Xi Chen

Waters Corporation, Милфорд, Massachusetts, United States

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Publications (42)109.07 Total impact

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    ABSTRACT: We detected internal proper motions of the methanol maser features at 6.7 GHz in a high-mass star-forming region G006.79−00.25 with the East-Asian VLBI Network. The spatial distribution of the maser features shows an elliptical morphology. The internal proper motions of 17 methanol maser features relative to the barycenter of the features were measured. The amplitude of the internal motions ranged from 1.30 to 10.25 km s−1. Most of the internal proper motions of the maser features seem to point counterclockwise along the elliptical morphology of the maser features. We applied the disk model, which includes both rotating and expanding components, to the observed positions, l.o.s. velocities, and proper motions. The derived rotation, expansion, and systemic velocities are $+3^{+2}_{-2}$, $+6^{+2}_{-2}$, and $+21^{+2}_{-2}\:$km s−1, respectively, at the radius of 1260 au on the disk with a position angle of the semi-major axis of − 140° and an inclination of 60°. The derived rotating motion suggests that the methanol maser emissions showing the elliptical spatial morphology possibly trace the rotating disk. The derived expanding motion might be caused by the magnetic-centrifugal wind on the disk, which was estimated on the basis of the typical magnetic field strength at emitting zones of a methanol maser.
    No preview · Article · Oct 2015 · Publications- Astronomical Society of Japan
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    ABSTRACT: Comprehensive analysis of fatty alcohol ethoxylates has been conducted by coupling ultra high pressure hydrophilic interaction chromatography and ion mobility spectrometry mass spectrometry. A custom-designed sub-2 μm column was used for the chromatographic separation of fatty alcohol ethoxylates by hydrophilic interaction chromatography. Ion mobility spectrometry provided a post-ionization resolution during a very short period of 6.4 ms. Distinguishable families of singly, doubly and triply charged fatty alcohol ethoxylates were clearly observed. By virtue of the combination of hydrophilic interaction chromatography and ion mobility spectrometry, comprehensive resolution based on both hydrophobicity difference and mobility disparity has been achieved for fatty alcohol ethoxylates. The orthogonality of the developed separation and analysis system was evaluated with the correlation coefficient and peak spreading angle of 0.0224 and 88.72°, respectively. The actual peak capacity obtained was individually 40 and 193 times than those when hydrophilic interaction chromatography and ion mobility spectrometry were used alone. The collision cross-sections of fatty alcohol ethoxylates were calculated by calibrating the traveling wave ion mobility device with polyalanine. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
    Full-text · Article · Apr 2015 · Journal of Separation Science
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    ABSTRACT: This paper presents a supervised polarimetric synthetic aperture radar (PolSAR) change detection method applied to specific land cover types. For each pixel of a PolSAR image, its target scattering vector can be modeled as having a complex multivariate normal distribution. Based on this assumption, the joint distribution of two corresponding vectors in a pair of PolSAR images is derived. Then, a generalized likelihood ratio test statistic for the equality of two likelihood functions of such joint distribution is considered and a maximum likelihood distance measure for specific land cover types is presented. Subsequently, the Kittler and Illingworth minimum error threshold segmentation method is applied to extract the specific changed areas. Experiments on two repeat-pass Radarsat-2 fully polarimetric images of Suzhou, China, demonstrate that the proposed change detection method gives a good performance in determining the specific changed areas in PolSAR images, especially the areas that have changed to water.
    No preview · Article · Nov 2014 · International Journal of Digital Earth
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    Jixing Ge · Jinhua He · Xi Chen · S. Takahashi
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    ABSTRACT: In order to investigate the physical and chemical properties of massive star forming cores in early stages, we analyse the excitation and abundance of four organic species, CH3OH, CH3OCH3, HCOOCH3 and CH3CH2CN, toward 29 Extended Green Object (EGO) cloud cores that were observed by our previous single dish spectral line survey. The EGO cloud cores are found to have similar methanol J_3-J_2 rotation temperatures of ~44 K, a typical linear size of ~0.036 pc, and a typical beam averaged methanol abundance of several 10^(-9) (the beam corrected value could reach several 10^(-7)). The abundances of the latter three species, normalized by that of methanol, are found to be correlated also across a large variety of clouds such as EGO cloud cores, hot corinos, massive hot cores and Galactic Center clouds. The chemical properties of the EGO cloud cores lie between that of hot cores and hot corinos. However, the abundances and abundance ratios of the four species can not be satisfactorily explained by recent chemical models either among the EGO cloud cores or among the various types of cloud cores from literature.
