Spatial Differencing Technique Research Articles

Direction Finding for Passive Bistatic Radar in the Presence of Multipath Propagation

In the case of multipath propagation for passive bistatic radar (PBR) using uncooperative frequency agile-phased array radar as an illuminator, a new direction-finding method is proposed to deal with the scenario where the coherent and uncorrelated signals are closely spaced or in the same direction. Firstly, spatial difference technique is used to eliminate uncorrelated signals. Then, in order to avoid the cross-terms effect and improve the resolution of coherent signal, the iterative adaptive method (IAA) is adopted for the rearranged spatial difference matrix. Finally, the direction of arrival (DOA) of the target signal is obtained by the reconstruction of the interference-plus-noise covariance matrix. Compared with previous studies, this method has better performance in the case of low signal-to-noise ratio (SNR) and limited number of snapshots.

DOA Estimation Method in Multipath Environment for Passive Bistatic Radar

Direction-of-arrival (DOA) estimation in multipath environment is an important issue for passive bistatic radar (PBR) using frequency agile phased array VHF radar as illuminator of opportunity. Under such scenario, the main focus of this paper is to cope with the closely spaced uncorrelated and coherent signals in low signal-to-noise ratio and limited snapshots. Making full use of the characteristics of moduli of eigenvalues, the DOAs of the uncorrelated signals are firstly estimated. Afterwards, their contributions are eliminated by means of spatial difference technique. Finally, in order to improve resolution and accuracy DOA estimation of remaining coherent signals while avoiding the cross-terms effect, a new beamforming solution based iterative adaptive approach (IAA) is proposed to deal with a reconstructed covariance matrix. The proposed method combines the advantages of both spatial difference method and the IAA algorithm while avoiding their shortcomings. Simulation results validate its effectiveness; meanwhile, the good performances of the proposed method in terms of resolution probability, detection probability, and estimation accuracy are demonstrated by comparison with the existing methods.

Spatial Differencing Method for Mixed Far-Field and Near-Field Sources Localization

In this letter, we present a covariance difference algorithm to cope with the mixed far-field and near-field sources localization problem. By exploiting the eigenstructure differences between the far-field covariance matrix and the near-field one, the spatial differencing technique can be adopted to classify the signals types. Based on the symmetric property of the uniform linear array geometry, a near-field estimator without any spectral search or parameter-pairing is performed. Compared to the previous works, the resultant algorithm can realize a more reasonable classification of the signals types, as well as provide the improved estimation accuracy. Computer simulations are carried out to evaluate the performance of the proposed algorithm.

Comparison of Motion-blurred Image Restoration Using Wiener Filter and Spatial Difference Technique

Motion blur of image is caused by relative motion between camera and photographed object during protographing. Motion-blurred image restoration is an important part of the image restoration. Image restoration is the process of recovering original image from its degraded version. This paper focuses on image restoration blurred by uniform linear motion. Principle of image restoration technique is introduced. Degradation model and blurred model of horizontal motion are analyzed. Two methods to solve image restoration caused by uniform linear motion are described in detail: Wiener filter in frequency domain and difference method in spatial domain. Comparisons are made between spatial method and the method in frequency domain. Experimental results on test image show that the spatial difference technique has better performance than the method based on Wiener filter.

Characterization of chemical composition and electronic structure of Pt/YSZ interfaces by analytical transmission electron microscopy

Platinum/yttria stabilized zirconia (Pt/YSZ) interfaces with two different orientation relationships were characterized using advanced analytical transmission electron microscopy methods. Quantitative X-ray energy dispersive spectroscopy (XEDS) was performed by the recently developed ζ-factor method. Neither interdiffusion nor segregation was detected across the Pt/YSZ interfaces of both orientation conditions within the resolution limit of the technique. The interface specific components of the electron energy-loss near-edge structures (ELNES) were extracted by employing the spatial difference technique. Features of the O–K ELNES at the interface are distinctly different from that at bulk YSZ indicating Pt–O bonding. The experimentally observed changes in the O–K ELNES are in good agreement with the results of ab-initio full multiple scattering calculation, based on structure modeling at the interfaces.

Successful application of spatial difference technique to electron energy-loss spectroscopy studies of Mo/SrTiO3 interfaces.

