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Tang R.,CAS Beijing Institute of Geographic Sciences and Nature Resources Research | Li Z.-L.,CAS Beijing Institute of Geographic Sciences and Nature Resources Research | Jia Y.,CAS Academy of Opto Electronics | Li C.,CAS Academy of Opto Electronics | And 3 more authors.
Remote Sensing of Environment | Year: 2011

This paper compares three remote sensing-based models for estimating evapotranspiration (ET), namely the Surface Energy Balance System (SEBS), the Two-Source Energy Balance (TSEB) model, and the surface temperature-vegetation index Triangle (TVT). The models used as input MODIS/TERRA products and ground measurements collected during the wheat and corn growth period in a subhumid climate at a measurement station in Yucheng, China. MODIS land surface temperature (LST) and leaf area index (LAI) products, corrected using ground-truth observations, were used in the three models. The TSEB model output of sensible (H) and latent (LE) heat fluxes were in good agreement with Large Aperture Scintillometer (LAS)-measured H and LE derived by residual (RMSD<45W/m2). Reasonable agreement was also obtained with the SEBS model output yielding RMSD for H of ~40W/m2 and LE ~55W/m2. However, the TVT model output resulted in poor agreement with the LAS-estimated H and LE with RMSD-values >110W/m2. Using the uncorrected MODIS LST and LAI products resulted in a deterioration of the agreement in H and LE with LAS-estimated values for both the TSEB and SEBS models, whereas TVT performance improved marginally. These results indicate that the TSEB model yielded the closest agreement with the LAS-estimated fluxes using either the corrected or uncorrected MODIS inputs (LST and LAI). The SEBS model also computed reasonable H and LE values but was significantly more sensitive to errors in MODIS LST and LAI inputs than the TSEB model. In the TVT model, output of H and LE was unacceptable in either scenario of MODIS input which was attributable to errors in selection of the dry edge. With the TVT method, accurate determination of the dry edge end member is critical in regional ET estimation, but for humid and subhumid regions this end member may often be quite difficult to identify or encompass within a satellite scene. © 2011 Elsevier Inc.


Yi L.,China University of Mining and Technology | Zhang G.,CAS Academy of Opto Electronics | Wu Z.,Wuhan University
IEEE Transactions on Geoscience and Remote Sensing | Year: 2012

Multiscale segmentation is always needed to extract semantic meaningful objects for object-based remote sensing image analysis. Choosing the appropriate segmentation scales for distinct ground objects and intelligently combining them together are two crucial issues to get the appropriate segmentation result for target applications. With respect to these two issues, this paper proposes a simple scale-synthesis method which is highly flexible to be adjusted to meet the segmentation requirements of varying image-analysis tasks. The main idea of this method is to first divide the whole image area into multiple regions; each region consisted of ground objects that have similar optimal segmentation scale. Then, synthesize the suboptimal segmentations of each region to get the final segmentation result. The result is the combination of suboptimal scales of objects and is therefore more coherent to ground objects. To validate this method, the land-cover-category map is used to guide the scale synthesis of multiscale image segmentations for the Quickbird-image land-use classification. First, the image is coarsely divided into multiple regions; each region belongs to a certain land-cover category. Then, multiscale-segmentation results are generated by the Mumford-Shah function based region-merging method. For each land-cover category, the optimal segmentation scale is selected by the supervised segmentation-accuracy-assessment method. Finally, the optimal scales of segmentation results are synthesized under the guide of land-cover category. It is proved that the proposed scale-synthesis method can generate a more accurate segmentation result that benefits the latter classification. The land-use-classification accuracy reaches to 77.8%. © 2012 IEEE.


