最新刊期
卷 18 ， 期 5 ， 2014
- 摘要：China successfully landed Chang’e-3（ CE-3） lander and rover on December 14,2013,and the rover Yutu had traversed 114. 8 m and accomplished a series of scientific investigations under teleoperation. To fulfill teleoperation requirements,the techniques of remote sensing mapping and localization based on images have been developed and used to produce high precision topographic and rover localization products. These techniques and derived products are critical to support teleoperation tasks such as obstacle avoidance and rover path planning.Based on consecutive image registration between CE-3 descent images and Chang’e-2（ CE-2） Digital Ortho Map（ DOM）（ 1.5 m/pixel）,the CE-3 lander has been localized on high-resolution lunar orbital and descent images. A sequence of descent i mages are firstly selected at intervals from 2 km altitude,and then the geometric relationships among the selected sequence images are established through SIFT matching. Lunar craters are subsequently extracted by image segmentation with mean-shift method and then applied to registration between CE-2 DOM and the descent images. The landing point of CE-3 is finally localized in CE-2 DOM and a Lunar Reconnaissance Orbiter（ LRO） image（ 1. 6 m /pixel） acquired before CE-3 landing.High-precision terrain products covering landing area of CE-3 have also been produced using the descent sequence images.First,image matching among the selected sequence images is performed with SIFT matching,during which gross errors are eliminated by an RANSAC algorithm. Second,the remaining matched points are used as tie points to construct an image network,and bundle adjustment is then performed with ground control points from the CE-2 Digital Elevation Model（ DEM） and DOM. Third,dense matching among the sequence images is performed to generate dense matched points,and the three-dimensional coordinates of the dense points is subsequently calculated with space intersection. Finally,the DEM is generated by Kriging interpolation and DOM is generated by back projection.Along the traverse,Yutu Rover acquired stereo images covering 120° field of view using its navigation cameras（ Navcam） at waypoints that are about 10 m apart. Epipolar images of the Navcam stereo images are generated and SIFT-based matching is p erformed among the Navcam images to extract tie points. Block adjustment is then performed to refine the exterior orientation p arameters of the Navcam images. Finally,the DEMs and DOMs are generated by the method similar to producing the topographic products from the descent images. These topographic products are used to support path planning at every waypoint.High precision localization of Yutu Rover has been accomplished by cross-site visual localization and DOM matching. Crosssite visual localization is based on Affine-SIFT matching algorithm,which obtains matched points in Navcam images between adjacent sites. Then bundle adjustment is applied to resolve the exterior orientation parameters of image at the present site so that to l ocalize the rover. The DOM matching localization is based on SIFT matching between the DOM derived from Navcam stereo i mages and the DOM generated previously from the descent images. As a result,the rover position is localized in CE-2 DOM b ecause the DOM from descent images has been registered to CE-2 DOM.The lander localization result based on consecutive registration of descent images is proved very precise（ within one pixel） by comparison with a LRO image acquired on December 25,2013,in which the lander can be directly identified. High precision localization and mapping results based on the descent images and Navcam stereo images have supported path planning and obstacle a voidance during CE-3 mission. These techniques have been routinely used in mission operations of CE-3 and have been proved e ffective and reliable.关键词：Chang’e-3 mission;remote sensing mapping;rover navigation and localization;teleoperation;descent camera;navigation camera
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发布时间：2021-06-10 - 摘要：Chang’e-3（ CE-3） was successfully soft landed on the Moon on Saturday,December 14,2013. The descent camera onboard lander acquired a large amount of descent images during the landing process. In addition to visualization of the landing process to the public,these images are supportive for pinpointing locations of lander and rover after landing. With a detailed analysis of the geometric characteristics of these sequence descent images,we propose a method to recover the descending and landing trajectory using descent images with bundle adjustment techniques.Descending and landing trajectory can provide critical information of the motion state of lander