Traditional artificial neural networks （ANN） such as back-propagation neural networks （BPNN） provide good predictions of length-of-day （LOD）. However, the determination of network topology is difficult and time ...Traditional artificial neural networks （ANN） such as back-propagation neural networks （BPNN） provide good predictions of length-of-day （LOD）. However, the determination of network topology is difficult and time consuming. Therefore, we propose a new type of neural network, extreme learning machine （ELM）, to improve the efficiency of LOD predictions. Earth orientation parameters （EOP） C04 time-series provides daily values from International Earth Rotation and Reference Systems Service （IERS）, which serves as our database. First, the known predictable effects that can be described by functional models-such as the effects of solid earth, ocean tides, or seasonal atmospheric variations--are removed a priori from the C04 time-series. Only the residuals after the subtraction of a priori model from the observed LOD data （i.e., the irregular and quasi-periodic variations） are employed for training and predictions. The predicted LOD is the sum of a prior extrapolation model and the ELM predictions of the residuals. Different input patterns are discussed and compared to optimize the network solution. The prediction results are analyzed and compared with those obtained by other machine learning-based prediction methods, including BPNN, generalization regression neural networks （GRNN）, and adaptive network-based fuzzy inference systems （ANFIS）. It is shown that while achieving similar prediction accuracy, the developed method uses much less training time than other methods. Furthermore, to conduct a direct comparison with the existing prediction tech- niques, the mean-absolute-error （MAE） from the proposed method is compared with that from the EOP prediction comparison campaign （EOP PCC）. The results indicate that the accuracy of the proposed method is comparable with that of the former techniques. The implementation of the proposed method is simple.展开更多
Based on four intra-technique combined solutions in SINEX format, we tried at first to assess the accuracy of each single-technique Earth Orientation Parameters(EOPs) series over a past time interval of at least 15 ye...Based on four intra-technique combined solutions in SINEX format, we tried at first to assess the accuracy of each single-technique Earth Orientation Parameters(EOPs) series over a past time interval of at least 15 years by comparison with IERS EOP 08C04 combined solution as the reference in this paper. The EOPs studied here mainly consist of four elements, i.e. polar motion(XPO, YPO), Universal Time(UT1-UTC) and length-of-day(LOD). We combined these intra-technique EOP series, each of them associated with a given space geodetic technique by taking advantage of the relationship of the first three EOP components and three rotational parameters which carry the orientation of technique-related reference frame with respect to the estimated weekly inter-technique combined reference frame. Results indicated that the discrepancy between the pole coordinates(XPO, YPO) series, extracted from the intra-SLR combined loose-constraint solutions and the IERS EOP 08C04, seemed to be clearly characterized by systematic errors. Although both the XPO and YPO series determined by intra-VLBI combination had no significant characteristic of system error, they had relatively large difference values at some point with respect to the IERS EOP 08C04, which may be limited by the quantity of observation stations. Since the number of GPS stations is on the increase aimed at better global coverage, the accuracy of pole coordinates provided by IGS was superior to that derived from other space-geodetic techniques. As for DORIS XPO and YPO series from intra-DORIS combined minimal-constraint solutions, the discrepancy range of the former with respect to IERS EOP 08C04 was a little smaller than that of the latter. The objective of this study is twofold: on the one hand to analysis individual EOP series derived from the various space-geodetic techniques, on the other hand to present the new inter-technique combined EOP solution consistent with weekly inter-technique combined reference frame.展开更多
介绍了国际测地/天体测量学甚长基线干涉测量服务(International Very Long Baseline Interferometry(VLBI)Service for Geodesy and Astrometry,IVS)组织机构及下属分析中心概况.系统归纳了目前IVS发布的地球定向参数(Earth Orientatio...介绍了国际测地/天体测量学甚长基线干涉测量服务(International Very Long Baseline Interferometry(VLBI)Service for Geodesy and Astrometry,IVS)组织机构及下属分析中心概况.系统归纳了目前IVS发布的地球定向参数(Earth Orientation Parameters,EOP)产品类型及不同观测类型的用途.利用2010—2019年公开发布的观测资料,对IVS不同分析中心的EOP日常监测和服务能力进行了评估.通过构造观测台站所构成的几何体积,分析了EOP精度与测站数量、测站网分布的关系,统计了IVS不同观测类型的EOP解算精度.此外,综合公开发布的美国、欧洲等区域网观测数据,分析了不同地区区域网的常规及加强观测结果与IVS结果的差异.结果表明:EOP的解算精度与观测台站的分布密切相关,IVS常规观测确定的极移分量的外符合精度优于0.2 mas,世界时(Universal Time,UT1)与协调世界时(Coordinated Universal Time,UTC)之差(UT1-UTC)的精度在0.015 ms左右,加强观测的UT1-UTC值与国际自转服务组织(International Earth Rotation Service,IERS)的C04之间存在0.02–0.03 ms的差异.区域观测网的精度受观测网形和基线长度制约,总体劣于IVS观测精度,其中,美国甚长基线干涉阵列(Very Long Baseline Array,VLBA)的常规及加强观测结果与IVS全球观测结果最接近.展开更多
