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    20 July 2018, Volume 47 Issue 7
    Resilient PNT Concept Frame
    YANG Yuanxi
    2018, 47(7):  893-898.  doi:10.11947/j.AGCS.2018.20180149
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    The concept of resilient positioning, navigation and timing (PNT) is described.The definition of resilient PNT is given from the point of view of sensors integration, functional model and stochastic model.The relationship between the integrated (or comprehensive) PNT and resilient PNT is analyzed. It is pointed out that the integrated PNT is the foundation of the resilient PNT.The resilient PNT should be divided into the resilient sensors integration, resilient functional model and resilient stochastic model.The strategy and the principle of resilient integration of sensors are discussed.The resilient integration of sensors should be designed following the optimal, available and compatible as well as interoperable principles.The concepts of resilient functional model and the possible modification strategies of the resilient functional model are also described.Several possible optimal routes for resilient stochastic model improvements are also set forth.It is also pointed out that the optimal improvements of stochastic models for multi PNT sources should follow the same variance scale.At last, the resilient PNT data fusion for state parameters are given based on the resilient functional and stochastic models.
    Comparison and Analysis of Crustal Vertical Deformation in Mainland China Observed by GPS from CMONOC and GRACE
    JIA Lulu, WANG Yuebing, LIAN Weiping, XIANG Longwei
    2018, 47(7):  899-906.  doi:10.11947/j.AGCS.2018.20170281
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    Based on continuous GPS data from crustal movement observation network of China (CMONOC), the vertical deformations on Earth's surface observed by 234 GPS stations and GRACE in mainland China are compared with each other.GPS-observed and GRACE-derived vertical deformations are in good agreements in general, which indicates that mass loading is an important factor to GPS nonlinear variations, but there are some differences between GPS and GRACE.For further analyzing the difference between GPS and GRACE vertical deformations, we take into account the thermal expansion effects to GPS vertical deformations and the effect of regional crustal structure on vertical loading deformations derived from GRACE.It is found that the annual amplitudes of vertical deformations induced by thermal expansion are not less than 1 mm at more than 50% of GPS stations.After considering thermal deformations, the vertical deformations of GPS and GRACE are with higher consistency in mainland China.The ratio of annual amplitudes from GPS and GRACE vertical deformations changes from 1.07±0.06 to 1.01±0.05, which is closer to the ideal value of 1.Thermal expansion can explain 6.2% of the difference between GPS and GRACE vertical deformations.And it can increase the consistency of vertical deformations between GPS and GRACE by 11.2% relatively after thermal expansion correction.The regional crustal structure has little effect on GRARE-derived vertical loading deformations in mainland China and the relative difference is 2.5%.
    EEMD Interval Threshold De-noising Method for Inertial Navigation
    LIU Tao, XU Aigong, SUI Xin
    2018, 47(7):  907-915.  doi:10.11947/j.AGCS.2018.20170391
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    The stochastic error of gyro is the significant factor that affect the precision of inertial navigation system (INS).On the basis of empirical mode decomposition (EMD) method and threshold de-noising method, proposed an ensemble empirical mode decomposition(EEMD) interval threshold de-noising method for gyro signal.The gyro signal is decomposed into several intrinsic mode function(IMF) components and one residual component, then rejecting the IMFs that contain pure noise by the 2-norm measures between the probability density function of IMFs and signal, using an improved interval threshold de-noising method to complete the signal de-noising process.Simulation and real test experiments show that the proposed method can not only effectively restrain the mode mixing phenomenon in EMD, but also effectively reducing the stochastic error of gyro, thus improving the precision and reliability of INS.
    Satellite Clock Estimation with Between-satellite Single Difference Phase Ambiguity Fixing
    RUAN Rengui, WEI Ziqing, FENG Laiping
    2018, 47(7):  916-923.  doi:10.11947/j.AGCS.2018.20170458
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    A large number of researches suggest that integer ambiguity resolution(IAR) significantly improve the precision and reliability of precise point positioning (PPP) and providing “integer” satellite clock corrections is one of the available approaches proposed in the past ten years. In this paper, we propose a novel approach to estimate integer satellite clock corrections to support IAR for PPP application, Our approach is based on between-satellite single difference (BSSD) ambiguity fixing which contains two key steps, namely to estimate wide-lane fractional cycle bias (FCB) for satellites and to select and fix the BSSD ambiguity datum, which would recover the integer property of the wide- and narrow-lane BSSD ambiguities, respectively. This approach has been implemented in the SPODS software developed at Xi'an Research Institute of Surveying and Mapping. Experiments for clock estimation with data collected at about 66 IGS stations have been carried out to validate the proposed approach. It is demonstrated that, in the clock estimation, 73%, on average, of the independent BSSD ambiguities were successfully fixed to integers and the mean RMS and STD of differences between our clocks and the IGS final clocks are 0.170 ns and 0.012 ns respectively. The fractional parts of the wide-lane and narrow-lane BSSD ambiguities from about 448 IGS stations were analyzed, which proves that the obtained satellite clocks together with the wide-lane FCB products have the ability to support IAR in PPP. Using our products, experiments for simulated kinematic PPP with data collected at 20 IGS stations were carried out. It is shown that, with IAR, the positioning accuracy (RMS) in N, E, U and 3D are 0.009, 0.010, 0.023 and 0.027 m, corresponding to improvements of 30.8%, 61.5%, 23.3% and 37.2%, respectively, compared with that without IAR or with IGS final clocks.
