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Shi C.,Wuhan University | Shi C.,State Key Laboratory of Information Engineering in Surveying | Gu S.,Wuhan University | Lou Y.,Wuhan University | Ge M.,German Research Center for Geosciences
Advances in Space Research | Year: 2012

PPP with low-cost, single-frequency receivers has been receiving increasing interest in recent years because of its large amount of possible users. One crucial issue in single-frequency PPP is the mitigation of ionospheric delays which cannot be removed by combining observations on different frequencies. For this purpose, several approaches have been developed, such as, the approach using ionospheric model corrections with proper weight, the GRAPHIC (Group and Phase Ionosphere Calibration) approach, and the method to model ionospheric delays over a station with a low polynomial or stochastic process. From our investigation on the stochastic characteristics of the ionospheric delay over a station, it cannot be precisely represented by either a deterministic model in the form of a low-order polynomial or a stochastic process for each satellite, because of its strong irregular spatial and temporal variations. Therefore, a novel approach is developed accordingly in which the deterministic representation is further refined by a stochastic process for each satellite with an empirical model for its power density. Furthermore, ionospheric delay corrections from a constructed model using GNSS data are also included as pseudo-observations for a better solution. A large data set collected from about 200 IGS stations over one month in 2010 is processed with the new approach and several commonly adopted approaches for validation. The results show its significant improvements in terms of positioning accuracy and convergence time with a negligible extra processing time, which is also demonstrated by data collected with a low-cost, handheld, single-frequency receiver. © 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.

Chen W.,Wuhan University | Chen W.,State Key Laboratory of Information Engineering in Surveying | Shen W.,Wuhan University | Shen W.,State Key Laboratory of Information Engineering in Surveying | Shen W.,Key Laboratory of Geospace Environment and Geodesy
Journal of Geophysical Research: Solid Earth | Year: 2010

The Earth's rotation is perturbed by mass redistributions and relative motions within the Earth system, as well as by the torques from both the internal Earth and celestial bodies. The present study aims to establish a theory to incorporate all these factors perturbing the rotation state of the triaxial Earth, just like the traditional rotation theory of the axial-symmetric Earth. First of all, we reestimate the Earth's inertia tensor on the basis of two new gravity models, EIGEN-GL05C and EGM2008. Then we formulate the dynamic equations and obtain their normal modes for an Earth model with a triaxial anelastic mantle, a triaxial fluid core, and dissipative oceans. The periods of the Chandler wobble and the free core nutation are successfully recovered, being ∼433 and ∼430 mean solar days, respectively. Further, the Liouville equations and their general solutions for that triaxial nonrigid Earth are deduced. The Liouville equations are characterized by the complex frequency-dependent transfer functions, which incorporate the effects of triaxialities and deformations of both the mantle and the core, as well as the effects of the mantle anelasticity, the equilibrium, and dissipative ocean tides. Complex transfer functions just reflect the fact that decays and phase lags exist in the Earth's response to the periodic forcing. Our theory reduces to the traditional rotation theory of the axial-symmetric Earth when assuming rotational symmetry of the inertia tensor. Finally, the present theory is applied to the case of atmospheric-oceanic excitation. The effective atmospheric-oceanic angular momentum function (AMF) χeff = χeff1 + iχeff2 for the present theory is compared with the AMF χeff sym = χeff1 sym + iχeff2 sym for the traditional theory and the observed AMF χobs = χ1 obs + iχ2 obs; we find that the difference between χeff and χeff sym is of a few milliseconds of arc (mas) and can sometimes exceed 10 mas. In addition, spectrum analyses indicate that χeff is in good agreement with χeff sym and, further, show an increase of coherency with χobs especially in the low-frequency band. The obvious advantage of χeff in the low-frequency band with respect to χeff sym is the critical support of the present theory. However, still better performance of our theory can be expected if the models of the mantle anelasticity and oceanic dynamics were improved. Thus we conclude that the traditional Earth rotation theory should be revised and upgraded to include the effects of the Earth's triaxiality, the mantle anelasticity, and oceanic dynamics. The theory presented in this study might be more appropriate to describe the rotation of the triaxial Earth (or other triaxial celestial bodies such as Mars), though further studies are needed to incorporate the effects of the solid inner core and other possible influences. Copyright 2010 by the American Geophysical Union.