PRLSENTLY there are three kinds of realization of conventional celestial reference frames (CCRFs), i. e. the optical frame, the planetary frame and the extragalactic radio frame. 'Though the CCRFs are high in prec...PRLSENTLY there are three kinds of realization of conventional celestial reference frames (CCRFs), i. e. the optical frame, the planetary frame and the extragalactic radio frame. 'Though the CCRFs are high in precision, due to the difference in (ⅰ) the constants, parameters, formulae and force models used to define the frames; (ⅱ) the methods and techniques展开更多
甚长基线干涉测量(very long baseline interferometry,VLBI)是测量地球定向参数(earth orientation parameters,EOP)的主要空间测地技术之一,中国正在建设名为VLBI全球观测系统(VLBI global observing system,VGOS)的新一代测地VLBI站...甚长基线干涉测量(very long baseline interferometry,VLBI)是测量地球定向参数(earth orientation parameters,EOP)的主要空间测地技术之一,中国正在建设名为VLBI全球观测系统(VLBI global observing system,VGOS)的新一代测地VLBI站,通过国际联测优化站网构型是实现高精度EOP测量的必由之路。以3个中国VGOS站为核心站,通过引入2个国外站构建5站联合观测网,分析评估了不同站网构型的EOP测量能力。针对每个站网构型,通过调整4个约束条件的权重因子批量生成相应的观测纲要,采用蒙特卡洛仿真方法选择最优的观测纲要,评价指标为EOP解算值的可重复性。仿真结果表明,由中国站、南非哈特比站以及澳大利亚霍巴特站组成的网型EOP测量能力最强,相对于中国3站组成的网型,dUT1测量精度提高5.7倍,极移的X、Y分量的测量精度分别提高2.8倍和18.3倍。仿真结果可为后续开展高精度EOP组网观测提供参考依据。展开更多
We elaborate an error budget for the long-term accuracy of IGS（International Global Navigation Satellite System Service） polar motion estimates, concluding that it is probably about 25-30 μas（1-sigma）overall, alt...We elaborate an error budget for the long-term accuracy of IGS（International Global Navigation Satellite System Service） polar motion estimates, concluding that it is probably about 25-30 μas（1-sigma）overall, although it is not possible to quantify possible contributions（mainly annual） that might transfer directly from aliases of subdaily rotational tide errors. The leading sources are biases arising from the need to align daily, observed terrestrial frames, within which the pole coordinates are expressed and which are continuously deforming, to the secular, linear international reference frame. Such biases are largest over spans longer than about a year. Thanks to the very large number of IGS tracking stations, the formal covariance errors are much smaller,around 5 to 10 μas. Large networks also permit the systematic frame-related errors to be more effectively minimized but not eliminated. A number of periodic errors probably also influence polar motion results, mainly at annual, GPS（Global Positioning System） draconitic, and fortnightly periods, but their impact on the overall error budget is unlikely to be significant except possibly for annual tidal aliases. Nevertheless, caution should be exercised in interpreting geophysical excitations near any of the suspect periods.展开更多
基金supported by the West Light Foundation of the Chinese Academy of Sciences
文摘Traditional artificial neural networks （ANN） such as back-propagation neural networks （BPNN） provide good predictions of length-of-day （LOD）. However, the determination of network topology is difficult and time consuming. Therefore, we propose a new type of neural network, extreme learning machine （ELM）, to improve the efficiency of LOD predictions. Earth orientation parameters （EOP） C04 time-series provides daily values from International Earth Rotation and Reference Systems Service （IERS）, which serves as our database. First, the known predictable effects that can be described by functional models-such as the effects of solid earth, ocean tides, or seasonal atmospheric variations--are removed a priori from the C04 time-series. Only the residuals after the subtraction of a priori model from the observed LOD data （i.e., the irregular and quasi-periodic variations） are employed for training and predictions. The predicted LOD is the sum of a prior extrapolation model and the ELM predictions of the residuals. Different input patterns are discussed and compared to optimize the network solution. The prediction results are analyzed and compared with those obtained by other machine learning-based prediction methods, including BPNN, generalization regression neural networks （GRNN）, and adaptive network-based fuzzy inference systems （ANFIS）. It is shown that while achieving similar prediction accuracy, the developed method uses much less training time than other methods. Furthermore, to conduct a direct comparison with the existing prediction tech- niques, the mean-absolute-error （MAE） from the proposed method is compared with that from the EOP prediction comparison campaign （EOP PCC）. The results indicate that the accuracy of the proposed method is comparable with that of the former techniques. The implementation of the proposed method is simple.