The electron energy-loss near-edge structure (ELNES) of Mo/SrTiO3 interfaces has been studied using high spatial resolution electron energy-loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope. Thin films of Mo with a thickness of 50 nm were grown on (001)-orientated SrTiO3 surfaces by molecular beam epitaxy at 600 degrees C. High-resolution transmission electron microscopy revealed that the interfaces were atomically abrupt with the (110)Mo plane parallel to the substrate surface. Ti-L2,3 ( approximately 460 eV), O-K ( approximately 530 eV), Sr-L2,3 ( approximately 1950 eV) and Mo-L2,3 ( approximately 2500 eV) absorption edges were acquired by using the Gatan Enfina parallel EELS system with a CCD detector. The interface-specific components of the ELNES were extracted by employing the spatial difference method. The interfacial Ti-L2,3 edge shifted to lower energy values and the splitting due to crystal field became less pronounced compared to bulk SrTiO3, which indicated that the Ti atoms at the interface were in a reduced oxidation state and that the symmetry of the TiO6 octahedra was disturbed. No interfacial Sr-L2,3 edge was observed, which may demonstrate that Sr atoms do not participate in the interfacial bonding. An evident interface-specific O-K edge was found, which differs from that of the bulk in both position (0.8 +/- 0.2 eV positive shift) and shape. In addition, a positive shift (0.9 +/- 0.3 eV) occurred for the interfacial Mo-L2,3, revealing an oxidized state of Mo at the interface. Our results indicated that at the interface SrTiO3 was terminated with TiO2. The validity of the spatial difference technique is discussed and examined by introducing subchannel drift intentionally.

Electron energy-loss near-edge structure studies at the atomic level: reliability of the spatial difference technique.

The spatial difference technique was applied to determine the interface-specific components of electron energy-loss near-edge structures and the results are in good agreement to those obtained by atomic column resolved measurements averaged over one atomic layer. In previous studies an experimental set-up had been chosen where the scanning areas, which are used to measure electron energy-loss spectra, were symmetrical in respect to the location of the investigated interfaces. In the present study, an asymmetric setting of the scanning areas was applied, which allows the interfacial signals to be determined directly. Comparing the results of the different measurements shows that the spatial difference technique is valid and can be used to obtain information about the electronic structure of interfaces where single atomic column resolved measurements are not yet possible.

Characterization of Intergranular Phases in Doped Zirconia Polycrystals

AbstractAnalytical electron microscopy at high spatial resolution in a scanning-transmission mode has been used to investigate the effects of glassy or crystalline material additions on grain boundary chemistry in yttria-stabilized zirconia polycrystals. Powders of additive phase were mixed into 3-mol% yttria-stabilized tetragonal zirconia polycrystals (‘3Y-TZP’) or 8-mol% yttria-stabilized cubic zirconia polycrystals (‘8Y-CSZ’). Zirconias processed without additive phases were also examinedWithout additives, grain boundaries were depleted in zirconium and enriched in yttrium. In 3Y-TZP with I wt% borosilicate glass, silicon was observed only at triple points, but not in grain boundaries. In 3Y-TZP with 1 wt% barium silicate glass, barium was observed both along grain boundaries and at triple points, whereas silicon was detected only within the triple points. This suggests either the composition of the additive phase at the grain boundary is different from that at the triple points, or that barium ions segregate to grain boundaries during processing. In 8Y-CSZ with I wt% silica, silicon was observed in grain boundaries by an EDS spatial differencing technique. In 8Y-CSZ with 10 wt% alumina, EDS revealed aluminum at all grain boundaries examined

Atomic Structure of Si-SiO2 Interface

Abstract Investigations of the atomic structure of Si-SiO2 interfaces have mostly been performed with high resolution transmission electron microscopy. However, the interpretation of the phase contrast in the amorphous phase at the interface is not unique. While Ourmazd et al. concluded on a crystalline phase at the Si-SiO2 interface, Akatsu and Ohdomari attributed the same contrast to an interface roughness parallel to the incident electrons. We investigated the Si-SiO2 interface by studying the ELNES of the O-K edge with the spatial difference technique with a dedicated STEM with l00kV (VG HB501 UX). Also the interface was studied by Z-contrast imaging with a 300 kV dedicated STEM (VG HB603 U). Silicon wafers (110) were first thermally oxidised to produce a SiO2 layer. The thermally grown oxide was used as a substrate for liquid phase epitaxy of silicon, given two {111} Si-SiO2 interfaces in the sample grown by two different techniques (see fig. 1).

Atomic Resolution Electron Energy Loss Spectroscopy of Interfaces

The ability of high resolution STEM instruments to provide EELS data at the ultimate atomic resolution offers significant new insights into interfacial phenomena. Not only can composition profiles now be collected plane-by-plane across an interface, but in addition, the EELS near edge fine structure can be analysed to determine the valence states of atoms at the interface, and hence the nature of the bonding across the interface plane. by applying the spatial difference technique, these effects can be studied quantitatively with high sensitivity.Atomic resolution EELS played a critical role in explaining a completely unexpected interfacial structure found recently at CdTe/Si interfaces grown by a particular molecular beam epitaxy growth procedure. The high resolution Z-contrast image in Fig. 1 shows the CdTe film to be terminated by Te, but in addition, for several monolayers into the Si substrate, occasional columns show brighter than the surrounding Si, although remaining in the correct position.

