基于小光斑复合激光足印的植被属性研究

用机载激光测高仪对单木树冠进行自动提取和轮廓描绘,是获取森林信息的一种快速有效的方法。机载激光测高仪可通过记录回波波形来描绘单树植被冠盖等细节信息,但由于激光足印较小,很难给出树木种群的细节尺度参数,如树的冠盖大小,树径等。提出把若干小激光足印复合成大激光足印的方法,并通过高斯分解算法导出植被冠层高度,植被冠层反射率和中值能量高度等大尺度参数。实验结果表明,通过把固定数量的小光斑激光足印复合成一个大光斑激光足印的方法,能反演出更多的单树外围尺度参数,为植被垂直结构分析提供更多的可靠保证。     

Modeling lidar waveforms in heterogeneous and discrete canopies

This study explores the relationship between laser waveforms and canopy structure parameters and the effects of the spatial arrangement of canopy structure on this relationship through a geometric optical model. Studying laser waveforms for such plant canopies is needed for the advanced retrieval of three-dimensional (3D) canopy structure parameters from the vegetation canopy lidar (VCL) mission. For discontinuous plant canopies, a hybrid geometric optical and radiative transfer (GORT) model describing the effects of 3D canopy structure parameters of discrete canopies on the radiation environment has been modified for use with lidar. The GORT model is first used to describe the canopy lidar waveforms as a function of canopy structure parameters and then validated using scanning lidar imager of canopies by echo recovery (SLICER) data collected in conifer forests in central Canada during the boreal ecosystem-atmosphere study (BOREAS). Model simulations show that the clumping in natural vegetation, such as leaves clustering into tree crowns causes larger gap probability and smaller waveforms for discontinuous plant canopies than for horizontally homogeneous plant canopies. Ignoring the clumping effect can result in significantly lower values for the estimated foliage amount in the profile and in turn lower estimated biomass. Because of clumping, only the gap probability and apparent vertical projected foliage profile can be directly retrieved from the canopy lidar data. The retrieval is sensitive to the ratio of the volume backscattering coefficients of the vegetation and background, and this ratio depends on canopy architecture as well as foliage spectral characteristics     

星载激光测高仪多模式回波参数提取方法（特邀）

全波形星载激光测高仪的接收波形特征参数可以用于反演目标的形貌信息,传统的波形处理算法不能用于混叠严重以及偏离高斯形态的多模式波形特征参数提取。针对混叠严重的多模式回波,提出一种基于偏正态拟合模型,使用激励Richardson-Lucy反卷积算法、逐层分解算法、梯度下降法和非线性最小二乘拟合算法相组合的波形特征参数提取方法。采用已知参数的波形数据集、机载仿真波形数据集和全球生态系统动态调查（GEDI）激光雷达波形数据,基于波形相关系数与均方根误差（RMSE）、波形特征参数相对误差、波形分量个数提取正确率等评价指标开展波形处理试验,并将处理结果与传统的高斯分解结果进行比较分析。已知参数波形数据集处理结果的平均波形相关系数提升了约2%,RMSE降低了约47%,波形特征参数相对误差平均降低了约5%,分量个数提取正确率提升了约34%;机载仿真数据和GEDI波形数据处理结果的平均波形相关系数分别提升了约1%和2%,RMSE分别降低了约56%和54%。同时,开展了陡坡区域植被高度解算的仿真试验,得到的植被高度准确程度明显高于传统方法。所有处理结果均表明该方法更有利于多模式回波特征参数的提取以及目标参数的反演。     

基于IDL的激光测高雷达回波时域仿真算法研究

从激光测高雷达回波能量分布模型和地表模型入手,详细介绍了基于IDL的激光测高雷达回波的仿真过程及仿真程序各个模块的功能,用Pearson相关性方法将仿真所得波形与实测回波波形进行了分析比对,二者的相关性系数达到90%以上,并分析了回波脉冲波形随测高系统参数和地表参数的变化情况.该方法可以直观有效地对不同地表的回波进行仿真,为激光测高雷达的回波分析提供了重要的技术手段.     