    Full-text · Article · Aug 2014
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    ABSTRACT: Very-long-baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol maser allows us to measure the internal proper motion of maser spots and therefore study the gas motion around high-mass young stellar objects. To this end, we have begun monitoring observations with the East-Asian VLBI Network. In this paper we present the results of the first epoch observation for 36 sources, including 35 VLBI images of the methanol maser. Since two independent sources were found in three images, images of 38 sources were obtained. In 34 sources, 10 or more spots were detected. The observed spatial scale of the maser distribution was from 9 to 4900 astronomical units, and the following morphological categories were observed: elliptical, arched, linear, paired, and complex. The position of the maser spot was determined with an accuracy of approximately 0.1 mas, which is sufficiently high to measure the internal proper motion from two years of monitoring observations. The VLBI observation, however, detected only approximately 20% of all maser emissions, suggesting that the remaining 80% of the total flux was spread into an undetectable extended distribution. Therefore, in addition to high-resolution observations, it is important to observe the whole structure of the maser emission including extended low-brightness structures, in order to reveal the associated site of the maser and gas motion.
    No preview · Article · Apr 2014 · Publications- Astronomical Society of Japan
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    ABSTRACT: Data for a number of OH maser lines have been collected from surveys. The positions are compared with recent mid-infrared (MIR) surveys such as the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) and Wide-field Infrared Survey Explorer (WISE) surveys, restricting the comparison to point sources. The colours and intensities of the IR sources are compared. There are many 18-cm OH masers, but far fewer in lines arising from higher energy levels. We also make a comparison with 5-cm Class II methanol masers. We have divided the results into three subsamples: those associated with OH masers only, those associated with OH masers and Class II methanol masers and those associated with Class II methanol masers only. There are no obvious differences in the colour–colour or colour–magnitude results for the GLIMPSE point sources. However, according to the results from the WISE 22-μm survey, the sources associated with OH masers are brighter than those associated with methanol masers. We interpret the presence of OH and methanol masers as marking the locations of regions where stars are forming. The OH masers are located on the borders of sharp features found in the IR. These are referred to as bubbles. If the OH masers mark the positions of protostars, the result provides indirect evidence for triggered star formation caused by the expansion of the bubbles.
    Full-text · Article · Apr 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Class I methanol masers are important tools for investigations of star formation throughout the Universe. Recently, a series of surveys for class I methanol masers at the 95 GHz transition toward samples of young stellar objects have been undertaken. These surveys have resulted in the detection of about 200 new class I methanol masers and combined with previous observations they have increased the number of known class I methanol masers in our Galaxy to ~400. From analysis of the relationship between class I methanol maser emission and associated astrophysical objects, it has been shown that the intensity of the class I methanol maser emission is closely correlated with the properties of the 1.1 mm Bolocam Galactic Plane Survey (BGPS) dust continuum sources and outflow properties. This further supports the hypothesis that class I methanol masers are collisionally pumped and excited in shocked regions. Based on these observations, it can be inferred that the BGPS catalog is likely to provide more reliable samples for targeting further class I methanol maser searches. A new survey for class I methanol masers toward a larger size sample of BGPS sources to test this is currently underway. The prospects for detecting class I methanol megamasers in extragalactic sources is discussed, and observations constituting the first sensitive search have been proposed.