    Fusion the Altimetric and Shipborne Gravity Data Based on Point Mass Fit Method
    KE Baogui, ZHANG Liming, ZHANG Chuanyin, DANG Yamin
    2018, 47(7):  924-929.  doi:10.11947/j.AGCS.2018.20170299
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    Using the point mass fit method, the position, size and other parameters of the point mass source are inversed by fitting the shipborne gravity anomaly data with the minimum variance between the point mass model and observed value.Avoiding the instability posed by the point mass method, the shipborne residual gravity anomalies are calculated by using these parameters based on the idea of remove-restore technique.Shipborne gravity residuals can be obtained by minusing the gravity anomaly in the track line.The altimetric residual gravity data are also obtained by using the similar method.Weighted the two residual gravity anomaly by the minimum curvature gridding method, and restored the model gravity anomaly calculated by the point mass size, depth and other parameters, then realized the satellite altimetry and shipborne gravity data fusion.The mean bias is 1~2 mGal and the root mean square is about 4 mGal, which is verified by the international shipborne gravity measurement data.The proposed method can be used as a reference for processing the various gravity data fusion in coastal zone and for satellite gravity downward continuation problem.
    Ionosphere-free Multi-carrier Ambiguity Resolution Method Based on Ambiguity Linear Constraints
    JIA Chun, ZHAO Lin, LI Liang, CHENG Jianhua, LI Hui
    2018, 47(7):  930-939.  doi:10.11947/j.AGCS.2018.20170296
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    Ionospheric delay is a crucial factor affecting the reliability of narrow-lane ambiguity resolution in the traditional multi-carrier ambiguity resolution (MCAR) method.To obtain a better narrow-lane ambiguity resolution performance, a new ionosphere-free MCAR method based on ambiguity linear constraints is proposed.The method constructs a geometry-based and ionosphere-free model by using the equality lemma, and then formats a narrow-lane ambiguity resolution model to resolve narrow-lane ambiguity and precise position depending on the ambiguity linear constraints formed by reliable extra wide-lane/wide-lane ambiguity value.The performance of the proposed method is tested by a series of experiments.The results denote that the proposed method achieves higher than ambiguity success rate of 96% and the position precision of about 15 cm, even though the double-differenced ionospheric delay increases up to 73.2 cm.
    North China Plain Water Storage Variation Analysis Based on GRACE and Seasonal Influence Considering
    LI Zhen, ZHANG Chuanyin, KE Baogui, LIU Yang, LI Wanqiu, YIN Cai
    2018, 47(7):  940-949.  doi:10.11947/j.AGCS.2018.20170171
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    Based on the remove-restore method, the GRACE satellite gravity data are used to study the variation of terrestrial water storage from January 2003 to June 2014 in North China Plain. A method for calculating the scale factor considered seasonal influence is proposed to reduce the GRACE post-processing error. The results obtained by the proposed method are compared with those from hydrological model and the precipitation model. The results indicate that serious drought occurred in North China Plain before 2008, and the terrestrial water and surface water decrease at the rate of (-7.9±2.4)mm/a and (-7.3±2.8)mm/a respectively while the groundwater decrease only at the rate of (-0.6±1.4)mm/a. However since 2008, the terrestrial water and surface water show an upward trend of (4.3±1.3)mm/a and (10.9±2.1)mm/a respectively.On the other hand, the over-exploitation of groundwater began to aggravate, and groundwater level showed a downward trend of (-6.5±1.2)mm/a, but the descent rate was reduced at the rate of -0.9 mm/a2.During the study period, the terrestrial water, surface water and groundwater storage in North China Plain show a trend of (-2.0±0.6)mm/a, (2.9±0.7)mm/a and (-4.8±0.7)mm/a respectively.Finally, the phenomenon of GRACE lagging behind precipitation is analyzed using cross-wavelet spectrum.Research revealed that the two most crucial factors influencing the water storage variation in North China Plain are precipitation and groundwater exploitation.