基于深度学习的机载激光海洋测深海陆波形分类

机载激光雷达的海陆波形分类对于沿海地区及其变化性质的研究至关重要。提出了一种在原始的机载激光雷达回波上使用深度学习进行分类的方法。构建全连接神经网络和一维卷积神经网络（CNN）,在一个测量海域的数据集上进行训练和测试,最优模型获得了99.6%的分类精度。该最优模型对来自不同测量海域的数据进行分类,分类精度达到了95.6%,相比支持向量机方法,处理速度提高了约52%。结果表明:深度学习方法对机载激光雷达回波波形的分类具有较高的精度和速度,它可以进一步作为通过机载激光测深技术对海底种类进行分类的候选方法。     

任意波形相关性的机载MIMO雷达杂波建模与分析

为了分析任意发射波形相关性影响下的机载MIMO雷达杂波分布特性,从信号空间与阵元空间变换的数学角度构建了发射波形合成模型。在此基础上,提出了任意发射波形合成影响下的机载MIMO雷达杂波统一模型,论述了发射波形合成与杂波统一模型之间的关系。仿真揭示了统一模型下发射波形从全正交、相关到全相干的演变过程中杂波的分布特性。研究得出,发射波形互相关矩阵的结构决定了杂波谱展宽的程度,而该互相关矩阵的秩决定了杂波自由度大小。该模型为波形设计影响下的机载MIMO雷达STAP算法的设计提供了理论依据。     

Polarized Lidar Reflectance Measurements of Vegetation at Near-Infrared and Green Wavelengths

There is growing interest in the use of lidar for remote sensing of vegetation owing to the emergence of reliable and rugged lasers and highly sensitive detectors. Lidar remote sensing has a distinct advantage over conventional techniques in vegetation remote sensing due to its capability for three-dimensional characterization of vegetative targets. The Multiwavelength Airborne Polarimetric Lidar (MAPL) system was developed primarily for vegetation remote sensing applications from an airborne platform of up to 1,000 -m altitude. The lidar system has full waveform capture and polarimetric measurement capability at two wavelengths in the near-infrared (1064 nm) and the green (532 nm) spectral regions. This study presents preliminary ground-based lidar reflectance measurements on a variety of deciduous and coniferous trees under fully foliated conditions with a view towards tree species discrimination. Variations in the reflectance characteristics of selected deciduous trees under unfoliated and fully foliated conditions were also investigated. Our study reveals distinct differences in the reflectance characteristics of various trees.     

Radiative transfer model for microwave bistatic scattering from forest canopies

A bistatic forest scattering model is developed to simulate scattering coefficients from forest canopies. The model is based on the Michigan Microwave Canopy Scattering (MIMICS) model (hence called Bi-MIMICS) and uses radiative transfer theory, where the first-order fully polarimetric transformation matrix is used. Bistatic radar systems offer advantages over monostatic radar systems because of the additional information provided by the diversity of the geometry. By simulating the forest canopy scattering from multiple viewpoints, we can better understand how the forest scatterers' shape, orientation, density, and permittivity affect the canopy scattering. Bi-MIMICS is parametrized using selected forest stands with different canopy compositions and structure. The simulation results show that bistatic scattering is more sensitive to forest biomass changes than backscattering. Analyzing scattering contributions from different parts of the canopy gives us a better understanding of the microwave's interaction with the tree components. The ground effects can also be studied. Knowledge of the canopy's bistatic scattering behavior combined with additional synthetic aperture radar measurements can be used to improve forest parameter retrievals. The simulation results of the model provide the required information for the design of future bistatic radar systems for forest sensing applications.     

Radar backscattering model for multilayer mixed-species forests

A multilayer canopy scattering model is developed for mixed-species forests. The multilayer model provides a significantly enhanced representation of actual complex forest structures compared to the conventional canopy-trunk layer models. Multilayer Michigan Microwave Canopy Scattering model (Multi-MIMICS) allows overlapping layer configuration and a tapered trunk model applicable to forests of mixed species and/or mixed growth stages. The model is the first-order solution to a set of radiative transfer equations and includes layer interactions between overlapping layers. It simulates SAR backscattering coefficients based on input dimensional, geometrical, and dielectric variables of forest canopies. The Multi-MIMICS is an efficient realization of actual forest structures and can be shaped for specific interest of forest parameters. We present the model's application and validation in the paper. The model is parameterized using data collected from a 220,000-ha area of forests in central Queensland, Australia. Fifteen 50/spl times/50 m test sites representing the general forest diversity and growth stages are chosen as ground truth. Polarimetric backscattering airborne SAR (AIRSAR) data of the same area are acquired to validate the model simulations. The model predicts SAR backscattering coefficients of the test areas. Simulation results show a good agreement with AIRSAR data at most frequencies and polarizations. The simulated backscattering coefficient from the multilayer model and the standard MIMICS are also compared and significant improvements are observed.     