    No preview · Article · Apr 2014 · Chinese Science Bulletin
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    ABSTRACT: We present the results of a 6.7 GHz methanol maser survey from the Effelsberg 100 m radio telescope. A sample of 404 sources from the Bolocam Galactic Plane Survey (BGPS) 1.1 mm dust clump survey that met specific Galactic Legacy Infrared Mid-Plane Survey Extraodinaire (GLIMPSE) point-source color criteria was selected and 318 of these were observed. The new observations resulted in the detection of 29 methanol masers, including 12 new ones. Together with the additional 74 detections from the literature, this means that a total of 103 methanol masers are coincident with 1.1 mm dust clumps, yielding an overall detection rate of 26%. A comparison of the properties of a 1.1 mm dust clump and a 6.7 GHz methanol maser indicates that methanol masers with a higher flux density and/or luminosity are generally associated with more massive but less dense 1.1 mm dust clumps. The overall detection rate of 26% appears to vary as a function of the derived H2 column density of the associated 1.1 mm dust clump. The methanol masers were primarily detected toward the brighter and more massive 1.1 mm dust clumps. A subsample of 194 sources that overlapped sources with observations of the 95 GHz methanol line was investigated in more detail for the properties of 1.1 mm dust clumps. The statistical analysis reveals that 1.1 mm dust clumps with both class I and II counterparts have much higher mean and median values of mass, column density, and flux density than those with only class I or II counterparts. Based on our much larger sample, we slightly revise the boundary defined previously for selecting BGPS sources associated with a class II methanol maser, wherein ~80% of expected class II methanol masers will be detected with a detection rate in the range of 40-50%.
    Preview · Article · Feb 2014 · Astronomy and Astrophysics
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    ABSTRACT: Corynebacterium glutamicum uses 4-cresol as sole carbon source for growth. Protocatechuate 3,4-dioxygenase activity had been detected when C. glutamicum was grown with 4-cresol. In this work, we found that 4-cresol was catabolized via 4-hydroxybenzoate and protocatechuate as intermediate metabolites, and a genetic cluster called cre (designated for 4-cresol, creABCDEFGHIR, tagged as ncgl0521-ncgl0531 in NCBI) was identified. The cre gene cluster comprises of 11 genes, and six of them were experimentally confirmed to be involving in 4-cresol catabolism. The genes creD, creE, and creJ were involved in oxidation of 4-cresol into 4-hydroxybenzyl alcohol. The creD encoded a protein showing Mg(2+)-dependent phosphohydrolase activity. The genes creE, creF, creJ encoded a putative P450 system. The creG encoded a NAD(+)-dependent dehydrogenase and catalyzed 4-hydroxybenzyl alcohol to 4-hydroxybenzaldehyde. Two other genes creH and creI were involved in conversion of 4-hydroxybenzyl alcohol to 4-hydroxybenzoate, but their catalytic function is still unknown. Similar genetic clusters with high DNA sequence identity were identified in Arthrobacter and additional Corynebacterium species, suggesting that this genetic organization for 4-cresol catabolism might be more widely distributed in Gram-positive bacteria.
    No preview · Article · Jan 2014 · Journal of Biotechnology
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    ABSTRACT: Very long baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol maser allows us to measure the internal proper motions of the maser spots and therefore study the gas motion around high-mass young stellar objects. To this end, we have begun monitoring observations with the East-Asian VLBI Network. In this paper we present the results of the first epoch observation for 36 sources, including 35 VLBI images of the methanol maser. Since two independent sources were found in three images, respectively, images of 38 sources were obtained. In 34 sources, more than or equal to 10 spots were detected. The observed spatial scale of the maser distribution was from 9 to 4900 astronomical units, and the following morphological categories were observed: elliptical, arched, linear, paired, and complex. The position of the maser spot was determined to an accuracy of approximately 0.1 mas, sufficiently high to measure the internal proper motion from two years of monitoring observations. The VLBI observation, however, detected only approximately 20% of all maser emission, suggesting that the remaining 80% of the total flux was spread into an undetectable extended distribution. Therefore, in addition to high-resolution observations, it is important to observe the whole structure of the maser emission including extended low-brightness structures, to reveal the associated site of the maser and gas motion.