Zhang K.,CAS Academy of Opto Electronics | Zhang K.,University of Chinese Academy of Sciences | Li X.,Chinese Academy of Sciences | Zhang J.,Chinese Academy of Sciences
IEEE Geoscience and Remote Sensing Letters | Year: 2014

Feature point matching is a critical step in feature-based image registration. In this letter, a highly robust feature-point-matching algorithm is proposed, which is based on the feature point descriptor calculated by the triangle-area representation (TAR) of the K nearest neighbors (KNN-TAR). The affine invariant descriptor KNN-TAR is used to find the candidate outliers, and then, the real outliers will be removed by the local structure and global information. The experimental results show that the proposed method can remove the outliers from the initial matching result even when the outliers are of high proportion. Compared with graph transformation matching and restricted spatial-order constraints, KNN-TAR outperforms these methods with higher stability and precision. © 2013 IEEE.


Li D.,Hubei Engineering University | Zhang G.,CAS Academy of Opto Electronics | Wu Z.,Hubei Engineering University | Yi L.,Hubei Engineering University
IEEE Transactions on Image Processing | Year: 2010

This correspondence proposes an edge embedded marker-based watershed algorithm for high spatial resolution remote sensing image segmentation. Two improvement techniques are proposed for the two key steps of maker extraction and pixel labeling, respectively, to make it more effective and efficient for high spatial resolution image segmentation. Moreover, the edge information, detected by the edge detector embedded with confidence, is used to direct the two key steps for detecting objects with weak boundary and improving the positional accuracy of the objects boundary. Experiments on different images show that the proposed method has a good generality in producing good segmentation results. It performs well both in retaining the weak boundary and reducing the undesired over-segmentation. © 2010 IEEE.


Li D.,CAS Academy of Opto Electronics
Chinese Journal of Aeronautics | Year: 2015

Multi-angle synthetic aperture radar (SAR) image matching is very challenging, because the same object may cause different backscattering patterns, heavily depending on the radar incident angle. A technique based on the relations between the invariant positions of ground targets among the reference and sensed images is proposed to accommodate the nonmatching patterns. It involves a target extraction using wavelet coefficient fusion, as well as a geometric voting matching routine for searching the matched control points (CPs) in the reference and sensed images, respectively. To accelerate the speed of the search, a robust, rapidly corresponding CPs determination strategy is exploited by utilizing the global spatial transformation model, as well as a procedure of outlier removal to ensure the desired accuracy. Meanwhile, the positions of the matched point pairs are relocated using mutual information. The final warping of the images according to the CPs is performed by using a polynomial function. The results show the possibility of matching multi-angle SAR images in general cases. © 2015 Production and hosting by Elsevier Ltd.


Gao Y.,CAS Academy of Opto Electronics | Li X.,CAS Academy of Opto Electronics
Acta Astronautica | Year: 2010

Based on the dynamic evolution of mean equinoctial orbital elements, we developed a direct method to optimize low-thrust many-revolution orbit transfers and proposed a Lyapunov-based guidance, in which a Lyapunov control law with time-varying gains is employed. Within each transfer revolution, a parameterized control law, in the form of the optimal control derived from the calculus of variations, is formulated, and a periapsis- and apoapsis-centered burn structure is proposed in order to effectively solve fuel-saving orbit transfers. The parameters governing the control law and the burn structure within each transfer revolution are interpolated through a finite number of discrete nodes. The optimal orbit transfer problem is converted to the parameter optimization problem that is solved by nonlinear programming. Subsequently, a mapping between the parameterized control law and the Lyapunov control law is revealed, in terms of which the time-varying gains of the Lyapunov control law, called Lyapunov gains, can be obtained using trajectory optimization solutions. However, this mapping does not guarantee that all obtained Lyapunov gains are positive so that the Lyapunov-based guidance may not be strictly stable during an entire transfer period. Nevertheless, we showed that the Lyapunov-based guidance that is not strictly stable may still successfully guide the spacecraft for certain orbit transfer cases. Negative Lyapunov gains can be re-defined as appropriate positive values to warrant both stability and acceptable performance for the Lyapunov-based guidance. © 2009 Elsevier Ltd. All rights reserved.


