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星载激光雷达测绘设备最早在深空探测中得到应用,其探测环境、精度要求等与对地观测差异较大。如表1所示,国内外已有和正在规划的对地观测激光雷达系统[10]主要有SLA、ICESat、ICESat-2、ZY3-02、GF-7、GEDI和LIST。
表 1 国内外对地观测激光测高设备主要技术指标及用途
Table 1. Main technical specifications and applications of earth observation laser altimetry equipment at home and abroad
Satellite/
payloadLaunch time Collect method Beam number Pulse
width/nsPoint
interval/mElevation
accuracy/mApplication SLA-01/02 1996/97 Full waveform 1 10 750 1.5 Global elevation control point ICESat/GLAS 2003 Full waveform 1 6 170 0.15 Sea ice, atmosphere, land,
vegetation, etc.ZY3-02 2016 Full waveform 1 7 3500 1.0 Experimental measurement ICESat-2/ATLAS 2018 Photon counting 6 1.5 0.7 0.1 Sea ice, atmosphere, land,
vegetation, etc.GEDI 2018 Full waveform 8 14 60 1.0 Forest biomass GF-7 2019 Full waveform 2 4-8 2900 1.0 Elevation control point LIST To be launched Photon counting 1000 1 0.7 0.1 Earth observation SLA (Shuttle Lidar Altimeter)是NASA开展的航天飞机搭载Lidar试验,用于制作全球控制点数据[11];ICESat卫星是全球首个以冰冻圈为重点的天基激光测高任务,该数据在极地冰盖变化估计、全球冠层高度测量等多个领域得到成功应用[12];ICESat-2是ICESat后续星,详细性能在后文介绍;GEDI (Global Ecosystems Dynamics Investigation)设备安装在国际空间站上,用作测量地球表面三维结构[13];ZY3-02星在国内首次搭载对地观测试验性激光测高仪,可用作1∶50 000立体测图高程控制点,辅助提升立体影像定位精度[8, 10];GF-7激光测高数据用于广义稀疏控制点测量,满足在少控制点条件下实现1∶10000立体测绘的应用需求[5]。LIST拟采用光子计数探测体制,用1000波束获得全球5 m格网大小和10 cm高程精度的地形信息,以及森林、湖泊、冰盖等地表高程变化[14],该卫星尚在计划发射中。
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ICESat-2与ICESat最显著的变化是采用光子计数激光雷达替代传统线性探测体制,后者需返回成千上万个光子,并通过全波形采样和波形分析才能获得点云坐标,而ICESat-2光电倍增管接收器探测灵敏度在单个光子级别,能够对每个返回到接收器的光子都标记时间,并计算坐标,大大提升了探测效率。
ICESat-2基本配置参数如表2所示,卫星轨道高约500 km,倾角92°,能够覆盖南北88°范围。ICESat-2地面波束几何结构设置如图1所示,6束激光一强一弱组成2组,强弱激光组内垂轨方向间隔90 m,可以借此进行坡度测量。由于卫星首要科学目标在于测量冰冻圈变化,要求前后多次测量尽量在同一/相近位置。ICESa-2设置了1387个地面参考轨迹RGT (Reference ground tracks)固定不变,为了保证得到参考轨迹线的高程值(内插),必须让强弱光束分别位于RGT的左右两侧,因此要求激光的指向控制精度优于45 m[6]。由于采用了光子计数探测体制,发射激光的能量大幅减小,强波束为120 μJ (弱波束为四分之一),激光重频大幅提高,达10 kHz,沿轨点间隔仅0.7 m,数据密度相比ICESat得到极大提升。
表 2 ICESat-2基本配置参数
Table 2. Basic parameters of ICESat-2
Item Value Orbital inclination/coverage 92°;Covering 88° between north and south Orbit type 91 days repeat orbit Orbit eight 500 km Pointing control accuracy 45 m Horizontal accuracy 6.5 m Laser wavelength 532 nm Laser repetition 10 kHz Number of beams 6 bundles, one strong and one weak to form 3 groups Beam spacing (vertical track) 90 m within the strong and weak group, 3.3 km between the groups Point cloud interval (along track) 0.7 m Footprint size <17.5 m -
光子计数激光雷达探测灵敏度极高,也导致噪声很多,数据信噪比差。虽然ICESat-2接收器安装了窄带滤波片,将波段范围限制在(532.272±0.15) nm,但该范围内仍有大量背景太阳光。在某些高太阳角和高地面反射率场景下,背景光噪声率达到约10 MHz (即每秒1千万个,换算到高程方向每3米1个噪声点)[15],因此点云去噪至关重要。
目前已有的光子计数激光雷达设备多数只沿飞行方向记录数据(摆扫较为少见),因此通常在二维剖面进行处理。ICESat-2基础理论算法文档ATL03[16]、ATL08[17]中分别提供了直方图和空间密度两种去噪算法:直方图法认为在垂直方向点出现次数最多的位置更可能是信号[15];空间密度法认为信号点在空间分布上更密集,密度直方图会呈现“噪声在左、信号在右”,“噪声高窄、信号低矮”的分布特点[18]。二者去噪对比效果见图2,ATL03算法在平原冰盖区效果较好,在植被区会出现明显的信号点漏提,ATL08更适合植被地形区域[19]。此外,在搜索核形状、地形相关、方向自适应性等问题上的针对性设计能进一步提升算法性能,获得优于98%去噪精度[20-21]。
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大多数ICESat-2指标是以高度变化来表示,要求在整个飞行任务期间每年监测0.2 cm的高度偏差变化,激光载荷精确在轨标定对达到上述指标至关重要。对500 km轨道高度的ICESat-2而言,1″的指向角误差会导致约2.4 m的平面误差,若地面坡度为2°,则会产生8.3 cm的高程误差。
激光测高仪在轨标定与传统光学卫星和机载激光雷达有一定相似之处,主要有基于地面标定场法和基于自然地形法。地面标定场通过激光接收器直接测量足印点,精度最高,但需预估足印点位置并选择合适地点建设标定场。自然地形法通过提前精确测量的局部地形与卫星激光数据进行剖面配准,实现激光定标参数解算。如图3所示,具体解算时,可简化为标定2个角度参数和1个距离参数。有研究表明,使用1 km长度激光测线与高精度地形匹配,角度标定精度优于0.3″,测线长度增加到2.5 km,角度标定精度优于0.1″[22]。此外,ICESat-2还采取了与ICESat相似的策略,在海洋区域进行姿态机动,通过锥形扫描将姿态和距离分开标定,标定后的测距值长期漂移小于1 mm/a[6]。
