光束发散角对星载激光测高仪森林回波的影响
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摘要
植被树高是生物量评估和森林生态监测的重要参数,但是大区域的植被树高数据获取困难.由大光斑星载激光测高系统森林回波可反演大区域植被树高,然而大坡度山区的植被和地面回波混叠严重,难以准确提取波形参数反演植被树高.为此建立森林目标回波的解析模型,推导出从混叠回波中分离植被和地面回波的必要条件,指出导致回波混叠的因素除了目标粗糙度和坡度,还有发射脉冲的发散角.分析了地表粗糙度和地形坡度对植被冠层回波的展宽效应及其对波形分解的影响,通过比较实测GLAS波形和仿真回波波形,验证了压缩光束发散角可降低森林回波对大坡度地形的敏感性.对于卫星轨道高度在500~600 km之间时,激光测高仪光束发散角一般在40~60μrad,更有利于森林植被树高的反演.所得结论为提高大坡度山地森林树高反演精度,从星载激光测高仪的系统设计角度提供了有意义的参考.
Vegetation height is an important parameter for biomass assessment and ecosystem structure monitoring. However,vegetation height data over large areas are difficult to obtain. Vegetation heights can be retrieved by using large-footprint satellite altimeter system GLAS waveforms,but the current method can only be used in relatively flat forest regions because over sloping mountainous areas,the effects of ground and canopy are mixing on the waveforms and cannot be distinguished. By establishing analytical model of forest target echo,the separation requirement of vegetation echo and ground echo from aliasing waveforms is derived. It is pointed out that the factors of leading to echo aliasing in addition to target roughness and slope,and beam divergence angle is also a factor. The broadening effect and waveform decomposition related to the beam divergence angle,surface roughness and topography slope is analyzed. By using the waveform simulation and GLAS actual waveforms,it is verified to decrease beam divergence angles can reduce the sensitivity of forest echo to sloping mountainous terrains and be more conducive to the retrieval of canopy heights. For laser altimeter to carry on satellite in the orbit of 500 ~ 600 km,the beam divergence angles is 40 ~ 60 μrad better. The result provides a meaningful reference for designing of satellite laser altimeter system for the retrieval of canopy heights over high sloping terrains.
Vegetation height is an important parameter for biomass assessment and ecosystem structure monitoring. However,vegetation height data over large areas are difficult to obtain. Vegetation heights can be retrieved by using large-footprint satellite altimeter system GLAS waveforms,but the current method can only be used in relatively flat forest regions because over sloping mountainous areas,the effects of ground and canopy are mixing on the waveforms and cannot be distinguished. By establishing analytical model of forest target echo,the separation requirement of vegetation echo and ground echo from aliasing waveforms is derived. It is pointed out that the factors of leading to echo aliasing in addition to target roughness and slope,and beam divergence angle is also a factor. The broadening effect and waveform decomposition related to the beam divergence angle,surface roughness and topography slope is analyzed. By using the waveform simulation and GLAS actual waveforms,it is verified to decrease beam divergence angles can reduce the sensitivity of forest echo to sloping mountainous terrains and be more conducive to the retrieval of canopy heights. For laser altimeter to carry on satellite in the orbit of 500 ~ 600 km,the beam divergence angles is 40 ~ 60 μrad better. The result provides a meaningful reference for designing of satellite laser altimeter system for the retrieval of canopy heights over high sloping terrains.
引文
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[23]Gardner C S.Target signatures for laser altimeters:an analysis[J].Applied Optics,1982,21(3):448-53.
[24]Gardner C S.Ranging Performance of Satellite Laser Altimeters[J].IEEE Transactions on Geoscience and Remote Sensing,1992,30(5):1061-1072.
[25]Blair J B,Hofton M A.Modeling Laser Altimeter Return Waveforms Over Complex Vegetation Using High-Resolution Elevation Data[J].Geophysical Research Letters,1999,26(16):2509-2512.
[2]Lefsky M A,Cohen W B,Harding D J,et al.Lidar Remote Sensing of Above-Ground Biomass in Three Biomes[J].Global Ecology&Biogeography,2002,11(5):393-399.
[3]Xing Y Q,de Gier A,Zhang J J,et al.An improved method for estimating forest canopy height using ICESat-GLASfull waveform data over sloping terrain:A case study in Changbai mountains,China[J].International Journal of Applied Earth Observation&Geoinformation,2010,12(5):385-392.
[4]WANG Cheng,XI Xiao-Huan,LUO She-Zhou,et al.Data processing and application of spaceborne LiDAR[M].Beijing:Science Press(王成,习晓环,骆社周,等.星载激光雷达数据处理与应用.北京:科学出版社),2015.
