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The first Sentinel-1 SAR image of a typhoon

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Acta Oceanologica Sinica 2015年第1期34卷 1-2页

作者： li xiaofeng GST National Oceanic and Atmospheric Administration(NOAA)/NESDISCollege ParkMaryland 20740USA

In this note, we present the first Sentinel-1 synthetic aperture radar （SAR） typhoon image acquired in the northwest Pacific on October 4, 2014. The eye shape and sea surface wind patterns associated with Typhoon Phanfone are clearly shown in the high-quality SAR image. SAR winds retrieval procedure was given but the actual wind estimates will only be available after the European Space Agency （ESA） releases the official calibration coefficients in order to accurately derive the SAR-measured normalized radar cross section. This study demonstrates the advantage of Sentinel- 1 SAR with regards to imaging fine scale typhoon patterns on the sea surface beneath storm clouds. This paper also advocates the use of Sentinel-1 SAR data that is made freely and openly available worldwide for the first time in civilian SAR history.

关键词： SAR Sentinel-1 MODIS typhoon sea surface wind

Unveiling three-dimensional sea surface signatures caused by internal solitary waves:insights from the surface water ocean topography mission

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Journal of Oceanology and limnology 2024年第3期42卷 709-714页

作者： Xudong ZHANG xiaofeng li Key Laboratory of Ocean Observation and Forecasting Key Laboratory of Ocean Circulation and WavesInstitute of OceanologyChinese Academy of SciencesQingdao 266071China

Internal solitary waves(ISW),characterized by large amplitude and long propagation distance,are widespread in global *** remote sensing images have played an essential role in studying ISWs,they mainly exploit two-dimensional image ***,with the launch of the surface water ocean topography(SWOT)satellite on December 16,2022,a unique opportunity has emerged to capture wide-swath three-dimensional ISW-induced sea surface *** this study,we examine ISWs in the Andaman Sea using data from the Ka-band Radar Interferometer(KaRIN),a crucial sensor onboard *** not only provides backscattering satellite images but also employs synthetic aperture interferometry techniques to retrieve wide-swath two-dimensional sea surface height *** observations in the Andaman Sea revealed the presence of ISWs characterized by dark-bright strips and surface elevation *** surface soliton has an amplitude of 0.32 m,resulting in an estimation of ISW amplitude of approximately 60 *** contrast to traditional two-dimensional satellite images or nadir-looking altimetry data,the SWOT mission’s capability to capture threedimensional sea surface information represents a significant *** breakthrough holds substantial promise for ISW studies,particularly in the context of ISW amplitude inversion.

关键词： internal solitary wave(ISW) surface water ocean topography(SWOT) altimeter

Data-Based Optimal Tracking of Autonomous Nonlinear Switching Systems

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IEEE/CAA Journal of Automatica Sinica 2021年第1期8卷 227-238页

作者： xiaofeng li Lu Dong Changyin Sun the School of Automation Southeast UniversityNanjing 210096 the Key Laboratory of Measurement and Control of Complex Systems of Engineering Ministry of EducationSoutheast UniversityNanjing 210096China the College of Electronics and Information Engineering Tongji UniversityShanghai 201804China

In this paper,a data-based scheme is proposed to solve the optimal tracking problem of autonomous nonlinear switching *** system state is forced to track the reference signal by minimizing the performance ***,the problem is transformed to solve the corresponding Bellman optimality equation in terms of the Q-function(also named as action value function).Then,an iterative algorithm based on adaptive dynamic programming(ADP)is developed to find the optimal solution which is totally based on sampled *** linear-in-parameter(liP)neural network is taken as the value function *** the presence of approximation error at each iteration step,the generated approximated value function sequence is proved to be boundedness around the exact optimal solution under some verifiable ***,the effect that the learning process will be terminated after a finite number of iterations is investigated in this paper.A sufficient condition for asymptotically stability of the tracking error is ***,the effectiveness of the algorithm is demonstrated with three simulation examples.

关键词： Adaptive dynamic programming approximation error data-based control Q-learning switching system

Hybrid Q-learning for data-based optimal control of non-linear switching system

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Journal of Systems Engineering and Electronics 2022年第5期33卷 1186-1194页

作者： li xiaofeng DONG Lu SUN Changyin School of Automation Southeast UniversityNanjing 210096China School of Artificial Intelligence Anhui UniversityHefei 230601China School of Cyber Science and Engineering Southeast UniversityNanjing 211189China

In this paper,the optimal control of non-linear switching system is investigated without knowing the system ***,the Hamilton-Jacobi-Bellman(HJB)equation is derived with the consideration of hybrid action ***,a novel data-based hybrid Q-learning(HQL)algorithm is proposed to find the optimal solution in an iterative *** addition,the theoretical analysis is provided to illustrate the convergence and optimality of the proposed ***,the algorithm is implemented with the actor-critic(AC)structure,and two linear-in-parameter neural networks are utilized to approximate the *** results validate the effectiveness of the data-driven method.

关键词： switching system hybrid action space optimal control reinforcement learning hybrid Q-learning(HQL)

A state-of-the-art review of mechanical characteristics and cracking processes of pre-cracked rocks under quasi-static compression

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Journal of Rock Mechanics and Geotechnical Engineering 2022年第6期14卷 2034-2057页

作者： liwang liu Haibo li xiaofeng li State Key Laboratory of Geomechanics and Geotechnical Engineering Institute of Rock and Soil MechanicsChinese Academy of SciencesWuhan430071China University of Chinese Academy of Sciences Beijing100049China

The mechanical characteristics and failure behavior of rocks containing flaws or discontinuities have received wide attention in the field of rock *** external loads are applied to rock materials,stress-induced cracks would initiate and propagate from the flaws,ultimately leading to the irreversible failure of *** investigate the cracking behavior and the effect of flaw geometries on the mechanical properties of rock materials,a series of samples containing one,two and multiple flaws have been widely investigated in the *** this paper,the experimental results for pre-cracked rocks under quasistatic compression were systematically *** progressive failure process of intact rocks is briefly described to reveal the background for experiments on samples with ***,the nondestructive measurement techniques utilized in experiments,such as acoustic emission(AE),X-ray computed tomography(CT),and digital image correlation(DIC),are *** mechanical characteristics of rocks with different flaw geometries and under different loading conditions,including the geometry of pre-existing flaws,flaw filling condition and confining pressure,are ***,the cracking process is evaluated from the perspective of crack initiation,coalescence,and failure patterns.