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上一代激光测高卫星ICESat标称定位精度平面约10 m,高程约15 cm。ICESat-2作为后续星,平面定位精度标称6.5 m (后处理),高程方向需满足高程变化监测精度。验证评估ICESat-2点云精度对测绘应用意义重大,主要方法有基于轨迹交叉点数学分析法[23]、与地面实测点比较法、与机载数据比较法和与地面角反射器比较法等。交叉点分析法利用多轨交叉点的高程变化评估数据相对精度,常选择极区冰盖平坦区域。地面实测数据方面,NASA开展了2项地面GPS实地勘测用来检核ICESat-2高程精度,一是在格林兰岛Summit站附近进行的约7 km测线,二是在南极洲南极站以北约224 km的88°纬线(记作88S)附近,测量长度约300 km,与约227条ICESat-2地面轨迹相交。利用2018-2019阶段的88S数据评估结果显示:ICESat-2在极区高程精度为(5±13) cm (均值±
$1\sigma $ )[24]。此外,高程精度与地物类型有关,研究显示在汉中平地区域中误差约为0.59 m[25],在芬兰植被区域高程中误差约为0.85 m[26],因此有理由认为ICESat-2在不同类型陆地区域高程精度优于1 m。地面实测点法仅用作评估高程精度,评估水平精度时可采用角反射器和机载数据。88S测区布设了角反射器,但并未公开报道ICESat-2实际水平定位精度。机载数据检核时常采用精度较高的光学立体或激光雷达作为真值,让ICESat-2剖面点云在水平方向以一定步长移动,计算剖面点云与机载点云高程差绝对值,高程差最小时表示二者配准。如图4所示,汉中区域机载数据的评估结果表明,该试验数据水平偏差为−0.1 m (东)、−4.1 m (北),此处满足标称6.5 m指标。
Development and application of lidar mapping satellite
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摘要: 星载激光测绘技术的迅猛发展对传统卫星对地观测领域带来了新的突破,高精度对地观测数据有利于提升整体几何精度、高效率数据处理特性有利于实现快速应用,因此激光测绘卫星数据获取、处理和应用成为研究热点。首先介绍了激光测绘卫星原理特点和发展现状,然后以目前在轨激光雷达卫星ICESat-2为例,分析了载荷配置特点、数据处理方法以及在影像联合精确定位、多源地形融合、全球植被测量、浅海水深测绘等多个领域的测绘应用情况,最后就我国下一步发展激光测绘卫星谈一些思考。Abstract: The rapid development of laser altimetry satellite has a great impact on the traditional satellite earth observation field. The accurate elevation data can effectively compensate the lack of optical satellites, while laser altimetry satellite data processing and mapping applications are worthy of attention. Firstly, the principle and characteristics of laser altimetry satellite was introduced, and the its development history was presented. Then, the current on-orbit LiDAR satellite ICESat-2 was focued on, which had a high degree of attention. The satellite configuration characteristics and data processing methods were discussed, and its surveying and mapping capabilities in many fields, such as image joint adjustment, multi-source terrain fusion, global vegetation survey, and shallow water bathymetry et al. were analyzed and revealed. Finally, some thoughts about the development and construction of laser altimetry satellite in China were shared.
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表 1 国内外对地观测激光测高设备主要技术指标及用途
Table 1. Main technical specifications and applications of earth observation laser altimetry equipment at home and abroad
Satellite/
payloadLaunch time Collect method Beam number Pulse
width/nsPoint
interval/mElevation
accuracy/mApplication SLA-01/02 1996/97 Full waveform 1 10 750 1.5 Global elevation control point ICESat/GLAS 2003 Full waveform 1 6 170 0.15 Sea ice, atmosphere, land,
vegetation, etc.ZY3-02 2016 Full waveform 1 7 3500 1.0 Experimental measurement ICESat-2/ATLAS 2018 Photon counting 6 1.5 0.7 0.1 Sea ice, atmosphere, land,
vegetation, etc.GEDI 2018 Full waveform 8 14 60 1.0 Forest biomass GF-7 2019 Full waveform 2 4-8 2900 1.0 Elevation control point LIST To be launched Photon counting 1000 1 0.7 0.1 Earth observation 表 2 ICESat-2基本配置参数
Table 2. Basic parameters of ICESat-2
Item Value Orbital inclination/coverage 92°;Covering 88° between north and south Orbit type 91 days repeat orbit Orbit eight 500 km Pointing control accuracy 45 m Horizontal accuracy 6.5 m Laser wavelength 532 nm Laser repetition 10 kHz Number of beams 6 bundles, one strong and one weak to form 3 groups Beam spacing (vertical track) 90 m within the strong and weak group, 3.3 km between the groups Point cloud interval (along track) 0.7 m Footprint size <17.5 m -
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