[5]Bye I J,North P R J,Los S O,et al.Estimating forest canopy parameters from satellite waveform LiDAR by inversion of the FLIGHT three-dimensional radiative transfer model[J].Remote Sensing of Environment,2017,188:177-189.
[6]Nie S,Wang C,Zeng H,et al.A revised terrain correction method for forest canopy height estimation using ICESat/GLAS data[J].Isprs Journal of Photogrammetry&Remote Sensing,2015,108:183-190.
[7]Nelson R,Margolis H,Montesano P,et al.Lidar-based estimates of aboveground biomass in the continental US and Mexico using ground,airborne,and satellite observations[J].Remote Sensing of Environment,2017,188:127-140.
[8]Tian J,Wang L,Li X,et al.Differentiating Tree and Shrub LAI in a Mixed Forest With ICESat/GLAS Spaceborne LiDAR[J].IEEE Journal of Selected Topics in Applied Earth Observations&Remote Sensing.2016,10(1):87-94.
[9]Hilbert C.and Schmullius C.,Influence of Surface Topography on ICESat/GLAS Forest Height Estimation and Waveform Shape[J].Remote Sensing,2012,4(8):2210-2235.
[10]Mallet C,Bretar F.Full-waveform Topographic Lidar:State-of-the-art[J].ISPRS Journal of Photogrammetry and Remote Sensing,2009,64(1):1-16.
[11]WANG Cheng,TANG Fu-Xin,LI Li-Wei,et al.Wavelet analysis for ICESat/GLAS waveform decomposition and its application in average tree height estimation[J].IEEEGeoscience&Remote Sensing Letters.2013,10(1):115-119.
[12]LAI Xu-Dong,QIN Nan-Nan,HAN Xiao-Shuang,et al.Iterative decomposition method for small foot-print LiDARwaveform[J].J.Infrared Millim.Waves(赖旭东,秦楠楠,韩晓爽,等.一种迭代的小光斑LiDAR波形分解方法.红外与毫米波学报),2013,32(4):319-324.
[13]Harding D J,Carabajal C C.ICESat waveform measurements of within‐footprint topographic relief and vegetation vertical structure[J].Geophysical Research Letters,2005,32(21):741-746.
[14]MA Yue,LI Song,ZHANG Wen-Hao,et al.Waveform width of a satellite laser altimeter illuminating on sea surface.Applied Optics,2017,56(22),6130-6137.
[15]MA Yue,WANG Ming-Wei,LI Guo-Yuan,et al.Waveform model of a laser altimeter for an elliptical Gaussian beam.Applied Optics,2016,55(8):3567-3574.
[16]Lefsky M A,Michael A.A Global Forest Canopy Height Map from the Moderate Resolution Imaging Spectroradiometer and the Geoscience Laser Altimeter System[J].Geophysical Research Letters,2010,37(15):78-82.
[17]Chen Q.Retrieving vegetation height of forests and woodlands over mountainous areas in the Pacific Coast region using satellite laser altimetry[J].Remote Sensing of Environment,2010,114(7):1610-1627.
[18]Pang Y,Lefsky M,Sun G,et al.Impact of footprint diameter and off-nadir pointing on the precision of canopy height estimates from spaceborne lidar[J].Remote Sensing of Environment,2011,115(11):2798-2809.
[19]Huang H,Liu C,Wang X,et al.Mapping vegetation heights in China using slope correction ICESat data,SRTM,MODIS-derived and climate data[J].ISPRSJournal of Photogrammetry and Remote Sensing.2017,129:189-199.
[20]Brenner A C,Zwally H J,Bentley C R,et al.,Derivation of Range and Range Distributions from Laser Pulse Waveform Analysis for Surface Elevations,Roughness,Slope,and Vegetation Heights[R].US:GLAS Algorithm Theoretical Basis Document.Version 5.0,2011.
[21]Wagner W,Ullrich A,Ducic V,et al.Gaussian decomposition and calibration of a novel small-footprint full-waveform digitising airborne laser scanner[J].ISPRS Journal of Photogrammetry&Remote Sensing,2006,60(2):100-112.
[22]Hofton M A,Minster J B,Blair J B.Decomposition of laser altimeter waveforms[J].IEEE Transactions on Geoscience&Remote Sensing,1999,38(4):1989-1996.
[23]Gardner C S.Target signatures for laser altimeters:an analysis[J].Applied Optics,1982,21(3):448-53.
[24]Gardner C S.Ranging Performance of Satellite Laser Altimeters[J].IEEE Transactions on Geoscience and Remote Sensing,1992,30(5):1061-1072.
[25]Blair J B,Hofton M A.Modeling Laser Altimeter Return Waveforms Over Complex Vegetation Using High-Resolution Elevation Data[J].Geophysical Research Letters,1999,26(16):2509-2512.