Table of Content

20 July 2017, Volume 46 Issue 7

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Precision Analysis of Chinese VLBI Network Software Correlator for Geodetic Applications

LIU Lei, ZHENG Weimin, ZHANG Juan, SHU Fengchun, TONG Fengxian, TONG Li

2017, 46(7):  805-814.  doi:10.11947/j.AGCS.2017.20160600

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To support international VLBI geodetic data processing, Chinese VLBI Network (CVN) software correlator has been updated for better performance and more features, including geodetic Mk4 data format output and improvement of signal to noise ratios. After updating, the visibility output of the software correlator can be sent to the standard VLBI geodetic postprocessing software to derive observables. To verify the correlation result, we have carried out correlation with both CVN software correlator and the popular DiFX correlator by using the same data set. We have compared the derived multi-band residual delay/rate, total delay/rate, signal to noise ratio, and the solved VLBI site positions. The comparison results demonstrate that the CVN software correlator has achieved the required precision, and therefore it can be used for IVS geodetic data processing.

Evaluation of the Geopotential Value for the Local Vertical Datum of China Using GRACE/GOCE GGMs and GPS/Leveling Data

HE Lin, LI Jiancheng, CHU Yonghai

2017, 46(7):  815-823.  doi:10.11947/j.AGCS.2017.20160643

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The main purpose of this paper is to estimate the zero height geopotential value for the China Local Vertical Datum. The method used is based on a gravimetric approach and benefits from the significant improvements in the determination of the global gravity field by the recent satellite gravity missions such as GRACE and GOCE. The performance of GOCE-based or GRACE/GOCE-based Global Geopotential Models (GGMs) is assessed for estimation of geopotential of the regional vertical datum of China using the GPS/Levelling BMs (benchmarks) from vertical control network. We should conver the normal height to orthometric height. From the evaluation with the GPS/Levelling BMs, it is concluded that the GOCE-based or GRACE/GOCE-based GGMs provide an absolute accuracy at of 40 cm, up to their maximum degree and order. Factors that affect the geopotential value of vertical datum include the GOCE commission and omission errors. Among these factors, the effect of the GOCE omission error is investigated by extending the models with the high resolution gravity field model EGM2008. When extending the GOCE-based or GRACE/GOCE-based models with the high resolution gravity field model EGM2008, we should provide the same parameters. At the same time we should unify the frame and tide system between the GGMs and GPS/Levelling dataset. The result shows that, the effect of the GOCE omission error is at the level of 1.6 m²s^-2 (about 16 cm), in China, is at decimetre-level when computing local vertical datum geopotential, the effect of the tide system is about 4-6 cm. The vertical datum geopotential of China is estimated as 62 636 852.704 5 m²s^-2.

Results and Analysis of BDS Precise Orbit Determination with the Enhancement of Fengyun-3C

ZENG Tian, SUI Lifen, JIA Xiaolin, JI Guofeng, ZHANG Qinghua

2017, 46(7):  824-833.  doi:10.11947/j.AGCS.2017.20170033

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Global navigation satellite system occultation sounder (GNOS) Fengyun-3C was launched successfully on September 23, 2013, which carried GPS/BDS receiver for the first time. This provides the convenience to study the enhancement results of low earth orbiter satellite (LEO) to BDS precise orbit determination (POD). First the data characteristics and code observation noise of GNOS are analyzed. Then the enhancement experiments in the case of global and regional ground observation stations layout are processed with four POD schemes: BDS single system, GPS/BDS double system, BDS single system with GNOS observations, GPS/BDS double system with GNOS observations. The precision of BDS orbits and clock are compared via overlapping arcs. Results show that in the case of global station layout the along directional precision of GEO satellite has the biggest improvement, with the improvement percentage 60%. Then the precision of cross direction and the along direction of remaining satellites shows the second biggest improvement. The orbit precision of only BDS POD in part of arcs some satellite even suffers a slight decline. The root mean square (RMS) of overlapping clock difference of visible arcs in GPS/BDS POD experiments is improved 0.1 ns level. As to the experiments of regional station layout with 7 ground observation stations, the orbit and clock overlapping precision and orbit predicting precision are analyzed. Results show that the predicting precision of BDS GEO satellites in the along direction is improved 85%. The remaining also has a substantial improvement, with the average percentage 21.7%. RMS of overlapping clock difference of visible arcs is improved 0.5 ns level.