during safe landing. So far,there is no report of recovery of descending and landing trajectory using descent images for Lunar or Mars landers. In this paper,we study the capability of trajectory recovery using CE-3 descent images. By fully utilizing the information of descent images,we r econstruct the trajectory and the three-dimensional terrain model of the CE-3 landing area. We provide a new approach to obtain-i ng the lander’s states in entry,descending,and landing phases. The derived Digital Terrain Model（ DTM） of landing area is of very high resolution and valuable in mission operations and scientific investigations.The method includes the following steps:（ 1） SIFT-based image matching is performed among the sequence images.（ 2）Match-i ng gross errors are eliminated by a RANSAC algorithm and the remaining matched points are used as tie points to form an image network.（ 3） Bundle adjustment of the image network is performed with control points which were acquired by matching the d escent images with a Digital Ortho Map（ DOM） generated from CE-2 images previously. As the output of bundle adjustment,the descending and landing trajectory is obtained,which includes the positions and attitudes of the lander when the descent images were taken. Based on the recovered trajectory,the lander’s states on entry,descending,and landing can be analyzed. DOM and DEM with much higher resolution than orbiter images have also been generated from the descent images.We obtain 26 control points and 18 check points from 1. 5 m resolution DOM and 4 m resolution DEM. The points are incorporated in the bundle adjustment. The RMS of control points and check points are 0. 60 m and 0. 75 m respectively. Based on the trajectory obtained from bundle adjustment,the lander’s motion state is analyzed,in which the speed curve and acceleration curve can reveal the engine thrust changes. The Digital Surface Model（ DSM） with a resolution of 0. 4 m has been produced and has been used in rover path planning and other mission operations. This method has been proved effective through applications in CE-3 m ission.关键词：Chang’e-3 mission;descent image;trajectory recovery;bundle adjustment;three-dimensional reconstruction
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发布时间：2021-06-10 - 摘要：Chang’e-3（ CE-3） mission,which includes China’s first lunar lander and rover,was successfully soft landed on theMoon on December 14,2013. After landing,the separation of the rover and the lander is a critical step of the mission operation,and is a lso the basis for subsequent explorations by the lander and the rover. As there exist terrain obstacles and communication blackout,it is important to calculate parameters of obstacles and communication envelope,which will be helpful for rover to avoid obstacles and drive smoothly after separation.To satisfy the requirements of fast and reliable decision making for rover-lander separation,we performed monoscopic measurements of the obstacles and calculated the communication envelope based on an image acuired by the monitoring camera.（ 1） We define three coordinate systems including rover body coordinate system,local coordinate system and monitoring camera coordinate system. These coordinate systems can be transformed to each other by matrix operations according to landing attitudes,rotation matrices and translation vectors.（ 2） Through coordinate system transformation,object coordinates can be calculated from image corodinates by using collinearity equations. In particular,sizes of the obstacles（ e. g.,rocks） and distances to the lander are caculated and labeled on the image.（ 3） Projection transformation is applied to obtain the communication envelope.The monoscopic measurement method developed in this paper is verified with a Digital Ortho Map（ DOM） of 2 mm resolution,which is generated by Navcam stereo images. The measurement accuracy is obtained by comparing monoscopic measurement results with ground truth,which is dervied from obstacle size measurement on the Navcam DOM. Experimental results show that the measurement error in object space is less than 10% and can meet the requirements of decision support.These measurements contributed to evaluation of terrain obstacles and communication blackout directly. The proposed method was successfullly used in CE-3 mission to support decision making for rover-lander separation.关键词：Chang’e-3 mission;rover-lander separation;monoscopic measurement;lunar surface obstacle;communication envelope