基金supported by the National Natural Science Foundation of China (grants 11773058)
文摘Based on four intra-technique combined solutions in SINEX format, we tried at first to assess the accuracy of each single-technique Earth Orientation Parameters(EOPs) series over a past time interval of at least 15 years by comparison with IERS EOP 08C04 combined solution as the reference in this paper. The EOPs studied here mainly consist of four elements, i.e. polar motion(XPO, YPO), Universal Time(UT1-UTC) and length-of-day(LOD). We combined these intra-technique EOP series, each of them associated with a given space geodetic technique by taking advantage of the relationship of the first three EOP components and three rotational parameters which carry the orientation of technique-related reference frame with respect to the estimated weekly inter-technique combined reference frame. Results indicated that the discrepancy between the pole coordinates(XPO, YPO) series, extracted from the intra-SLR combined loose-constraint solutions and the IERS EOP 08C04, seemed to be clearly characterized by systematic errors. Although both the XPO and YPO series determined by intra-VLBI combination had no significant characteristic of system error, they had relatively large difference values at some point with respect to the IERS EOP 08C04, which may be limited by the quantity of observation stations. Since the number of GPS stations is on the increase aimed at better global coverage, the accuracy of pole coordinates provided by IGS was superior to that derived from other space-geodetic techniques. As for DORIS XPO and YPO series from intra-DORIS combined minimal-constraint solutions, the discrepancy range of the former with respect to IERS EOP 08C04 was a little smaller than that of the latter. The objective of this study is twofold: on the one hand to analysis individual EOP series derived from the various space-geodetic techniques, on the other hand to present the new inter-technique combined EOP solution consistent with weekly inter-technique combined reference frame.
文摘介绍了国际测地/天体测量学甚长基线干涉测量服务(International Very Long Baseline Interferometry(VLBI)Service for Geodesy and Astrometry,IVS)组织机构及下属分析中心概况.系统归纳了目前IVS发布的地球定向参数(Earth Orientation Parameters,EOP)产品类型及不同观测类型的用途.利用2010—2019年公开发布的观测资料,对IVS不同分析中心的EOP日常监测和服务能力进行了评估.通过构造观测台站所构成的几何体积,分析了EOP精度与测站数量、测站网分布的关系,统计了IVS不同观测类型的EOP解算精度.此外,综合公开发布的美国、欧洲等区域网观测数据,分析了不同地区区域网的常规及加强观测结果与IVS结果的差异.结果表明:EOP的解算精度与观测台站的分布密切相关,IVS常规观测确定的极移分量的外符合精度优于0.2 mas,世界时(Universal Time,UT1)与协调世界时(Coordinated Universal Time,UTC)之差(UT1-UTC)的精度在0.015 ms左右,加强观测的UT1-UTC值与国际自转服务组织(International Earth Rotation Service,IERS)的C04之间存在0.02–0.03 ms的差异.区域观测网的精度受观测网形和基线长度制约,总体劣于IVS观测精度,其中,美国甚长基线干涉阵列(Very Long Baseline Array,VLBA)的常规及加强观测结果与IVS全球观测结果最接近.
文摘PRLSENTLY there are three kinds of realization of conventional celestial reference frames (CCRFs), i. e. the optical frame, the planetary frame and the extragalactic radio frame. 'Though the CCRFs are high in precision, due to the difference in (ⅰ) the constants, parameters, formulae and force models used to define the frames; (ⅱ) the methods and techniques
文摘甚长基线干涉测量(very long baseline interferometry,VLBI)是测量地球定向参数(earth orientation parameters,EOP)的主要空间测地技术之一,中国正在建设名为VLBI全球观测系统(VLBI global observing system,VGOS)的新一代测地VLBI站,通过国际联测优化站网构型是实现高精度EOP测量的必由之路。以3个中国VGOS站为核心站,通过引入2个国外站构建5站联合观测网,分析评估了不同站网构型的EOP测量能力。针对每个站网构型,通过调整4个约束条件的权重因子批量生成相应的观测纲要,采用蒙特卡洛仿真方法选择最优的观测纲要,评价指标为EOP解算值的可重复性。仿真结果表明,由中国站、南非哈特比站以及澳大利亚霍巴特站组成的网型EOP测量能力最强,相对于中国3站组成的网型,dUT1测量精度提高5.7倍,极移的X、Y分量的测量精度分别提高2.8倍和18.3倍。仿真结果可为后续开展高精度EOP组网观测提供参考依据。
文摘We elaborate an error budget for the long-term accuracy of IGS（International Global Navigation Satellite System Service） polar motion estimates, concluding that it is probably about 25-30 μas（1-sigma）overall, although it is not possible to quantify possible contributions（mainly annual） that might transfer directly from aliases of subdaily rotational tide errors. The leading sources are biases arising from the need to align daily, observed terrestrial frames, within which the pole coordinates are expressed and which are continuously deforming, to the secular, linear international reference frame. Such biases are largest over spans longer than about a year. Thanks to the very large number of IGS tracking stations, the formal covariance errors are much smaller,around 5 to 10 μas. Large networks also permit the systematic frame-related errors to be more effectively minimized but not eliminated. A number of periodic errors probably also influence polar motion results, mainly at annual, GPS（Global Positioning System） draconitic, and fortnightly periods, but their impact on the overall error budget is unlikely to be significant except possibly for annual tidal aliases. Nevertheless, caution should be exercised in interpreting geophysical excitations near any of the suspect periods.