An Algorithm in Adjustment Model with Uncertainty

WANG Zhizhong, CHEN Danhua, SONG Yingchun

2017, 46(7):  834-840.  doi:10.11947/j.AGCS.2017.20160522

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The uncertainty of observation often affects the validity of parameter estimation, and the effects of uncertainty can be reduced effectively by incorporating uncertainty into the adjustment model as an observation error parameter. An adjustment criterion is proposed under the bound constrain of uncertainty, in which the sum of squares of random error and uncertainty error should be minimized, and provided an iteration algorithm to solve the adjustment model. With simulation examples, the estimation results of uncertainty least-square method are compared with that of total least-square method. The results show that the estimation results of uncertainty least-square method are better than that of total least-square method to a certain extent and more applicable when uncertainty is greater.

Correction Model of BeiDou Code Systematic Multipath Errors and Its Impacts on Single-frequency PPP

WANG Jie, HE Xiyang

2017, 46(7):  841-847.  doi:10.11947/j.AGCS.2017.20170020

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There are systematic multipath errors on BeiDou code measurements, which are range from several decimeters to larger than 1 meter. They can be divided into two categories, which are systematic variances in IGSO/MEO code measurement and in GEO code measurement. In this contribution, a methodology of correcting BeiDou GEO code multipath is proposed base on Kalman filter algorithm. The standard deviation of GEO MP Series decreases about 10%~16% after correction. The weight of code in single-frequency PPP is great, therefore, code systematic multipath errors have impact on single-frequency PPP. Our analysis indicate that about 1 m bias will be caused by these systematic errors. Then, we evaluated the improvement of single-frequency PPP accuracy after code multipath correction. The systematic errors of GEO code measurements are corrected by applying our proposed Kalman filter method. The systematic errors of IGSO and MEO code measurements are corrected by applying elevation-dependent model proposed by Wanninger and Beer. Ten days observations of four MGEX (Multi-GNSS Experiment) stations are processed. The results indicate that the single-frequency PPP accuracy can be improved remarkably by applying code multipath correction. The accuracy in up direction can be improved by 65% after IGSO and MEO code multipath correction. By applying GEO code multipath correction, the accuracy in up direction can be further improved by 15%.

A Fast Parallel Processing Strategy of Double Difference Model for GNSS Huge Networks

CUI Yang, LÜ Zhiping, LI Linyang, CHEN Zhengsheng, SUN Dashuang, KUANG Yingcai

2017, 46(7):  848-856.  doi:10.11947/j.AGCS.2017.20160585

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In view of the existing problem of poor timeliness for larger GNSS network data processed by the double-difference (DD) model, a new approach for the independent DD observations selection and integrated processing of huge GNSS networks is presented by using the parallel algorithm design and parallel programming technology in this paper. The new approach is implemented by using the parallel computing strategy of multi-core parallel level and network parallel level. Validation experiment with GPS observation data collected from about 375 IGS stations was carried out, which demonstrated that the new approach has an average about 530,000 more independent DD ambiguities than the traditional method. The average ENU(East North Vertical) positioning precision of the new approach was improved by 14.0%、12.9% and 29.2%. In the whole network processing of 375 stations, the efficiency of parallel solution with four quad-core nodes is increased about 14 times than the traditional serial computing scheme, significantly improves the efficiency of the entire network processing with DD model.

Non-negative Least Squares Variance Component Estimation of Partial EIV Model

WANG Leyang, WEN Guisen

2017, 46(7):  857-865.  doi:10.11947/j.AGCS.2017.20160501

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As an extended form of the errors-in-variables (EIV) model, and the weight matrix is easy to structure, the applicability is stronger when both non-random elements and random elements exist in the coefficient matrix. According to the inaccurate stochastic model in the Partial EIV model, the coefficient matrix and observation vector are used as a kind of data respectively. The least squares variance component estimation method based on Partial EIV model is used and the relevant calculated formulas and iterative algorithm are derived. Then the corresponded variance components are estimated. The non-negative least squares is used when the negative variance appears, then add constraint condition to correct the rand model, so the estimated parameters are more reasonable. The experiments show that the results obtained by the method of this paper and other methods are equivalent.