Ding H.,Wuhan University | Shen W.-B.,Wuhan University | Shen W.-B.,State Key Laboratory of Information Engineering in Surveying
Journal of Geophysical Research: Solid Earth | Year: 2013

The inner core translational mode (1S1), known as the Slichter triplet, can provide important information particularly on the Earth's inner core and its dynamics. A time domain spectral analysis method, the optimal sequence estimation (OSE), is devised and applied to search for the Slichter modes. Applications of the OSE and the multistation experiment (MSE) technique for detection of the singlets of 3S1 and 2S1 show that both OSE and MSE can isolate their singlets, but OSE provides a higher signal-to-noise ratio. OSE also can completely isolate the modes 0S2 and 1S2. Using two superconducting gravimeter data sets from nine Global Geodynamic Project stations before and after the 2004 Sumatra earthquake, we search for the possible Slichter triplet via OSE and provide a simple validation using the product spectrum. We set two preliminary criteria in order to identify possible signals; results show three signals as candidates for the Slichter triplet, more likely in the data after the 2004 Sumatra earthquake. While choosing the most reasonable criteria needs further studies, we emphasize here the effectiveness of the OSE in searching for the Slichter modes rather than claiming actual detections. Key Points Optimal sequence estimation (OSE) for search for Slichter modes is put forwardValidity of OSE is confirmed by isolating all singlets of 2S1, 3S1, 0S2 and 1S2Three possible singlets of Slichter modes (m equals -1,0 and +1) were suggested © 2013. American Geophysical Union. All Rights Reserved.

Zhang Y.,Wuhan University | Zhang Z.,Wuhan University | Sun M.,State Key Laboratory of Information Engineering in Surveying | Ke T.,State Key Laboratory of Information Engineering in Surveying
Photogrammetric Engineering and Remote Sensing | Year: 2011

Wall painting plays an important role in the culture relics of the Mogao Caves in Dunhuang, P.R. China. A novel approach of generating a high-resolution orthoimage of the wall painting is proposed. Since the photographic object is nearly flat and also the forward overlap between adjacent images is smaller than 60 percent, the main difficulty to be resolved is the high correlation problem among the unknowns. Improved models of relative orientation and bundle adjustment with virtual constraints have been developed to resolve the high correlation problem. A Voronoi diagram of projective footprints is applied to automatically determine the mosaic lines of ortho-rectified images. The color quality of the generated orthoimage is improved through global minimization of the color differences among overlapped images. The experimental results show that the proposed approach has great potential for conservation of wall paintings with sub-millimeter to millimeter precision. © 2011 American Society for Photogrammetry and Remote Sensing.

Li J.,Wuhan University | Li J.,Chinese Academy of Sciences | Shen W.,Wuhan University | Shen W.,State Key Laboratory of Information Engineering in Surveying
Journal of Earth Science | Year: 2011

Gravity Recovery and Climate Experiment (GRACE) observations have been used to detect the co-seismic and post-seismic gravity field variations due to the Mw=9.3 Sumatra-Andaman earthquake that occurred on December 26, 2004. This article focuses on investigating some gravitational effects caused by this huge earthquake. We computed the geoid height changes, the equivalent water height (EWH) changes, and the gravity changes using the GRACE Level-2 monthly spherical harmonic (SH) solutions released by University of Texas Center for Space Research (UTCSR). The GRACE results agree well with the prediction by a dislocation model and are consistent with the results obtained by some previous scholars. In particular, we calculated the three components of the gravity gradient variations and found that they can recover the seismic-related signature more sensitively due to a certain degree of amplification of the signals. A positive-negative-positive mode predominates in the spatial distribution of the horizontal components of the gravity gradient variations, which is possibly attributed to the anomalies in the crustal density distribution caused by the uplift-subduction effect of the dip-slip earthquake. Moreover, the latitude components of the gravity gradient changes show strong suppression of the north-south stripes, which is due to the along-orbit measurements of the two GRACE satellites. We conclude that the positive-negative-positive mode in latitude gravity gradient changes would be a more sensitive feature to detect the deformations of some major dip-slip earthquakes by GRACE data. © 2011 China University of Geosciences and Springer-Verlag Berlin Heidelberg.

Zhang Y.,Wuhan University | Zhang Y.,State Key Laboratory of Information Engineering in Surveying | Hu B.,Wuhan University | Zhang J.,Wuhan University | Zhang J.,State Key Laboratory of Information Engineering in Surveying
ISPRS Journal of Photogrammetry and Remote Sensing | Year: 2011

In digital photogrammetry, corresponding points have been widely used as the basic source of information to determine the relative orientation parameters among adjacent images. Sometimes, though, the conventional relative orientation process cannot be precisely implemented due to the accumulation of random errors or in the case of inadequate corresponding points. A new relative orientation approach with multiple types of corresponding features, including points, straight lines, and circular curves, is proposed in this paper. The origin of the model coordinate system is set at the projection center of the first image of a strip, and all of the exterior orientation parameters, except and ω of the first image, are set at zero. The basic models of relative orientation with corresponding points, straight lines, and circular curves are discussed, and the general form of a least squares adjustment model for relative orientation based on multi-features is established. Our experimental results show that the proposed approach is feasible and can achieve more reliable relative orientation results than the conventional approach based on corresponding points only. © 2011 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS).