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发布时间：2021-06-10 - 摘要：China’s Chang’e-3（ CE-3） probe,which includes a lander and a rover,successfully landed on the lunar surface on D ecember 14,2013. High precision and automatic mapping of the lunar surface surrounding the rover is of fundamental importance both for safe rover navigation and for achievement of scientific and engineering goals. During CE-3 mission,we routinely produced topographic maps,and rover traverse maps to support surface operations of the rover.In this research,we focused on mapping capability analysis and fast terrain reconstruction using Navcam（ navigation camera）stereo images acquired by Yutu rover. First,we performed topographic mapping capability analysis of Navcam stereo pairs using basic photogrammetric principles and error propagation law,and empirical formula. The expected measurement error of Navcam stereo images is less than 2 m within 44 m from the rover,and less than 1 m within a range of 31 m. To give a theoretical estimation of the mapping errors at different locations,a covariance matrix is used to depict the position error. The position errors are mainly in the radial directions. Given that the elevation differs in different distances,grid error in DEM is analyzed based on linear interpolation model,the elevation accuracy is higher than the horizontal accuracy.In order to complete terrain mapping,we presented a multi-threaded image processing method for fast generation of topographic products using rover images. The whole mapping process include automatic feature points extraction based on epipolar images,multi-threaded dense matching,three-dimensional compution,DEM interpolation by a parallel processing combined with multithread technique. First,epipolar images were generated using exterior orientation parameters,then bundle adjustment was used to refine the exterior orientation parameters of the rover images. Moreover,a triangulated irregular network-controlled dense image matching method was adopted for dense grid matching. After image dense matching,three-dimensional positions of the matched points were calculated using space intersection with co-linearity equations. The final DEM was generated using Kriging interpolation.To verify the accuracy of our method,experiments were conducted using simulated data. Seven pairs of stereo images were s imulated using resampled 0. 01 m CE-2 DEM and DOM. A new DEM with 0. 02 m resolution is generated from the simulated i mages; and 12 check points are used to verify the accuracy. The RMSEs（ Root Mean Square Errors） of these GCPs are 0. 036 m,0. 051 m and 0. 014 m in three directions. Experimental results using stereo images at waypoint D acquired by Yutu rover also d emonstrated the effectiveness and accuracy of the method.Until January 17,2014,Yutu rover has traveled 114. 8 m on lunar surface and performed a series of scientific investigations.The developed terrain mapping techniques and software have been routinely used in Chang’e-3 mission operations. Topographic products（ DEM and DOM） were generated quickly and at 17 waypoints using Navcam images based on multi-threaded image p rocessing to support mission operations.关键词：Chang’e-3 mission;lunar terrain reconstruction;DEM;accuracy analysis
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发布时间：2021-06-10 - 摘要：The HITRAN database is vital for building a retrieval algorithm of CO2 concentration from satellites. Six versions of HITRAN have been published,and HITRAN2012 is currently available. These HITRAN versions have different molecular spectral parameters. This study applies the line-by-line radiative transfer model to simulate gas optical depths and transmittances at the 1. 6μm channel of TanSat for three recent versions（ 04,08,and 12） of the HITRAN database. The differences of these databases are compared. The optical depth of version HITRAN2004 is larger than that of the other two versions in gas optical depth,leading to an underestimation of CO2 concentration by 38 ppm. Meanwhile,the result of version HITRAN2008 is similar to that of version HITRAN2012,with a maximum deviation of 2 ppm. The sensitivity of CO2 column density and its near-surface value to the different HITRAN versions is also analyzed. The CO2 column density is more sensitive to version HITRAN2004 than to the other two versions. This density shows similar sensitivity to versions HITRAN2008 and HITRAN2012. Minimal differences are also detected between the two convolved radiative signals with the same slit function.关键词：HITRAN;carbon dioxide;satellite inversion;transfer properties
