Time filter

Source Type

Zhang M.,CAS Institute of Remote Sensing | Tang J.,National Ocean Technology Center | Dong Q.,CAS Institute of Remote Sensing | Duan H.,CAS Nanjing Institute of Geography and Limnology | Shen Q.,CAS Institute of Remote Sensing
Optics Express | Year: 2014

We have presented an atmospheric correction algorithm for HJ-1 CCD imagery over Lakes Taihu and Chaohu with highly turbid waters. The Rayleigh scattering radiance (Lr) is calculated using the hyperspectral Lr with a wavelength interval 1nm. The hyperspectral Lr is interpolated from Lr in the central wavelengths of MODIS bands, which are converted from the band response-averaged Lr calculated using the Rayleigh look up tables (LUTs) in SeaDAS6.1. The scattering radiance due to aerosol (La) is interpolated from La at MODIS band 869nm, which is derived from MODIS imagery using a shortwave infrared atmospheric correction scheme. The accuracy of the atmospheric correction algorithm is firstly evaluated by comparing the CCD measured remote sensing reflectance (Rrs) with MODIS measurements, which are validated by the in situ data. The CCD measured Rrs is further validated by the in situ data for a total of 30 observation stations within ± 1h time window of satellite overpass and field measurements. The validation shows the mean relative errors about 0.341, 0.259, 0.293 and 0.803 at blue, green, red and near infrared bands. © 2014 Optical Society of America. Source

Sun X.J.,National Ocean Technology Center | Shi J.,Tianjin Institute of Urban Construction | Yang Y.,Tianjin Institute of Urban Construction
Advanced Materials Research | Year: 2013

Attitude control in three-dimensional space for AUV (autonomous underwater vehicle) with x-shaped fins is complicated but advantageous. Yaw, pitch and roll angles of the vehicle are all associated with deflection angle of each fin while navigating underwater. In this paper, a spatial motion mathematic model of the vehicle is built by using theorem of momentum and angular momentum, and the hydrodynamic forces acting on x-shaped fins and three-blade propeller are investigated to clarify complex principle of the vehicle motion. In addition, the nonlinear dynamics equation which indicates the coupling relationship between attitude angles of vehicle and rotation angles of x-shaped fins is derived by detailed deduction. Moreover, a decoupling controller based on artificial neural networks is developed to address the coupling issue exposed in attitude control. The neural networks based controller periodically calculates and outputs deflection angles of fins according to the attitude angles measured with magnetic compass, thus the vehicle's orientation can be maintained. By on-line training, twenty four weights in this controller converged according to index function. © (2013) Trans Tech Publications, Switzerland. Source

Zhao Y.,Ocean University of China | Zhao Y.,National Ocean Technology Center
Chinese Journal of Oceanology and Limnology | Year: 2013

WindSat/Coriolis is the first satellite-borne polarimetric microwave radiometer, which aims to improve the potential of polarimetric microwave radiometry for measuring sea surface wind vectors from space. In this paper, a wind vector retrieval algorithm based on a novel and simple forward model was developed for WindSat. The retrieval algorithm of sea surface wind speed was developed using multiple linear regression based on the simulation dataset of the novel forward model. Sea surface wind directions that minimize the difference between simulated and measured values of the third and fourth Stokes parameters were found using maximum likelihood estimation, by which a group of ambiguous wind directions was obtained. A median filter was then used to remove ambiguity of wind direction. Evaluated with sea surface wind speed and direction data from the U. S. National Data Buoy Center (NDBC), root mean square errors are 1. 2 m/s and 30° for retrieved wind speed and wind direction, respectively. The evaluation results suggest that the simple forward model and the retrieval algorithm are practicable for near-real time applications, without reducing accuracy. © 2013 Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag Berlin Heidelberg. Source

Zhang M.,Chinese Academy of Sciences | Tang J.,National Ocean Technology Center | Dong Q.,Chinese Academy of Sciences | Song Q.,State Oceanic Administration
IEEE Transactions on Geoscience and Remote Sensing | Year: 2013

A forward Monte Carlo 3-D (FMC3D) model is developed for simulating light fields in a volume of water where the boundary conditions for radiance values can be expressed by mathematical formulas, which cannot be done using the radiative transfer models currently available such as HydroLight. Water is assumed parallel homogenous for these models, which are incapable of investigating the sidewall reflectance effect on the light fields. These ones are called quasi-3-D radiative transfer models. The FMC3D model perfects the assumption and the incapability and is validated using the in situ data measured in the tank experiment. The FMC3D model is first applied to investigate the sidewall reflectance effect on the remote sensing reflectance R\rm rs for waters in a fabricated tank with infinite depth and different radii. The investigation shows that the effect is decreasing with the increase in the tank radius and that the minimum radius that the effect is negligible for highly scattering water is bigger than that for highly absorbing water. Taking the tank used in the experiment carried out in a previous work by Han and Rundquist as an example, the FMC3D model is second applied to investigate the combining effects on Rrs from bottom and sidewall reflectances. Compared with Rrs for open water, the Rrs for tank water having the same inherent optical properties is underestimated. The underestimation is increasing with the increase in the single scattering albedo ω and can be up to 32% for water with ω = 0.88, showing that the effects cannot be removed by the black inside wall, which is a method commonly used in tank experiments. The potential applications of the FMC3D model are discussed, taking the examples of the correction for the wall reflectance effect on apparent spectra measured in tank experiments and of the scattering error correction for the reflective tube absorption coefficient measured using a WET Labs AC-9 or AC-S device. © 2012 IEEE. Source

Li Z.,Hubei Engineering University | Zhou H.,CAS Wuhan Institute of Rock and Soil Mechanics | Song Y.-Z.,National Ocean Technology Center | Zhang C.-Q.,CAS Wuhan Institute of Rock and Soil Mechanics | Hu Q.-Z.,Hubei Engineering University
Yantu Lixue/Rock and Soil Mechanics | Year: 2013

Chlorite schist is a kind of soft rocks, and its large deformation and collapse due to excavation are greatly harmful to engineering safety. In order to investigate the mechanical property of chlorite schist, triaxial compression test is conducted and a new internal variable is defined considering the influence of stress state on plasticity evolution. According to test results, evolution characteristics of cohesion and friction angle are that the cohesion decreases firstly approximately linearly and then parabolically, but friction angle always decreases approximately parabolically. Meanwhile, based on test data, evolution of dilatancy angle is opposite to that of friction angle. Finally, mechanical model of chlorite schist is proposed considering hardening-softening and dilatancy characteristics; and then it is applied to simulate the triaxial test; the result shows a good agreement with the test data. Effort of this paper offers a rational mechanical model for engineering safety estimation and an important reference for the mechanical investigation of other similar soft rocks. Source

Discover hidden collaborations