关键词： Pre-cracked rock Crack initiation pattern Crack coalescence pattern Failure pattern

Oceanic internal waves generated by the Tongan volcano eruption

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Acta Oceanologica Sinica 2022年第8期41卷 1-4页

作者： Xudong Zhang xiaofeng li Key Laboratory of Ocean Circulation and Waves Institute of OceanologyChinese Academy of SciencesQingdao 266071China Center for Ocean Mega-Science Chinese Academy of SciencesQingdao 266071China

Internal waves(IW)are widely distributed at the marginal seas or continental shelves(liu et al.,2013;Zhao and Alford,2006;Zheng et al.,2007).They have an amplitude of up to hundreds of meters and wave crests of severa... 详细信息

关键词： eruption environments ocean

Coastal upwelling observed by multi-satellite sensors

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Science China Earth Sciences 2009年第7期52卷 1030-1038页

作者： li XiaoMing li xiaofeng HE MingXia Ocean Remote Sensing Institute Ocean University of ChinaQingdao 266003China IMSG at NOAA/NESDIS 5200 Auth RoadWWB/102Camp SpringsMD 20746USA

Coastal upwelling phenomenon along the China coast in the Yellow Sea during August 2007 is studied using ENVISAT Advanced Synthetic Aperture Radar (ASAR) data, NOAA Advanced AVHRR series Sea Surface Temperature (SST) data,and NASA QuikSCAT Scatterometer ocean surface wind data. A dark pattern in an ASAR image is interpreted as coastal upwelling. This is because the natural biogenic slicks associated with coastal upwelling damp the Bragg waves on the sea surface and thus make the surface smoother. Most of the incoming radar energy is reflected in the forward direction. As a result, the radar backscatter signal is very weak. Analyzing the concurrent AVHRR SST image, we find that the dark pattern in the ASAR image is indeed corresponding to the low SST area. The wind retrieval in the slicks dominant region is biased due to the low Normalised Radar Cross Section (NRCS) associated with the coastal upwelling. We applied a SST correction to the NRCS values to improve the accuracy of wind retrieval from ASAR data.

关键词： coastal upwelling SAR AVHRR/SST wind field retrieval

ALGORITHM FOR ACOUSTIC PASSIVE LOCAliZATION WITH DUAL ARRAYS

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Chinese Journal of Mechanical Engineering 2008年第6期21卷 14-17页

作者： JIA Jingjing liU Mingjie li xiaofeng School of Aerospace Science and Engineering Beijing Institute of Technology Beijing 100081 China

Aiming at the problem of 3D target localization by time delay estimation, this paper proposes a new acoustic passive localization method, which can provide high precision localization estimation. The first step of the two-stage algorithm is to measure the azimuth angle and pitch angle at each single array, which can obtain high precision angle estimation but low precision range estimation. And in the second step, the location of acoustic source is calculated from the angles measured above and geometry position of the two arrays. Then the accuracy of localization estimation is discussed in theory, and the influence factors and localization error are analyzed by simulation. The simulation results validate the performance of the proposed algorithm, and show the precision of localization estimation with dual arrays is superior to single array.

关键词： Acoustic passive localization Dual arrays Target tracking

Characteristics and generations of internal wave in the Sulu Sea inferred from optical satellite images

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Journal of Oceanology and limnology 2020年第5期38卷 1435-1444页

作者： ZHANG Xudong li xiaofeng ZHANG Tao CAS Key Laboratory of Ocean Circulation and Waves Institute of OceanologyChinese Academy of Sciences and Center for Ocean Mega-ScienceChinese Academy of SciencesQingdao 266071China College of Geomatics Shandong University of Science and TechnologyQingdao 266071China

Characteristics and generation of internal waves(IWs)in the Sulu Sea are studied using Moderate-Resolution Imaging Spectroradiometer(MODIS)and Visible Infrared Imaging Radiometer Suite(VIIRS)images taken from October 2016 to September *** observations show that IWs in the Sulu Sea mainly located in the shallower western areas with occasional observations in the deeper eastern *** dominant length of wave crest(LWC)of IWs is between 50 and 150 km with the largest LWC reaching over 300 *** analysis of temporal distributions of IWs shows that March has the most IWs and July has the *** analysis shows that the seasonal variation is mainly due to the cloud contamination of optical satellite *** generation sites of IWs are analyzed using satellite *** possible generation sites for IWs in the western Sulu Sea and one generation site for IWs in the eastern Sulu Sea are found using the ray-tracing *** IW sources in the same strait are found,which may be due to the seawater fl ow over the strait in diff erent *** analysis shows that IWs with long wave crest in the Sulu Sea is a combined eff ort of all straits between small islands in the Sulu *** generated IWs with long wave crest in the eastern Sulu Sea are studied,which are generated at the straits around(121.5°E,6°N)by the nonlinear evolution of internal tide originated from the Sulu Archipelago.

关键词： internal waves(IWs) Sulu Sea Moderate-Resolution Imaging Spectroradiometer(MODIS) Suomi National Polar-Orbiting Partnership(NPP)