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发布时间：2021-06-10 - 摘要：Uncertainties in validation cause errors in the validation of remote sensing products. Therefore,these uncertainties must be minimized to increase the accuracy of remote sensing validation. This study aims to summarize and analyze three main uncertainty sources in the remote sensing validation of Leaf Area Index（ LAI）. The three sources are ground-based measurement uncertainty,validation model uncertainty,and scale effects. This study also proposes a method to minimize each uncertainty.Field measurement and satellite observation errors can minimize the effect of LAI measurement errors. The uncertainties in the sampling,field measurement,and LAI model are minimized to measure LAI and obtain LAI map.The scale mismatch errors among ground-based LAI,fine-scale LAI map,and coarse-scale LAI map are primarily estimated by three methods to reduce the uncertainties. The Taylor expansion of the transfer function as one of the three methods is emphasized to compute the scale bias,which is a function of intra-pixel spatial heterogeneity and the degree of transfer function non-linearity. The coarse-scale LAI reference map,which can be compared with LAI remote sensing products,is obtained from fine-scale LAI aggregation after minimizing the uncertainties.An obvious scale mismatch can be observed between the ground-based LAI and coarse-scale remote sensing LAI in the case of land surface heterogeneity. A fine-scale LAI map should be used as a bridge between the ground-based LAI and coarse-scale r emote sensing LAI. The error information in field measurement and satellite observation can be used to derive the site-specific r elationship and reduce the uncertainties in the fine-scale LAI reference map. The scaling bias caused by the nonlinearity of the Normalized Difference Vegetation Index（ NDVI） compensates for the scaling bias between LAI and NDVI. This bias can be c orrected through Taylor expansion.The uncertainties in the validation of LAI remote sensing products are significant. The three main uncertainties,namely,ground-based measurement uncertainty,validation model uncertainty,and scale effects,can be minimized to show a rigorous and reliable validation of LAI remote sensing products. For relatively homogeneous land surfaces,the ground-based measurements are enough to represent the sample plot,and the scale effects of the measurements and the model can be ignored. For heterogeneous land surfaces,the scale effects can be ignored if the model is linear. However,the uncertainties caused by the spatial representation for model and ground measurements should be considered. All three types of uncertainties should be considered if the model is non-linear.关键词：Leaf Area Index（LAI）;validation;uncertainty analysis;scale effect;measurement;Model
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发布时间：2021-06-10 - 摘要：The application of gamma mixture model on Synthetic Aperture Radar（ SAR） image segmentation ignores spatial correlation between pixels. To solve this problem,this study proposes a new algorithm using regional gamma mixture model,which i mproves segmentation accuracy and reduces speckle noise effects on SAR images. The special imaging mechanism of SAR images results in severe distractions to speckle noise. The traditional gamma mixture model for SAR image segmentation only uses the gray information of images and ignores the spatial correlation between pixels,leading to segmentation result discontinuity and false segmentation. Watershed segmentation is a common initial segmentation algorithm. The defect of this algorithm is the over-segmentation phenomenon,which becomes more serious as the noise increases. Segmentation results cannot accurately represent the ground object features. Therefore,related algorithms should be used to combine segmentation results. A substantial amount of research has been conducted to solve the above problems. This study proposes a new segmentation algorithm that increases both segmentation accuracy and noise immunity. This algorithm involves two main steps: building of a regional gamma mixture model and designing of a neighborhood factor. SAR images are segmented by the watershed algorithm. The over-segmented results are used as initial clustering objects for the gamma mixture model. However,the watershed algorithm has a serious problem of over-segmentation. The watershed algorithm and gamma mixture model are combined to include regional information into calculation; however,inter-regional linkages remain missing,and segmentation accuracy remains poor. Therefore,considering the correlation between regional blocks in the parameter estimation,we include the neighborhood factor into the expectation-maximization iterative algorithm.The neighborhood factor is the weighted probability of each region belongs to each segmentation class. After several iterations,the factor is updated,and the image is segmented. Synthetic images and real SAR images demonstrate that the new algorithm can achieve more accurate segmentation results than the traditional gamma