    Full-text · Article · Nov 2013
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    ABSTRACT: We have started a VLBI monitor project of the 6.7 GHz methanol maser sources by using the Japanese VLBI Network (JVN) and the East-Asian VLBI Network (EAVN). The 6.7 GHz methanol maser emission can be one of the best probe to obtain 3-dimensional (3-D) velocity information, particularly on an accretion disk around forming high-mass protostars. The systematic VLBI survey monitor can provide us a chance to understand the evolution of the accretion disk, which is one of the basic issues in the high-mass star formation. Our VLBI monitor project for the 6.7 GHz methanol masers has been started since August 2010 toward 36 sources, which are at mainly southern hemisphere, brighter than 65 Jy in total flux densities, and no previous VLBI observations (including two previous VLBI sources for verification of imaging capabilities). In this presentation, we will show a result of the JVN/EAVN imaging survey for all of 36 target sources obtained in 2010-2012 as an initial result of this project. In this imaging survey, spatial distributions of the methanol maser spots in 35 sources were obtained, in which 33 sources provide new VLBI images. The new images increase the VLBI imaged sample of the 6.7 GHz methanol maser by 1.5 times compared to that so far. The spatial morphology was classified into five categories on the basis of the criteria used in the previous VLBI observations (Bartkiewicz et al. 2009), including elliptical, arched, and linear morphology which could be the best candidates associated with the disk (Fujisawa et al. to be submitted). The VLBI imaging survey result was compared to spatial distributions observed with the Australia Telescope Compact Array (ATCA). The ATCA observations provided us the spatial distribution without a missing flux, and solved ambiguities for some sources classified into the complex spatial morphology on the basis of the VLBI survey. We also present an initial result for a detection of relative proper motions and 3-D velocity information in the elliptical methanol maser sources, G006.79-00.25. The detected motions suggest that the methanol masers showing the elliptical morphology can trace the rotating disk.
    No preview · Article · Jul 2013
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    ABSTRACT: We have undertaken a survey of molecular lines in the 3 mm band toward 57 young stellar objects using the Australia Telescope National Facility Mopra 22 m radio telescope. The target sources were young stellar objects with active outflows (extended green objects (EGOs)) newly identified from the GLIMPSE II survey. We observe a high detection rate (50%) of broad line wing emission in the HNC and CS thermal lines, which combined with the high detection rate of class I methanol masers toward these sources (reported in Paper I) further demonstrates that the GLIMPSE II EGOs are associated with outflows. The physical and kinematic characteristics derived from the 3 mm molecular lines for these newly identified EGOs are consistent with these sources being massive young stellar objects with ongoing outflow activity and rapid accretion. These findings support our previous investigations of the mid-infrared properties of these sources and their association with other star formation tracers (e.g., infrared dark clouds, methanol masers and millimeter dust sources) presented in Paper I. The high detection rate (64%) of the hot core tracer CH3CN reveals that the majority of these new EGOs have evolved to the hot molecular core stage. Comparison of the observed molecular column densities with predictions from hot core chemistry models reveals that the newly identified EGOs from the GLIMPSE II survey are members of the youngest hot core population, with an evolutionary time scale of the order of 103 yr.
    No preview · Article · Jun 2013 · The Astrophysical Journal Supplement Series
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    ABSTRACT: We have produced a catalog containing 98 newly identified massive young stellar object (MYSO) candidates associated with ongoing outflows (known as extended green objects, or EGOs). These have been identified from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) II data set and our new identifications increase the number of known EGOs to ~400 in our Galaxy, adding to the ~300 previously identified EGOs reported by Cyganowski et al. from the GLIMPSE I survey. The high detection rate (~70%) of 95 GHz class I methanol masers achieved in a survey toward 57 of these new EGOs with the Mopra 22 m radio telescope demonstrates that the new EGOs are associated with outflows. Investigations of the mid-infrared properties and physical associations with other star formation tracers (e.g., infrared dark clouds, class I and II methanol masers, and millimeter Bolocam Galactic Plane Survey sources) reveal that the newly identified EGOs are very similar in nature to those in the sample of Cyganowski et al. All of the observational evidence supports the hypothesis that EGOs correspond to MYSOs at the earliest evolutionary stage, with ongoing outflow activity, and active rapid accretion.