Ding H.,Wuhan University | Shen W.,Wuhan University | Shen W.,State Key Laboratory of Information Engineering in Surveying
Annals of Geophysics | Year: 2013

Based upon SG (superconducting gravimeter) records, the autoregressive method proposed by Chao and Gilbert [1980] is used to determine the frequencies of the singlets of seven spheroidal modes (0S2, 2S1, 0S3, 0S4, 1S2, 0S0, and 3S1) and the degenerate frequencies of three toroidal modes (0T2, 0T3, and 0T4) below 1 mHz after two recent huge earthquakes, the 2010 Mw8.8 Maule earthquake and the 2011 Mw9.1 Tohoku earthquake. The corresponding quality factors Qs are also determined for those modes, of which the Qs of the five singlets of 1S2 and the five singlets (m=0, m=±2, and m=±3) of 0S4 are estimated for the first time using the SG observations. The singlet m=0 of 3S1 is clearly observed from the power spectra of the SG time series without using other special spectral analysis methods or special time series of polar station records. In addition, the splitting width ratio R of 3S1 is 0.99, and consequently we conclude that 3S1 is normally split. The frequencies and Qs of the modes below 1mHz can contribute to refining the 3D density and attenuation models of the Earth. © 2013 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.

Shen W.-B.,Wuhan University | Shen W.-B.,State Key Laboratory of Information Engineering in Surveying | Ding H.,Wuhan University
Geophysical Journal International | Year: 2014

Superconducting gravimeter (SG) records after the 2004 Sumatra Mw 9.0, 2010 Maule Mw 8.8 and 2011 Tohoku Mw 9.1 earthquakes are selected to observe the singlets of six spheroidal normal mode multiplets below 1 mHz (0S2, 2S1, 0S3, 0S4, 1S2 and 3S1). To clearly observe their spectral splitting, the ensemble empirical mode decomposition (EEMD) is applied to SG records as a dyadic filter bank. Comparisons of the product spectra obtained from the direct residual gravity records and those obtained after using EEMD clearly confirm the validity of EEMD. After using EEMD, all of the singlets of the six multiplets, particularly those of 0S4 and 1S2, are completely observed with high signal-to-noise ratio, whereas some of singlets could not be well resolved without the application of EEMD. This study demonstrates that EEMD may be important in the detection of the splitting of some weak and low-frequency seismic modes. The relevant observation results may improve the Earth's density models. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Wu L.,Zhongnan University of Economics and Law | Xu X.,State Key Laboratory of Information Engineering in Surveying | Ye X.,Kent State University | Zhu X.,State Key Laboratory of Information Engineering in Surveying
Cartography and Geographic Information Science | Year: 2014

Repeat and near-repeat phenomena indicate that victimization might form a contagion-like pattern in both near space and near time. The research community has built abundant empirical evidence of repeat and near-repeat patterns over the past 10 years. Along with these pattern analyses, scholars have also proposed theoretical explanation of the mechanisms of near-repeat burglaries. However, the empirical study of mechanisms is very limited and almost absent in China due to the data unavailability. Utilizing a recent burglary dataset in Wuhan (a large Chinese city), the current research examines repeat and near-repeat burglary patterns as well as the underlying theoretical hypotheses. Consistent with prior findings, the current study found significant and meaningful near-repeat patterns of burglary. In addition, the police detection data indicate that pairs of detected burglaries occurring close in space and time are more likely to involve the same offender than distant pairs. Finally, the implications of the findings and limitations are discussed. © 2014 © 2014 Cartography and Geographic Information Society.

Li F.,State Key Laboratory of Information Engineering in Surveying | Zhang G.,State Key Laboratory of Information Engineering in Surveying
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2015

Large Aperture Static Interference Imaging Spectrometer is a new type of spectrometer with light structure, high spectral linearity, high luminous flux and wide spectral range, etc,which overcomes the contradiction between high flux and high stability so that enables important values in science studies and applications. However, there're different error laws in imaging process of LASIS due to its different imaging style from traditional imaging spectrometers, correspondingly, its data processing is complicated. In order to improve accuracy of spectrum detection and serve for quantitative analysis and monitoring of topographical surface feature, the error law of LASIS imaging is supposed to be learned. In this paper, the LASIS errors are classified as interferogram error, radiometric correction error and spectral inversion error, and each type of error is analyzed and studied. Finally, a case study of Yaogan-14 is proposed, in which the interferogram error of LASIS by time and space combined modulation is mainly experimented and analyzed, as well as the errors from process of radiometric correction and spectral inversion. © 2015 SPIE.

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