mixture model,regional Gaussian m ixture model and regional Markov random field algorithm. Upon visual observation,the segmentation results of the new algorithm are clearer and less affected by noise. Moreover,the results of the new algorithm have larger Kappa coefficient and better segmentation accuracy. This study proposes the RGaMM algorithm,which includes regional information and interregional contextual information into calculation. This algorithm can also effectively segment SAR image. In particular,under the effects of heavy speckle noise,the RGaMM algorithm has more advantages than the traditional algorithm. The proposed algorithm not only can a chieve accurate segmentation on SAR images,but also reduce speckle noise effects.关键词：synthetic aperture radar;watershed;gamma mixture model;neighborhood factor;expectation-maximization algorithm
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发布时间：2021-06-10 - 摘要：This study aims to balance the contradiction caused by large satellite cloud image data and the limitations of transmission channels and storage space. We proposed a compressed sensing approach with highly reconstructive quality that utilizes temporal-spatial correlation of adjacent sequence cloud images based on Tetrolet transform. The approach introduces Tetrolet transform into the sparse representation step of compressed sensing for its remarkable representation of important image information,such as directional texture and edge,which can depict the detail and texture structure of satellite cloud images well. Meanwhile,considering the correlation between sequences of the satellite images,three adjacent cloud images are bound to form a Group Of Pictures（ GOP）,in which the intermediate time cloud image is the reference picture,and the difference with its adjacent cloud images is calculated to form the difference images. Finally,GOP is reconstructed by the projected Landweber algorithm with smoothing operation under the compressed sensing framework. Experimental results show that Tetrolet transform is appropriate for the sparse representation of satellite cloud images. The use of the temporal-spatial correlation of GOP can improve the visual quality and quantitative measurements of reconstructed cloud images. When the sampling rate for cloud images of infrared one,water vapor,and visible light channel is lower than 0. 2,the Peak Signal-to-Noise Ratio（ PSNR） of the reconstructed images is improved by an average of 7. 48 dB,13. 51 dB and 6. 15 dB,respectively,compared with that obtained by traditional methods. High-quality cloud image can be reconstructed by obtaining random measurements using the p roposed method. This method provides a feasible solution for the low-bit-rate compression of cloud images and is significant for the compressed sampling of other sequence images.关键词：sparse representation;compressed sensing;Tetrolet transform;temporal-spatial correlation;satellite cloud image
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发布时间：2021-06-10 - 摘要：Estimation of land surface energy balance has become increasingly important in science research and application. R emote sensing has long been identified as a technology capable of monitoring land surface energy,such as Landsat and Moderateresolution imaging spectroradiometer. This study introduced and discussed algorithms that use HJ-1 satellite data to estimate the e nergy budget of land surface including downward shortwave and longwave radiation,surface net radiation,and sensible and latent heat fluxes. Considering the error caused by using radiative surface temperature instead of aerodynamic surface temperature in the one-layer model for sensible heat flux estimation,we compared and analyzed two correction methods,the KB- 1coefficient method and the external resistance method. The surface energy balance components of Haihe River Basin were estimated by HJ-1B satellite data in 2010,and the results were validated by in suit measurements of the three sites. Error analysis indicates that the accuracy of the estimated net radiation is influenced by the uncertainties of input parameters,particularly downward shortwave radiation. The error of the sensible heat flux estimation is mainly caused by the errors in the retrieved surface temperature and roughness length.The results of the KB- 1coefficient method and external resistance method are similar,and the sensible heat flux estimated by the KB- 1coefficient method is usually smaller than that by the other method. Significant errors can be found under complex surfaces,indicating that surface heterogeneity should be considered under such conditions to accurately estimate surface heat fluxes.关键词：HJ-1B satellite;surface energy balance;evapotranspiration;roughness;resistance