Footprint Location Prediction Method of ZY3-02 Altimeter

TANG Xinming, XIE Junfeng, MO Fan, ZHU Guangbin, DOU Xianhui, ZHANG Qiang, LI Guoyuan, OUYANG Sida

2017, 46(7):  866-873.  doi:10.11947/j.AGCS.2017.20160639

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On-orbit geometric calibration is an essential way to improve plane and elevation accuracy of laser altimeter data, and the laser footprint location prediction is a prerequisite for calibration based on land infrared detector. This paper builds a laser footprint location prediction model for China's first space-borne laser altimeter carried by ZY3-02, which refers to the satellite optical camera rigorous geometry imaging model. The model takes full account of the law of platform movement, and correlates laser emission center with ground footprint point. We get the precise predicted laser pointing by Pyramid terrain matching, ephemerisby acceleration prediction, and attitude by frequency analysis. The laser footprint location can be predicted. This laser footprint location prediction model were successfully applied in the calibration test of the laser altimeter on ZY3-02 satellite, and the maximum error between the predicted location and real footprint location obtained by triggered detectors is less than 150 m, which proves the validity of the proposed model. The proposed method provides the precise point-to-point prediction from satellite to ground for Chinese remote sensing satellite, and offers a technological support for the space-borne laser altimeter calibration in future.

Block-adjustment without GCPs for Large-scale Regions only Based on the Virtual Control Points

YANG Bo, WANG Mi, PI Yingdong

2017, 46(7):  874-881.  doi:10.11947/j.AGCS.2017.20160588

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Mapping from optical satellite images without ground control points (GCPs) is one of the goals of photogrammetry. Aiming at the mapping without GCPs, a large-scale block adjustment method based on the virtual control points is proposed in this paper, in which the single image is regarded as an adjustment unit to be organized. To overcome the block distortion caused by unstable adjustment without GCPs and the excessive accumulation of errors, the virtual control points created by the initial RPC model of the images are regarded as the weighted observations and introduced into the adjustment model to refine the adjustment. To verify the availability and the accuracy of the presented method, 26 406 three-linear-array images of ZY-3 covering China are used as the basic data and 8000 uniformly distributed high precision check points are applied to evaluate the geometric accuracy of the DOM (digital ortho model) and DSM (digital surface model) production. The experimental results indicate that the standard deviations of plane and elevation are both better than 4 m and the mosaic accuracy of neighboring DOM is within better than 1 pixel, which could meet the demand of seamless mosaic.

Comparison of the Cross-calibration Methods between Image-based and RTM-BRDF for GF-1 Images

LI Juan, FENG Lian, PANG Xiaoping

2017, 46(7):  882-890.  doi:10.11947/j.AGCS.2017.20160315

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Four wide-field-of-view (WFV) instruments are on board the Gaofen-1 (or GF-1) satellite, providing a combined swath of ~800 km. Before appling to quantitative remote sensing, precision radiometric calibration is needed. Currently, there are two cross-calibration methods. One is the traditional cross-calibration method (image-based), the other is based on radiative transfer model and bidirectional reflectance distribution function(RTM-BRDF). In this study, the two methods were used to cross-calibrate the WFVs of GF-1, and the comparisons were made at the same time. The verification based on satellite data and in situ measurements have shown that, for the two approximately nadir imaging camera WFV2 and WFV3, Image-based method could get higher precision radiometric calibration coefficients, while for two non-nadir imaging cameras WFV1 and WFV4, high precision calibration coefficients would be obtained by the RTM-BRDF method. Finally, the calibration coefficients of GF-1 was derived by the combination of the two methods.

Fusion of Spaceborne Optical and SAR Images for Building Height Quick Extraction in Big Urban Areas

TIAN Feng, LI Hu

2017, 46(7):  891-899.  doi:10.11947/j.AGCS.2017.20160573

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The spaceborne high-resolution optical images and synthetic aperture radar (SAR) images are applied to extract building height in urban areas widely. But the lack of optical satellite parameters along with the SAR images' incomplete scattering characteristics and inefficient extraction make the application flawed. To cure the above problems, we investigated the joint use of the spaceborne high-resolution optical images and SAR images to extract building height information quickly in big urban areas. The chain is decomposed into the main following steps: First, the building shadows are extracted by integrating support vector machines (SVM) with morphological shadow index (MSI) and their lengths are measured automatically. Then, the height extraction from SAR images based on a model matching technique for some appropriate samples. Finally, obtain the other heights based on the simple linear regression analysis. This approach which combines the data and feature from different satellite systems to make up the flaws for each other is not only efficient and low-cost, but also satisfy the basic accuracy requirement.