    No preview · Article · May 2013 · The Astrophysical Journal Supplement Series
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    ABSTRACT: We performed polarization sensitive VLBI observations of 6.7 GHz methanol masers toward high-mass young stellar objects with clear outflow seen from Spitzer IRAC images in the 4.5 μm band (i.e. EGOs, see Cyganowski et al. 2008) with the EVN to investigate the birthplace of the masers. By comparing direction of the major axis of methanol maser distributions with directions of higher resolution outflow and magnetic field vector, we suggest that the methanol masers toward source G28.83-0.25 may arise from surrounding disk.
    No preview · Article · Mar 2013 · Proceedings of the International Astronomical Union
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    ABSTRACT: We have observed a sample of 288 molecular outflow sources including 123 high-mass and 165 low-mass sources to search for class I methanol masers at 95 GHz transition and to investigate relationship between outflow characteristics and class I methanol maser emission with the PMO-13.7m radio telescope. Our survey detected 62 sources with 95 GHz methanol masers above 3$\sigma$ detection limit, which include 47 high-mass sources and 15 low-mass sources. Therefore the detection rate is 38% for high-mass outflow sources and 9% for low-mass outflow sources, suggesting that class I methanol maser is relatively easily excited in high-mass sources. There are 37 newly detected 95 GHz methanol masers (including 27 high-mass and 10 low-mass sources), 19 of which are newly identified (i.e. first identification) class I methanol masers (including 13 high-mass and 6 low-mass sources). Statistical analysis for the distributions of maser detections with the outflow parameters reveals that the maser detection efficiency increases with outflow properties (e.g. mass, momentum, kinetic energy and mechanical luminosity of outflows etc.). Systematic investigations of relationships between the intrinsic luminosity of methanol maser and the outflow properties (including mass, momentum, kinetic energy, bolometric luminosity and mass loss rate of central stellar sources) indicate a positive correlations. This further supports that class I methanol masers are collisionally pumped and associated with shocks, where outflows interact with the surrounding ambient medium.
    Full-text · Article · Nov 2012 · The Astrophysical Journal
  • Zili Shan · Hong Zhang · Chao Wang · Wentao An · Tao Wu · Xi Chen
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    ABSTRACT: A four-component model-based decomposition for polarimetric synthetic aperture radar (SAR) images is proposed to deal with the ground objects with orientation angles around 45°. In the previous decompositions, these special targets are mixed with the vegetated areas. With the deficiency of the previous decompositions analyzed, the ambiguity between two scattering mechanisms is clarified. A rotated Fresnel dihedral reflection model is introduced in the proposed algorithm, to model the scattering characteristics of these special targets. The nonnegative eigenvalue decomposition is applied to the remainder coherency matrix to prevent negative powers of the decomposed scattering mechanisms. Another advantage of the proposed decomposition is that it makes use of all the information provided by the coherency matrix, which remains unachieved in the previous model-based decompositions. Experimental Synthetic Aperture Radar (E-SAR) L-band polarimetric SAR data acquired over Oberpfaffenhofen, Germany, are analyzed in this letter. Experimental results indicate that the special ground objects have acquired correct scattering mechanisms, which verifies the effectiveness of the proposed method.
    No preview · Article · Sep 2012 · IEEE Geoscience and Remote Sensing Letters
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    ABSTRACT: The industrially important organism Corynebacterium glutamicum has been characterized in recent years for its robust ability to assimilate aromatic compounds. In this study, C. glutamicum strain AS 1.542 was investigated for its ability to catabolize phenylacetic acid (PAA). The paa genes were identified; they are organized as a continuous paa gene cluster. The type strain of C. glutamicum, ATCC 13032, is not able to catabolize PAA, but the recombinant strain ATCC 13032/pEC-K18mob2::paa gained the ability to grow on PAA. The paaR gene, encoding a TetR family transcription regulator, was studied in detail. Disruption of paaR in strain AS 1.542 resulted in transcriptional increases of all paa genes. Transcription start sites and putative promoter regions were determined. An imperfect palindromic motif (5′-ACTNACCGNNCGNNCGGTNAGT-3′; 22 bp) was identified in the upstream regions of paa genes. Electrophoretic mobility shift assays (EMSA) demonstrated specific binding of PaaR to this motif, and phenylacetyl coenzyme A (PA-CoA) blocked binding. It was concluded that PaaR is the negative regulator of PAA degradation and that PA-CoA is the PaaR effector. In addition, GlxR binding sites were found, and binding to GlxR was confirmed. Therefore, PAA catabolism in C. glutamicum is regulated by the pathway-specific repressor PaaR, and also likely by the global transcription regulator GlxR. By comparative genomic analysis, we reconstructed orthologous PaaR regulons in 57 species, including species of Actinobacteria, Proteobacteria, and Flavobacteria, that carry PAA utilization genes and operate by conserved binding motifs, suggesting that PaaR-like regulation might commonly exist in these bacteria.