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发布时间：2021-06-10 - 摘要：Large terrain rendering is hotspot in Computer Graphics,especially the spherical terrain rendering,which is more complicated in shape and data organization. Based on researching in spherical terrain rendering method,we propose a new spherical terrain rendering method using Geometry Clipmap. First construct an icosahedron model,and then split the icosahedron into ten main diamonds,and use spherical Sphere Quad Trees（ SQT） model to ensure G eometry Clipmap can be used in every diamond,what else,grids around the main diamond are partitioned and c omposed to a bigger virtual diamond terrain in 3 × 3 size,which enlarge clipmaps’ extent and solve the boundary crossing problem to a extent,and the Geometry Clipmap can be used in the big diamond. The result illustrates feasibility and effectivity of the method.关键词：spherical terrain rendering;Geometry Clipmap;icosahedrons;Sphere Quad Trees（SQT）
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发布时间：2021-06-10 - 摘要：The microwave scatterometer onboard HY-2A ocean satellite primarily measures surface wind speed and direction over the global oceans. A precise in-orbit radiometric calibration of the scatterometer is crucial to accurately measure both wind speed and direction. This study is the first to perform the in-orbit radiometric calibration of the seawinds scatterometer on the QuikSCAT spacecraft by using global open ocean backscatter measurements. The ocean calibration method is analyzed and discussed on the basis of the result of weawinds that was precisely calibrated. Results show that the accuracy of the ocean calibration method is a pproximately 0. 15 dB for a specific Geographic Model Function（ GMF）. The deviation among different GMFs in ocean calibration is caused by systematic bias. The GMF must be accurate enough to realize the absolute calibration of the scatterometer. The calibration method and results from seawinds are used as a basis to implement the radiometric calibration of the HY-2A microwave scatterometer. Our results suggest that the calibration coefficients for the HY-2A microwave scatterometer are- 1. 79 dB for VV polarization and- 1. 73 dB for HH polarization. The wind vectors of the HY-2A microwave scatterometer are also validated by wind observations from NDBC ocean buoys. The accuracy of retrieving wind speed and direction is significantly improved after applying the obtained calibration coefficients.关键词：microwave scatterometer;radiometric calibration;ocean calibration;geophysical model function
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发布时间：2021-06-10 - 摘要：With the development of the BeiDou system,the features of mixed constellation have become more obvious with GEO and IGSO satellite orbits,the number of high elevation satellites has obviously increased,and the number of visible satellites has increased among high buildings. Therefore,we propose a new method wherein fewer satellites can also have better positioning a ccuracy with GEO /IGSO satellites. This method is also applicable to urban environment. To prove the effectiveness of this new method,we perform tests under open and urban environments. We also use two modes to verify the new method. The first mode u ses combinations from all BeiDou satellites with several GPS satellites,and the second mode uses combinations from several B eiDou and several GPS satellites. We select fewer satellites of BeiDou and GPS whose position dilution of precision is the smallest and then compare the positioning results with the BeiDou,GPS,and BeiDou /GPS systems under the two environments. Experimental results under the two environments show that the BeiDou system has better positioning performance than the GPS system. Under open environment,both modes of the new method can use fewer satellites to perform better positioning performance than the BeiDou /GPS system. The performance with the second mode is better than that with the first mode. Under urban environment,the features of the GEO and IGSO satellites of the BeiDou system can be maximized. Compared with the GPS system,the BeiDou system has more visible satellites because of the particularity of GEO and IGSO satellite orbits,as well as better positioning p erformance.In the first mode,the positioning error of our method is always less than that of the BeiDou or GPS system and is frequently less than that of the BeiDou /GPS system. In the second mode,the new method has less positioning error and fewer s atellites than the BeiDou /GPS system. Therefore,under urban environment,using fewer satellites can also have better positioning performance with the new method. This study proposes a new method to select fewer satellites. Two modes are used to verify the new method under open and urban environments. The BeiDou system displays better performance than the GPS system under both environments. Considering the full use of GEO and IGSO satellites in the two modes,we can conclude from the experiments that fewer satellites can also have better positioning performance. This research is necessary and effective particularly under urban environment.关键词：BeiDou;GPS;combination positioning;PDDP;Harsh environments between buildings
