes of the ripples corresponded to the maximum in sputter rate. Furthermore, the average distance between adjacent ripples increased with depth until saturation. It is concluded that the complexity in the topographies is caused by a delicate balance between several roughening and smoothing mechanisms.

Sensitivity of space-borne SAR data to forest parameters over sloping terrain. Theory and experiment

Recently, SAR data proved to be useful for the retrieval of forest biomass. However, the effects of terrain slope must be addressed towards the generalization of biomass retrieval for varied forest and environmental conditions. To this aim, we developed experimental and theoretical approaches allowing the study of multi-frequency/multi-polarization forest backscatter of a given forest type, as a function of forest parameters and SAR local incidence angle over the relief. The experimental results showed that the sensitivity of SAR data to biomass was similar to that obtained over a flat terrain, only if the backscatter data were calibrated for slope effects. Moreover, the backscatter must also be corrected for its angular decrease, which can be removed using a simple angular model developed under assumptions of theoretical equations. The highest correlation of corrected backscatter with forest parameters related to aboveground biomass (such as stand age and bole volume) was achieved at L-HV 55° (R 2

A simple method to account for topography in the radiometric correction of radar imagery

This article presents a method to study and correct radiometric distortions caused by topography in SAR images. The method is easy to implement and requires neither sophisticated software nor code-level programming. It also considers the case of a flat surface having an elevation different from the one for which calibration parameters were derived. An ortho-image of the slant range distance is used with a Digital Elevation Model to generate images of the local incident angle along the range and azimuth directions. The method compensates for variations in the terrain area of each pixel and for the angular dependence of backscatter, allowing the choice of either an empirical or semi-empirical scattering model. The method is applied to high-resolution C-SAR subsets of an agricultural area in the Central Cordillera of Costa Rica. The removal of topographic features appears excellent for local incident angles up to 80°, but small-scale structures have pronounced effects on the radar return for higher local incident angles and are not adequately corrected.

Sensitivity of space-borne SAR data to forest parameters over sloping terrain. Theory and experiment

Recently, SAR data proved to be useful for the retrieval of forest biomass. However, the effects of terrain slope must be addressed towards the generalization of biomass retrieval for varied forest and environmental conditions. To this aim, we developed experimental and theoretical approaches allowing the study of multi-frequency/multi-polarization forest backscatter of a given forest type, as a function of forest parameters and SAR local incidence angle over the relief. The experimental results showed that the sensitivity of SAR data to biomass was similar to that obtained over a flat terrain, only if the backscatter data were calibrated for slope effects. Moreover, the backscatter must also be corrected for its angular decrease, which can be removed using a simple angular model developed under assumptions of theoretical equations. The highest correlation of corrected backscatter with forest parameters related to aboveground biomass (such as stand age and bole volume) was achieved at L-HV 55° (R 2

Sand Dune Attributes Estimated from SAR Images

Dunes (such as linear dunes oriented in a constant direction), their size, and interdune spacing are very regular in the deserts. Therefore, they manifest a periodic pattern in terms of spatial distribution. There are several modulation transfer functions between SAR images and the actual periodic surface parameters, and the image brightness is sensitive to the local incidence angle. According to previous studies, a linear mapping process can describe these modulation processes. This study proposes an algorithm for estimating the linear dune features including linear dune direction (the prevailing wind direction), interdune spacing, and rms slope and dune height from SAR images. To estimate rms slope, two algorithms were developed, a local incidence angle method and a modulation transfer function method. The linear dune height was then calculated based on the rms slope of the surface. Finally, the obtained results were compared with the truth data from field investigations. The estimated physical quantities did not completely agree with the truth data, and possible error sources are discussed.