    Full-text · Article · Jun 2012 · Applied and Environmental Microbiology
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    ABSTRACT: We report a survey with the Purple Mountain Observatory 13.7 m radio telescope for class I methanol masers from the 95 GHz (80-71 A+) transition. Two hundred and fourteen target sources were selected by combining information from both the Spitzer GLIMPSE and 1.1 mm Bolocam Galactic Plane Survey (BGPS) catalogs. The observed sources satisfy the GLIMPSE mid-IR criteria of [3.6]-[4.5] > 1.3, [3.6]-[5.8] > 2.5, [3.6]-[8.0] > 2.5 and 8.0 μm mag less than 10; they also have an associated 1.1 mm BGPS source. Class I methanol maser emission was detected in 63 sources, corresponding to a detection rate of 29% for this survey. For the majority of detections (43), this is the first identification of class I methanol masers associated with these sources. We show that the intensity of the class I methanol maser emission is not closely related to mid-IR intensity or the colors of the GLIMPSE point sources; however, it is closely correlated with properties (mass and beam-averaged column density) of the BGPS sources. Comparison of measures of star formation activity for the BGPS sources with and without class I methanol masers indicates that the sources with class I methanol masers usually have higher column density and larger flux density than those without them. Our results predict that the criteria log (S int) ≤ –38.0 + 1.72log (N beamH2) and , which utilizes both the integrated flux density (S int) and beam-averaged column density () of the BGPS sources, are very efficient for selecting sources likely to have an associated class I methanol maser. Our expectation is that searches using these criteria will detect 90% of the predicted number of class I methanol masers from the full BGPS catalog (~1000), and that they will do so with a high detection efficiency (~75%).
    Full-text · Article · Feb 2012 · The Astrophysical Journal Supplement Series
  • Tao Wu · Qinan Ren · Xi Chen · Lei Niu · Xiangwei Ruan
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    ABSTRACT: This paper proposes a method for bridge detection in airborne multiband high resolution SAR images based on PCA fusion. Images are obtained simultaneously by three bands, which are L(HH), C(HH) and X(VV), with resolution 1 meter. The paper compares the efficiency of bridge detection from different band firstly. Then, the PCA fusion of multiband is implied to detect bridge. The results show that PCA fusion is an effective method in this case.
    No preview · Article · Aug 2011
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    ABSTRACT: Abstract-The objective of this paper is to assess the sub-aperture analysis technique for the detection of coherent scatterers (CSs) using polarimetric SAR data. The non normalized coherence is used to greatly improve the contrast between the clutter and the target, since it can preserve the strong intensity information of the target. Furthermore, by applying a way similar to the polarimetric interferometry coherence optimization, the optimal coherence can be achieved by tuning the polarization states using the full polarimetric SAR data. Finally, utilizing the ALOS PALSAR data, the preliminary results and analyses of the detection of CSs are demonstrated.
    No preview · Article · Aug 2011

Publication Stats

175 Citations
109.07 Total Impact Points

Institutions

  • 2015
    • Waters Corporation
      Милфорд, Massachusetts, United States
  • 2005-2014
    • Chinese Academy of Sciences
      • • Galaxies, Cosmology Research Department
      • • Graduate School
      Peping, Beijing, China
  • 2012
    • Bielefeld University
      Bielefeld, North Rhine-Westphalia, Germany
  • 2007
    • Beijing Normal University
      • State Key Laboratory of Remote Sensing Sciences
      Peping, Beijing, China