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发布时间：2021-06-10 - 摘要：The infrared channel of geostationary satellites can detect weak texture information resulting from the u neven distribution of atmospheric water vapor and dust aerosol in cloud-free regions. Atmospheric Motion Vectors（ AMVs） for cloud-free regions can be retrieved by tracking water vapor motion or dust aerosol motion. However,weak information is easily affected by changes in surface temperature. Therefore,a suitable algorithm must be developed to decrease surface temperature interference and to extract weak information. Basing on split-window infrared theory,we perform a numerical simulation of extracting weak information by using the split-window Brightness Temperature Difference（ BTD） algorithm. We also apply this algorithm to instance analysis. The numerical study indicates that u sing BTD images can weaken the influence of surface temperature and highlight the distribution characteristics of water vapor and aerosol. It also explains the feasibility and superiority of this algorithm. In cloud-free regions,low-level wind that is hard to obtain by only using IR1 or IR2 images can be obtained by using BTD images. The results of the cases agree with the wind field of the NCEP reanalysis data.关键词：geostationary satellite;Brightness Temperature Difference between split-window;AMVs in cloud-free region;tracer extract;numerical simulation
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发布时间：2021-06-10 - 摘要：The Kobresia pygmaea alpine meadow is a main vegetation type in the Qinghai-Tibetan Plateau. An accurate detection of the green-up dates for K. pygmaea is important to simulate and predict vegetation phenology shifts under the influence of c limate change in the Qinghai-Tibetan Plateau. Green-up date estimation methods from remote sensing data generally include two processes: reconstruction of high-quality vegetation index time-series data through noise removal and calculation of green-up dates from the reconstructed vegetation index time series. The reconstruction methods for vegetation index time-series d ata can be divided into two categories: filter fitting and curve fitting methods. The green-up date retrieval methods include the threshold,maximum slope,curvature,and moving average methods. The green-up date identification method is a combination of the reconstruction methods for vegetation index time-series data and the retrieval methods for green-up dates under different study conditions. The a ccuracy of the green-up date identification methods is usually affected by many factors,such as specific geographic location,prior experience,parameterization,and initial parameters. In this study,we adopted a simulated annealing algorithm to optimize the r econstruction process and thus avoid the problems of low efficiency and local optimum caused by traditional optimal methods. We first used the double-Gaussian,double-Logistic,and polynomial functions to reconstruct the Normalized Difference Vegetation I ndex（ NDVI）time series. After evaluations with visual inspections and root mean square error,we identified the most feasible r econstruction method. We then used the maximum slope,threshold,curvature,and dynamic threshold methods to derive the green-up dates from the best reconstructed NDVI time series. The performance of these three methods for green-up date identification were tested using the green-up data from 34 ground observation samples and their corresponding National Oceanic and A tmospheric Administration NDVI time-series data at 8-kilometer resolution. We selected additional 153 samples,which were e venly distributed in the K. pygmaea alpine meadow in the Qinghai-Tibetan Plateau,to test the identified optimal green-up estimation method and to investigate the changes in green-up dates in the study area. The reconstructed NDVI time series with the double-Gaussian function had the smallest deviation from the original NDVI time series,and the noises can be reduced effectively through the double-Gaussian fitting process.Therefore,the aforementioned method was the most suitable for describing the intra-annual growth cycle of the K. pygmaea alpine meadow. The reconstructed NDVI time series with the double-Gaussian function method indicated that the green-up dates identified with the maximum slope threshold method agreed with the observed ground phenology data. The correlation coefficients between the identified green-up dates and the observed dates were 0. 