    Road Extraction from High-resolution Remote Sensing Images Based on Adaptive Circular Template and Saliency Map
    LIAN Renbao, WANG Weixing, LI Juan
    2018, 47(7):  950-958.  doi:10.11947/j.AGCS.2018.20170596
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    In order to solve the problem that the existing template matching algorithms need to manually set template sizes, it is proposed that an adaptive circular template algorithm for extracting the road information in a high-resolution remote sensing image.Firstly, an improved local morphological gradient map is constructed to calculate the size of the circular template automatically;then, a modified road saliency map is made.It is designed that a new algorithm to search for the most likely center points of a road between the start and the end points by the way of iterative interpolation.The comprehensive utilization of the saliency information and the angles of geometric during the search process make the algorithm have the better recognition effect.The experimental results show that the proposed algorithm can be applied to high-resolution remote sensing images of different conditions, to extract the road information more effectively.
    Refinement of the 3D Mesh Model Driven by the Image Information
    ZHANG Chunsen, ZHANG Mengmeng, GUO Bingxuan
    2018, 47(7):  959-967.  doi:10.11947/j.AGCS.2018.20170733
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    A method of 3D mesh refinement driven by the image information is proposed to solve the existing problem of 3D re-construction constructed by the 3D mesh model.According to the image information, one image is reprojected to other visible images induced by the object triangular facet, adopt gradient descent method, take the derivative of object vertex coordinates with the images' NCC coefficient, and adjust the vertices coordinates of object 3D mesh in order to minimize regional matching cost, to obtain the optimal 3D mesh model, and to achieve the purposes of the 3D mesh model refinement.Results indicate that this method can greatly improve the accuracy of 3D mesh to reconstructing 3D model.
    Self-adaption Bundle Adjustment of Three-line Array Image with Smoothing Equation of Exterior Orientation Elements
    WANG Jianrong, WANG Renxiang, HU Xin
    2018, 47(7):  968-972.  doi:10.11947/j.AGCS.2018.20170217
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    Exterior orientation elements is a key factor to effect the location accuracy of optical satellite images, and its accuracy is depended on not only the attitude and orbit determination system, but also the stability of satellite platform.However, there is a mutation phenomenon in satellite engineering, which will lead to a big change of vertical parallax in processing strip.In this paper, based on the theory of EFP bundle adjustment of all three-line intersection, the strategy and mathematical model of self-adaption bundle adjustment with smoothing equation of exterior orientation elements are proposed.Firstly, the image is segmented automatically according to the vertical parallax difference data of the stereoscopic image.Then, the different weights of smoothing equation of external orientation element are given with different image segments, and participated in bundle adjustment.Finally, the experiments are performed and the results show that the proposed method can effectively restrain the influence of the satellite attitude observation mutation to the accuracy of positioning without control of stereo images, especially the elevation accuracy improved significantly.
    Inclination Angle Error Compensation Algorithm of MIMO Downward-looking Array SAR
    LIU Hui, LI Geshuang, XU Qing, LOU Liangsheng
    2018, 47(7):  973-985.  doi:10.11947/j.AGCS.2018.20170292
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    Combined with the real POS data form, elements of exterior orientation of traditional optical photogrammetry were introduced, and inclination angle error compensation methods of MIMO downward-looking array SAR were designed.The beam-independent phase error was compensated in the three-dimensional spatial domain firstly.Then the compensated echo data was transformed into the two-dimensional wavenumber domain in along-track and cross-track directions, and was equally divided into M and N segments.The MN subblocks were zeroed and extended to the original size, transformed into the two-dimensional spatial domain in along-track and cross-track directions and wavenumber domain in range direction, and the beam-dependent phase error was compensated.Then, the subblock echo data were transformed into three-dimensional spatial domain, and added directly to the final data.Finally, the correct 3D imaging results could be obtained by 3D RD imaging algorithm.The effectiveness of the proposed method was verified by simulation experiments.