Effect of thickness variations on EELS spatial-difference profiles

This paper concerns the influence of sample thickness on spatial-difference spectra, and seeks to identify if an interface dependent signal may be generated as an artifact of grain boundary grooving. The spatial-difference profiling technique may be used to identify variations in composition and electronic structure across interfaces at sub-nanometer length scales. The signal-to-background ratios and hence visibility of small changes to the near-edge structure and edge intensities are enhanced using this technique by removing intense energy dependent backgrounds. These backgrounds are assumed to be only slowly varying with respect to the electron probe position. A spatial-difference spectrum is generated from the difference between two spectra after suitable normalization or scaling. This scaling is achieved by either matching intensities of the background prior to a characteristic absorption edge (for compositional profiles) or by normalizing to some characteristic structure of the near-edge structure (for bonding profiles). The latter is performed typically after subtraction of a smooth power-law background modeled in the region immediately preceding the edge.

Spatially resolved electron energy‐loss studies of metal–ceramic interfaces in transition metal/alumina cermets

SummaryComposites consisting of an alumina matrix and 20 vol.% transition metal (Ni or Fe) particles, prepared by hot pressing powder blends, have been studied using spatially resolved transmission electron energy‐loss spectroscopy (EELS), and, to a lesser extent, by high‐resolution electron microscopy (HREM). Particular attention was paid to the elucidation of the chemical bonding mechanisms at the metal‐ceramic interface; EELS spectra from interfacial regions being obtained via a spatial difference technique. From both qualitative and quantitative interpretation of EELS near‐edge structures, as well as observed HREM images, the data appear to be consistent with the presence of an Al‐terminated alumina at the interface and the formation of direct transition metal – aluminium bonds in Al(O3M) (M = Ni or Fe) tetrahedral units, possibly as a result of the dissolution and interfacial reprecipitation of Al during processing. These results correlate well with similar model studies on diffusion‐bonded Nb/Al2O3 interfaces and may, in the light of recent theoretical electronic structure calculations, have implications for the resultant interfacial bond strength in such materials.

Investigations of the chemistry and bonding at niobiumsapphire interfaces

Spatially resolved electron energy-loss data have been recorded at the interface between niobium and sapphire (α-Al2O3), a model metal/ceramic couple. The spatial-difference technique is used to extract interface specific components of the energy-loss near-edge structure (ELNES), which are dependent on the chemistry and bonding across the interface. Multiple scattering calculations of aluminum, oxygen, and niobium clusters were performed to simulate the measured Al L2,3 ELNES. Two samples fabricated by different techniques were examined. The first interface was made by diffusion bonding pure crystals. Its interface spectrum is identified with tetrahedral coordination of the Al ions at the interface. The calculations match the experimental edge structures, supporting the notion of aluminum to niobium metal bonding and concurring with a structural model in which the basal plane of sapphire at the interface is terminated by a full monolayer (i.e., 67% excess) of aluminum. The second sample was produced by molecular beam epitaxy. The spectrum of this interface is consistent with an atomistic structure in which the interfacial basal plane of sapphire is terminated by oxygen. An unoccupied band of states within the band gap of Al2O3 is observed, signifying chemical bonding between metal and ceramic.

Numerical simulation of free shear flows and far-field sound pressure directivity

SUMMARY An implicit spatial differencing technique with fourth-order accuracy has been developed based on the Pade compact scheme. A dispersion-relation-preserving concept has been incorporated into the numerical scheme. Two-dimensional Euler computation of a spatially developing free shear flow with and without external excitation has been performed to demonstrate the capability of the numerical scheme developed. Results are in good agreement with theory and experimental observation regarding the growth rate of the fluctuating velocity, the convective velocity and the vortex-pairing process. The far-field sound pressure generated by the computed shear flow solution using Lighthill's acoustic analogy shows a strong directivity with a zone of silence at the flow angle.

The finite element formulation of the streaming ray method

Abstract A high order spatial differencing technique based on finite element and collocation methods is presented. Collocation is used at the intersection points of streaming rays with the mesh cell boundaries, where flux values are readily available. With all collocation points located on the cell edges, the maximum order of approximation is sixteen. )Here order means simply the number of trial functions per mesh cell.( An even higher order of approximation would be possible if interior collocation points were used. A set of trial functions suitable for an eighth order calculation is given. In the past, streaming ray (SR) computations have relied on first order (diamond difference and step characteristics) or third order (linear characteristics) differencing schemes.

Transient response of inelastic shells of revolution

The paper presents a transient response finite difference solution in space and time for the shell of revolution equations derived by Sanders. Inelastic constitutive relationships and nonlinear kinematics are included. The spatial differencing technique is based on a checkerboard half-spacing mesh configuration that allows the use of lower order difference expressions than those normally used for accuracy. Several example problems are considered which illustrate convergence for elastic and elastic-plastic problems. Experimental results for cylindrical shells are compared with predictions based on linear and nonlinear kinematics, various mesh spacings, and constitutive differences.