823（ P < 0. 001） and 0. 646（ P < 0. 01） at the Haiyan and Gande stations,respectively. The average green-up dates for the K. pygmaea alpine meadow in the Qinghai-Tibetan Plateau were mainly located between DOY（ Day of Year） 120（ i. e.,30 April） and 140（ i. e.,20 May）. The green-up onset date advanced by an average of 7 days from 1982 to 2011 in the study region.关键词：Kobresia pygmaea;alpine meadow;NDVI;green-up date;simulated annealing algorithm
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发布时间：2021-06-10 - 摘要：The Volume Velocity Processing（ VVP） retrieval method can be used in the three dimensional structure analysis of wind field,but the accuracy of retrieval results is always affected by the ill-conditioned coefficient matrix. Error analysis and tests with synthetic data were performed by using a simplified VVP method to analyze the characteristics of fitted parameters and to confirm the fitted wind field in retrieval. In the simplified method,only three parameters with particularly large magnitude were s elected. The coefficient matrix is a function of the analysis volume position. Hence,the retrieval errors and condition number v aried accordingly. Selected partial parameters were substituted for full model retrieval and retrieval errors caused by neglected p arameters were qualitatively analyzed to reduce solving difficulties. Observation and calculation errors are inevitable in retrieval.According to the spread of e rrors in the solving process,a demonstration through mathematical deduction was provided to explainthat the large analysis v olume can improve retrieval accuracy. The performance of the simplified VVP method was tested using synthetic data with r andom observation errors to find the fitted wind velocity. Wind model parameters with small magnitude or small position coefficient can be abandoned to simplify the wind field model. The results of the sensitive tests indicate that the r obustness of the simplified VVP method is not highly sensitive to random errors. The accuracy of retrieval results at different e levations is almost similar and may improve when the wind field velocity is large. Considering the observation and calculation e rrors with normal or uniform distribution,we can reduce the influence caused by these errors in an analysis volume. These errors are spread and amplified in the retrieval process. Therefore,preventing the spread of these errors at the beginning of retrieval can improve the accuracy of retrieval results. For the wind field that is suited to use the simplified VVP method,the test result shows that this method can be used to r etrieve the wind field with wind velocity above 20 m /s,which contains 1. 0 m /s random observation error. In addition,the analysis volume size is 10° * 20 Gates. The retrieval results of the 0608"Saomai"typhoon shows that the wind field can be retrieved reasonably and is consistent with the Rankine typhoon model. The retrieved wind profiles indicate that the point of inflexions for wind velocity and direction can be used to confirm the eye region and typhoon height,which are suited to analyze the three dimensional wind field structures of a typhoon. Parameters with small magnitude are abandoned to s implify the wind field model.Thus,difficulties in s olving are decreased and retrieval results become more robust. After confirming that the fitted wind field is suited to retrieval by using the simplified VVP method,the wind field of the 0608 "Saomai"t yphoon is retrieved for a real test case. The three dimensional wind field structures of the 0608"Saomai"typhoon can be obtained with this method and can be used for disastrous weather analysis.关键词：Single Doppler Weather radar;Volume Velocity Processing（VVP） method;wind retrieval;error analysis;Typhoon Saomai（0608）
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发布时间：2021-06-10 - 摘要：<正>大气气溶胶是悬浮于空气中的微小固体或液体颗粒状物质,其来源多种多样,包括自然源与人工源。气溶胶通过散射与吸收短波和长波辐射改变大气和地表的辐射能量收支,这称为直接气候效应。它还作为水云或冰晶凝结核影响云的形成与发展,带来间接气候效应。此外,落在冰雪地表上的黑碳气溶胶,可以改变地表反照率,并由此改变地表能量收支以致影响气候。因此,摸清气溶胶的时空分布特征并把握气溶胶对气候的影响在气候变化研究中非常重要。
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发布时间：2021-06-10 - 摘要：<正>高分一号卫星是中国高分辨率对地观测系统的首发星,配置了2台2m分辨率全色和8m分辨率多光谱相机以及4台16m分辨率多光谱宽幅相机,具有高、中空间分辨率对地观测和大幅宽成像结合的特点,其2m分辨率全色和8m分辨率多光谱图像组合幅宽优于60km,16m分辨率多光谱图像组合幅宽优于800km,为国际同类卫星观测幅宽的最高水平,大幅提升了观测能力,对大尺度地表观测和环境监测具有独特优势。封面图片是高分
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发布时间：2021-06-10
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