    Hybrid Geometric Calibration Method for Multi-platform Spaceborne SAR Image with Sparse GCPs
    LÜ Guannan, TANG Xinming, AI Bo, LI Tao, CHEN Qianfu
    2018, 47(7):  986-995.  doi:10.11947/j.AGCS.2018.20170283
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    Geometric calibration is able to provide high-accuracy geometric coordinates of spaceborne SAR image through accurate geometric parameters in the Range-Doppler model by ground control points (GCPs).However, it is very difficult to obtain GCPs that covering large-scale areas, especially in the mountainous regions.In addition, the traditional calibration method is only used for single platform SAR images and can't support the hybrid geometric calibration for multi-platform images.To solve the above problems, a hybrid geometric calibration method for multi-platform spaceborne SAR images with sparse GCPs is proposed in this paper.First, we calibrate the master image that contains GCPs.Secondly, the point tracking algorithm is used to obtain the tie points (TPs) between the master and slave images.Finally, we calibrate the slave images using TPs as the control points.We take the Beijing-Tianjin-Hebei region as an example to study SAR image hybrid geometric calibration method using 3 TerraSAR-X images, 3 TanDEM-X images and 5 GF-3 images covering more than 235 kilometers in the north-south direction.Geometric calibration of all images is completed using only 5 GCPs.The GPS data extracted from GNSS receiver are used to assess the plane accuracy after calibration.The results after geometric calibration with sparse GCPs show that the geometric positioning accuracy is 3 m for TSX/TDX images and 7.5 m for GF-3 images.
    A Cloud Detection Method for High Resolution Remote Sensing Imagery Based on the Spectrum and Texture of Objects
    DONG Zhipeng, WANG Mi, LI Deren, WANG Yanli, ZHANG Zhiqi
    2018, 47(7):  996-1006.  doi:10.11947/j.AGCS.2018.20170690
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    To solving the problems that the spectral threshold selection of image cloud detection and the influence of cloud-like ground objects on cloud detection results, a novel cloud detection method for HSRI based onthe spectrum and texture of objects is proposed.Firstly, histogram equalization is performed on the image, and then the appropriate image cloud detection spectral threshold is obtained according to the image equalization histogram.Secondly, the image is segmented to obtain superpixels using the simple linear iterative clustering algorithm.The cloud in the image is initially detected based on cloud detection threshold and spectral attributes of superpixels.Thirdly, the local binary patterns (LBP) texture image of histogram equalization image is obtained.The initial cloud detection image is refined based on the gray mean value and angular second moment of the superpixels LBP texture to eliminate the influence of cloud like objects.Finally, the cloud detection image is processed using region growing algorithm and expansion algorithm to obtain accurate cloud detection results.The experimental results show that the proposed method can obtain good cloud detection results.
    Multiscale Integer Coding and Data Index of 3D Spatial Grid
    LAI Guangling, TONG Xiaochong, DING Lu, QIN Zhiyuan
    2018, 47(7):  1007-1017.  doi:10.11947/j.AGCS.2018.20170364
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    This paper proposed a multiscale integer coding and index method available for 3D spatial grid area based on the exiting problems of 3D spatial grid.This method used integer to unify coding the region divided by regular grid, formed a tree structure showed the size relationship and scale variation of grid which embodied the spatial relationships in different scale grid:include, be included, adjacent and so on, and achieved the unified integer coding of multi-scale grid in the end.On this basis, a serious of basic operation methods were also studied like:level operation, coordinate transformation operation between coding and grid, parent-unit query and sub-unit query.The contrast experiment was designed to compare this method with 3D R-tree index of Oracle Spatial.The result showed that, multiscale integer coding of 3D spatial grid was superior to the 3D R-tree of Oracle Spatial in data importing, index establishing and region querying, and the efficiency were enhanced about two times, forty-six times and four times respectively.
    Encoding and Operation for the Planar Aperture 4 Hexagon Grid System
    WANG Rui, BEN Jin, DU Lingyu, ZHOU Jianbin, LI Zhuxin
    2018, 47(7):  1018-1025.  doi:10.11947/j.AGCS.2018.20170374
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    Discrete global grid system is a new data model which supports the fusion processing of multi-source geospatial information.Research into hexagon grid systems that have excellent geometric attributes has raised academic concern.Description of hierarchical relation and design of encoding scheme are research difficulties.According to the characteristics of the planar aperture 4 hexagon grid system, this paper designs an encoding scheme named Hexagon Lattice Quad Tree (HLQT).Code operations are defined, rules of them are generalized and based on these, transformation between 2-dimensional coordinates and addressing codes is implemented.Compared with similar schemes, HLQT overcomes the disadvantages caught by encoding schemes which divide the odd and even levels or mix the vertices and centers for encoding.In addition, operation rules of HLQT are simpler and easier for complementation.Contrast experiments show that the add operation efficiency of HLQT is about 6 times that of PYXIS and about 5 times that of HQBS, the efficiency of the transform algorithm from 2-dimensional coordinates to codes is about 5 times that of HQBS, and the efficiency of the transform algorithm from codes to 2-dimensional coordinates is about 3 times that of HQBS.
    Research on Modeling and Prediction of the Satellite Clock Bias and Performance Evaluation of GNSS Satellite Clocks
    WANG Yupu
    2018, 47(7):  1026-1026.  doi:10.11947/j.AGCS.2018.20170467
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