ICESat Full-Waveform Altimetry Compared to Airborne Laser Scanning Altimetry Over The Netherlands

Since 2003, the full-waveform laser altimetry system onboard NASA's Ice, Cloud and land Elevation Satellite (ICESat) has acquired a worldwide elevation database. ICESat data are widely applied for change detection of ice sheet mass balance, forest structure estimation, and digital terrain model generation of remote areas. ICESat's measurements will be continued by a follow-up mission. To fully assess the application possibilities of the full-waveform products of these missions, this research analyzes the vertical accuracy of ICESat products over complex terrain with respect to land cover type. For remote areas, validation of individual laser shots is often beyond reach. For a country with extensive geo-infrastructure such as The Netherlands, excellent countrywide validation is possible. Therefore, the ICESat full-waveform product GLA01 and the land elevation product GLA14 are compared to data from the Dutch airborne laser altimetry archive Actual Height model of the Netherlands (AHN). For a total population of 3172 waveforms, differences between ICESat- and AHN-derived terrain heights are determined. The average differences are below 25 cm over bare land and urban areas. Over forests, differences are even smaller but with slightly larger standard deviations of about 60 cm. Moreover, a waveform-based feature height comparison resulted in feature height differences of 1.89 m over forest, 1.48 m over urban areas, and 29 cm over low vegetation. These results, in combination with the presented processing chain and individual waveform examples, show that state-of-the-art ICESat waveform processing is able to analyze waveforms at the individual shot level, particularly outside urban areas.     

基于遗传算法的星载激光全波形分解

全波形激光雷达的回波中包含着地物目标的垂直结构特征信息,传统的全波形数据处理方法在提取这些信息时过于依赖初始参数,导致地形复杂地区的数据可利用率低、准确率低。针对这一问题,提出了一种基于遗传算法的波形分解方法。改进后的处理算法无需提供精确的初始参数,用概率性传递规则代替确定性规则,具有全局寻优特点。并以高分七号卫星激光全波形数据进行试验。结果证明,基于改进后的波形处理方法拟合的回波波形与预处理后波形的相关系数在99%以上。文中对森林地区最大树高的反演与ICESat-2的ATL08产品中的森林冠层高度参数进行对比,两者相关系数为0.85,中误差为1.1 m,表明该方法可以较准确地提取复杂波形的特征信息。     

基于克尔晶体的FROG光学任意波形测量技术

利用光克尔效应和交叉相位调制效应,采用基于矩阵的主元素广义投影(PCGP)算法,提出了基于克尔晶体的频率分辨光学开关法(FROG)光学任意波形测量方案.建立了基于克尔晶体的FROG光学任意波形测量的理论模型,并对该方案进行了仿真研究,讨论背景噪声处理、门函数形状、门波形幅度对波形恢复的影响.仿真结果表明:该方案可以实现对光学任意波形的幅度和相位的准确测量;门脉冲采用矩形波形时,光学任意波形的测量准确性较高.     

基于半解析模型的激光测高回波海水海冰波形分类方法

现有利用激光测高波形的地物分类方法绝大多数基于机器学习的分类原理,是一种基于经验的分类方法。从激光回波的理论模型出发,通过推导纯海水表面回波和含有海冰的表面回波的解析模型,对纯海水回波和含有海冰回波逐个采样点按时域距离加权计算总振幅差异值,以该差异值作为依据建立一种半解析型的海水、海冰分类方法;通过机载LiDAR将在格陵兰北部海冰区的实测点云数据判断GLAS激光脚点对应的地面类型,对GLAS在该区域实测波形进行基于论文方法的分类准确性验证;结果显示,在剔除饱和波形影响后,分类总体精度OA大于95%,Kappa系数接近0.89,具有非常好的分类效果。论文将使得激光测高仪地物类型分类方法由基于机器学习为依据向半理论解析模型为依据的分类方向延伸,为后续基于激光回波数据的地物分类方法提供重要的参考思路。     

基于FPGA芯片的机载波形动态重构设计

针对机载综合射频传感器系统高度综合化的实际需求，基于部分可重构技术提出了一种机载传感器功能波形重构设计方法，以实现在现场可编程门阵列（Field Programmable Gate Array，FPGA）芯片局部区域上时分复用机载功能波形。该方法引入一种便于功能波形移植部署的FPGA平台设计，并在此平台上完成机载功能波形在FPGA芯片局部区域的可重构具体设计。工程应用表明，该设计能够灵活有效复用可编程逻辑器件资源，提高了综合射频传感器系统的功能波形集成度，具有较好的实践意义。     