UAV Image Registration Algorithm Using Color Invariant and AKAZE Feature

LIANG Huanqing, XIE Yi, FU Sizhou

2017, 46(7):  900-909.  doi:10.11947/j.AGCS.2017.20160436

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Image matching based on feature was one of practical methods in UAV image matching. Since the conventional methods of image registration mainly used gray image as input that it could not take color features into account to distinguish the identical point. To address this problem, this paper designed a matching algorithm combined color invariant with AKAZE feature, which overcame the shortcoming of ignoring color information in traditional UAV image matching. Then gray level transformation was utilized to reduce the number of feature points and remain its reliability. Experimental results demonstrate that the proposed method can find the identical point accurately and enhance the efficiency.

Graph Cut Energy Driven Earthquake-damaged Building Detection from High-resolution Remote Sensing Images

LIU Ying, TAO Chao, YAN Pei, ZOU Zhengrong

2017, 46(7):  910-917.  doi:10.11947/j.AGCS.2017.20170035

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In order to make full use of the detail information provided by high-resolution remote sensing images to improve the detection accuracy of damaged buildings during earthquake,combining the shape, edge and corner characteristics of buildings, a novel method based on graph cut frame for damaged building detection is proposed in this paper. Firstly, the local image containing single building used to model the energy function is constructed by digital line graphic data. And the bound terms of the energy function are defined by the location, shape, edge and corner of buildings, respectively. Then, the energy function is minimized through max-flow/min-cut method, and the similarity of the buildings in the pre-and post-event images is measured by the minimum cut energy. Finally, the EM algorithm is exploited to select the classification threshold value of the minimum cut energy, and post-processing is performed according to the misclassification rate estimation to obtain the final detection result. Images taken in Ishinomaki before and after the 2011 off the Pacific coast of Tohoku Earthquake are used in this paper. The experimental results show that the proposed method can effectively detect the damaged buildings.

Refueling Stop Activity Detection and Gas Station Extraction Using Crowdsourcing Vehicle Trajectory Data

YANG Wei, AI Tinghua

2017, 46(7):  918-927.  doi:10.11947/j.AGCS.2017.20160657

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In view of the deficiencies of current surveying methods of gas station, an approach is proposed to extract gas station from vehicle traces. Firstly, the spatial-temporal characteristics of individual and collective refueling behavior of trajectory is analyzed from aspects of movement features and geometric patterns. Secondly, based on Stop/Move model, the velocity sequence linear clustering algorithm is proposed to extract refueling stop tracks. Finally, using the methods including Delaunay triangulation, Fourier shape recognition and semantic constraints to identify and extract gas station. An experiment using 7 days taxi GPS traces in Beijing verified the novel method. The experimental results of 482 gas stations are extracted and the correct rate achieves to 93.1%.

Cognitive Semantic Analysis and Dynamic Generation of Cartographic Symbols

TIAN Jiangpeng, YOU Xiong, JIA Fenli, XIA Qing

2017, 46(7):  928-938.  doi:10.11947/j.AGCS.2017.20160504

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Location-based services (LBS) requires generating real-time personalized and dynamic cartographic symbols. In order to tackle this problem, this paper proposes a structural description model and a dynamic generation method of cartographic symbols based on cognitive analysis. Following the cognitive semantics principle of‘reality-cognition-symbol’, the description model elaborates the mapping mechanism between symbol graphics and symbol semantics, which uses symbol morphemes as atomic units and semantic structure as description framework. The generation method is composed of a context-free grammar model which uses rules to generate cartographic symbols on the basis of morphemes. Through the modelling of graphic morphemes, structural morphemes and rule-based generation system, a grammar compiler for dynamic generation of cartographic symbols is redesigned. Lastly, experiments of dynamic generation of cartographic symbols in different contexts and semantics are performed to verify the feasibility of the proposed method. Therefore, this study leads to a further understanding of cognition and structure principles of cartographic symbols, and also pushes forward dynamic generation of cartographic symbols for related industries and applications.