Huang Z.,Jiangsu University | Yuan H.,CAS Academy of Opto Electronics
Radio Science | Year: 2014

In this article a radial basis function (RBF) neural network improved by Gaussian mixture model is developed to be used for forecasting ionospheric 30 min total electron content (TEC) data given the merits of its nonlinear modeling capacity. In order to understand more about the response of developed network model with respect to stations situated at different latitude, estimated TEC overhead of GPS ground stations BJFS (39.61°N, 115.89°E), WUHN (30.53°N, 114.36°E), and KUNM (25.03°N, 102.80°E) for 6 months in 2011 are used for training data set, validating data and test data set of RBF network model. The performance of the trained model is evaluated at a set of criteria. Our results show that the predicted TEC is in good agreement with observations with mean relative error of about 9% and root-mean-square error of less than 5 total electron content unit, 1 TECU = 1016 el m -2. Our comparison further indicates that RBF network offers a powerful and reliable tool for the design of ionospheric TEC forecast. ©2014. American Geophysical Union. All Rights Reserved.


Xu Y.,CAS Academy of Opto Electronics | Yuan H.,CAS Academy of Opto Electronics
Science China: Physics, Mechanics and Astronomy | Year: 2011

Signal structure design is an important part of satellite navigation system research, which directly affects navigation performance. Signal performance parameters are analyzed and performances of BPSK modulated signals and BOC modulated signals are compared. Aiming at requirements of high navigation precision and high anti-jamming ability, a new navigation signal structure based on complex carrier modulation is proposed and performances of the signal are researched with different parameters. A synchronization algorithm is put forward according to the signal characteristics, and the synchronization performance is qualitatively analyzed. Next, the applications of the complex carrier modulated signal are discussed, which include anti-jamming, navigation enhancement, power combing and so on. Simulations and analysis show that the proposed navigation signal structure based on complex carrier modulation has good navigation capabilities and anti-jamming abilities, which deserves further study. © 2011 Science China Press and Springer-Verlag Berlin Heidelberg.


Li D.,CAS Academy of Opto Electronics
Beijing Hangkong Hangtian Daxue Xuebao/Journal of Beijing University of Aeronautics and Astronautics | Year: 2015

The V-estimator (VE) for K-distribution shape parameter proposed by Oliver in 1993, bears the characteristics of without solving non-linear equations so it has a high estimating efficiency, but the estimating accuracy of it is lower than that of many other moment estimators, sometimes the VE even results in odd value. In order to make the best use of the advantages and bypass the disadvantages, on the basis of derivation and analysis of the VE bias, by means of a set of Monte-Carlo experiments, the V-estimator with corrective term (VCE) was discussed, which overcomes the shortcomings above of the VE. Simulation results show that not only the estimating accuracy of the VCE is significantly superior to the VE, but also, on both of estimating accuracy and efficiency, to the U-estimator considered as the moment estimator with the highest accuracy usually. Especially, experiment results demonstrate that the VCE is better suited to performing in the case of small samples, this feature makes it possible that the VCE is more applicable to the practice. ©, 2015, Beijing University of Aeronautics and Astronautics (BUAA). All right reserved.


Liu Y.-Y.,CAS Academy of Opto Electronics | Lu Q.-B.,CAS Academy of Opto Electronics | Zhang W.-X.,CAS Academy of Opto Electronics
Wuli Xuebao/Acta Physica Sinica | Year: 2012

Interference imaging system for space target has close relation with atmospheric environment, and atmospheric turbulence disturbance distorts terribly the wavefront phase of any transmission optics launched from any optics system. In the space target interference imaging system the phase-closure-principle is adopted, in order to eliminate possible effects of atmospheric turbulence on the target image. Based on the power spectrum method and the Fourier transform method, the numerical simulation of wavefront phase screen, which is distorted by atmospheric turbulence conforming to the statistical rules of kolmogonov model and modified Von Karmen model, is respectively implemented. Various images by several different phase-screen models on the interference imaging system are obtained. Simulation results show that, with the advantage of adopting phase-closure-principle, the possible effects of atmospheric turbulence imaging can be eliminated basically. © Chinese Physical Society.

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