L-band radiometer measurements of soil water under growing clover grass

A field experiment with an L-band radiometer at 1.4 GHz was performed from May-July 2004 at an experimental site near Zurich, Switzerland. Before the experiment started, clover grass was seeded. Thermal infrared, in situ temperature, and time-domain reflectometer (TDR) measurements were taken simultaneously with hourly radiometer measurements. This setup allowed for investigation of the microwave optical depths and mode opacities (parallel and perpendicular to the soil surface) of the clover grass canopy. Optical depths and opacities were determined by in situ analysis and remotely sensed measurements using a nonscattering radiative transfer model. Due to the canopy structure, optical depth and opacity depend on the polarization and radiometer direction, respectively. A linear relation between vegetation water-mass equivalent and polarization-averaged optical depth was observed. Furthermore, measured and modeled radiative transfer properties of the canopy were compared. The model is based on an effective-medium approach considering the vegetation components as ellipsoidal inclusions. The effect of the canopy structure on the opacities was simulated by assuming an anisotropic orientation of the vegetation components. The observed effect of modified canopy structure due to a hail event was successfully reproduced by the model. It is demonstrated that anisotropic vegetation models should be used to represent the emission properties of vegetation. The sensitivity of radiometer measurements to soil water content was investigated in terms of the fractional contribution of radiation emitted from the soil to total radiation. The fraction of soil-emitted radiation was reduced to approximately 0.3 at the most developed vegetation state. The results presented contribute toward a better understanding of the interaction between L-band radiation and vegetation canopies. Such knowledge is important for evaluating data generated from future satellite measurements.     

Backscattering properties of boreal forests at the C- and X-bands

The backscattering properties of boreal forests are studied using empirical airborne and spaceborne radar data from Finland. Airborne measurements were carried out in the summer of 1992 by the HUTSCAT scatterometer at the Teijo test area in southern Finland. The HUTSCAT scatterometer is an eight-channel helicopter-borne profiling radar operating at the C- and X-bands. The ranging capability of the HUTSCAT scatterometer was employed in the semiempirical modeling of forest backscatter. The backscatter profile information was used in the analysis of the canopy transmissivity and the canopy backscattering coefficient by distinguishing backscattering contributions from the canopy and the ground. Additionally, ERS-1 C-band satellite SAR measurements were obtained for the Teijo test area and for the reference test area in Sodankyla in northern Finland. The radar results were compared with operational ground-based forest assessment data on forest compartments (stands) of the area. The key parameter investigated was the stem (bole) volume per hectare. The results obtained show the behavior of the canopy transmissivity and the canopy backscatter as a function of stem volume (directly related to the forest biomass). The influence of seasonal and diurnal changes on, and the effects of the changes in soil moisture to the backscattering coefficient were also investigated     

基于星载激光测高仪的森林植被高度估算

为了提高星载激光测高仪估算森林植被高度的精度,本文在处理星载激光森林地区回波波形时,提出了一种基于高斯混合函数模型的波形分解方法,充分保留了波形中有效峰值信息,提高了利用波形估算植被高度的精度.本文以GLAS过中国长白山地区的20个激光点为例,先后进行了自适应波形背景噪声滤除与高斯平滑滤波的波形预处理,对预处理后波形进...     

ICESat-2/ATLAS数据反演林下地形精度验证

针对星载激光雷达数据反演林下数字地面模型（Digital Terrain Model,DTM）存在困难的问题,研究了冰云陆地高程卫星-2 （Ice, Cloud, and land Elevation Satellite,ICESat-2）/先进地形激光高度计系统(Advanced Opographic Laser Altimeter System,ATLAS）的强弱光束数据反演林下地形的精度,并探究了冠层高度及植被覆盖率对于ICESat-2/ATLAS反演林下DTM精度的影响。研究结果表明:强波束反演林下DTM的精度为R2=1,RMSE=0.74 m,弱波束反演林下DTM的精度为R2=1,RMSE=0.76 m,强波束相对弱波束表现出更优的反演精度,但是,强光束与弱光束的光子云数据均可为反演林下DTM提供科学数据。从研究区植被的整体情况来看,随冠层高度及植被覆盖率的增加,不同激光类型数据均出现误差逐步增加的情况。     

Inversion of the Li-Strahler canopy reflectance model for mappingforest structure

As part of a larger forest vegetation mapping process based on Landsat TM and digital terrain data, inversion of the Li-Strahler model provides estimates of tree size and cover for conifer stands. The vegetation maps are intended for use in natural resource management by the US Forest Service. Analysis of extensive field data in the form of “test” stands from four National Forests indicate the following about the Li-Strahler model: (1) the underlying assumptions of independence between tree size and crown shape are valid, (2) the means for tree geometry parameters vary between forest types, (3) estimates of forest cover are reliable, and (4) estimates of tree size are unreliable due to the breakdown in the relationship between image intra-stand variance and tree size. Improvements in estimates of tree size will require additional data beyond a single Landsat TM image, with multidirectional data a promising possibility