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空间激光测高主要依赖于脉冲激光测距技术的发展,其发展历史可以追溯到1962年,Mc Clung和Hellwarth发明调Q脉冲激光器[2],产生峰值功率足够高的巨脉冲激光,使得远距离飞行时间脉冲激光测距成为可能。调Q脉冲激光技术发明后即被应用于远距离测量,并于1971年随阿波罗15号开展首次空间应用,1971~1972年,阿波罗15-17号利用搭载在指令舱的红宝石脉冲激光器,开展多次绕月激光测距探测[1, 3],但由于技术限制,当时开展探测的激光器每20 s才发射一次激光脉冲,导致探测效率极低,3次搭载激光测高载荷的阿波罗探月任务,仅获取了几千次月球表面高程数据。20世纪90年代,随着激光器技术的进步,尤其是半导体泵浦技术极高提高了激光器的重复频率和寿命,飞行时间脉冲激光测距技术在空间探测领域中得到快速发展。1971年至今,国内外发射了大量的空间激光测高载荷(表1),大部分由美国国家航空航天局(NASA)主导,探测目标包括地球、月球、火星、小行星等,如2003年NASA发射的冰、云、陆地高程探测卫星搭载的GLAS地球科学激光测高系统[4]、NASA于2009年成功实施的月球勘测轨道飞行器(LRO)任务上首次搭载的多波束对月激光测高仪(LOLA)[5]、NASA于2018年成功实施的冰、云、陆地高程探测卫星二代上搭载的ATLAS先进地形测高系统[6-7]等。
表 1 1970~2019年国内外空间激光测高载荷发展历史
Table 1. Development history of space laser altimetric payloads at home and abroad from 1970 to 2019
Launch time Country Payload Space-based platform Major mission 1971-1972 USA Lunar laser altimetry Apollo 15-17 The world’s first lunar surface elevation measurement mission. Limited by the laser technology, only several thousand measurements were taken during the three missions 1992 USA Mars observer laser altimeter Mars observer This mission was planned to measure the Mars surface elevation. But the satellite was failed to enter the Mars orbit 1994 USA Lunar laser altimetry Mentine orbiter The world’s first lunar surface elevation measurement mission 1996-1997 USA Shuttle laser altimeter STS-72 The SLA mission carried out twice space laser altimetry experiments, acquired about 80 hours data, and established a global elevation control point database for the first time 1996 USA Mars orbiter laser altimeter Mars Global Surveyor The world's first successfully Martian elevation measurement mission 2003 USA Geoscience Laser Altimeter System Ice, cloud,and land elevation satellite World's first laser altimetry system for long-term continuous observation of the earth 2003 Japan Laser altimetry Hayabusa spacecraft The system successfully detected the three-dimensional shape of the Itokawa asteroid at 2005 2004 USA Mercury laser altimeter MESSENGER spacecraft The spacecraft entered the orbit of Mercury at 2011 and carried out three-dimensional survey of the northern hemisphere of Mercury during the observation period of one earth year 2007 China Laser altimeter Chang’e-1 China firstly carried out the lunar elevation observation. The payload acquired more than 9 million measurements until 2009 2007 Japan Laser altimeter SELENE satellite Japan firstly carried out the lunar elevation observation. 2008 India Lunar Lnging Instrument Chandrayaan-1 India firstly carried out the lunar elevation measurement. 2009 USA Lunar orbiter laser altimeter Lunar reconnaissance orbiter The 5-beam design is used to obtain the highest resolution 3D topographic map of the lunar surface so far. 2010 China Laser altimeter Chang’e-2 The second lunar elevation measurement program by China. 2013 China Laser three-dimensional imaging sensor Chang’e-3 The payload quickly scanned the lunar terrain at the final stage of Chang’e-3 soft landing. 2016 China Laser altimter Ziyuan-3 (02) satellite The first space-based lidar observation of earth surface elevation by China. 2018 USA Advanced topographic laser altimeter system Ice, cloud,and land elevation satellite-2 The world’s first photon counting lidar for earth surface elevation measurement. ATLAS largely improved the elevation measurement efficiency than previous space-based laser altimeter. It also has the ability to measure the atmospheric aerosol and clouds 2018 USA Global ecosystem dynamics investigation International space
stationISS-based lidar altimeter for forest observation with repetition rate of 242 Hz to achieve nearly successive laser footprint observation 2018 China Laser three-dimensional imaging sensor Chang’e-4 The terrain scanning lidar is used for the first Lunar backside soft landing 2019 China Laser altimeter GF-7 The laser altimeter was used for accurate elevation control points measurement of 3D surveying and mapping satellite 中国空间激光测高技术起步较晚,但也在嫦娥探月工程及高分辨率对地观测计划中得到了充分应用。嫦娥一号到四号均搭载了激光测高载荷,开展工程或者科学应用;2019年,高分七号卫星成功发射,其上搭载的激光测高分系统成功开机并获取数据,为中国立体测绘卫星地面高程控制点提供精确的三维坐标数据。下文将重点对国内外典型空间激光测高载荷进行描述。
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GLAS地球科学激光高度计是首个开展长期地球表面高程探测的载荷,搭载于2003年发射的冰、云、陆地高程卫星(图1)上(ICESat: Ice, Cloud and land Elevation Satellite),是美国于20世纪80年代制定的地球观测系统(EOS: Earth observation system)的一部分。ICESat卫星轨道高度大约600 km,GLAS载荷配备3台激光器,其中2台为备份激光器,GLAS开机后仅工作36天,其第一台激光器即由于故障原因停止工作,为了保证针对南北极冰盖厚度变化这一主要科学目标的正常实施,GLAS每年仅开展3次观测,每次观测持续30~50天,以保证激光器在载荷科学目标达成前仍能正常工作。
GLAS载荷的主要科学任务包括:(1)测量地球南北极冰盖高程,研究冰川消融对海平面变化的影响;(2)利用532 nm波段激光,测量云和气溶胶的垂直结构,获取云顶和云底高度、云后向散射系数垂直廓线、薄云的光学厚度等信息;(3)测量冰面高度、陆地地形和森林植被的树冠高度,获取全球尺度的地表粗糙度、反射率、植被高度等特征。
GLAS的激光器采用二极管泵浦调Q结构,同时输出1 064 nm基频和532 nm倍频激光脉冲,单脉冲能量分别为75 mJ和32 mJ。GLAS利用Nd:YAG激光器的1064 nm基频光开展地表高程探测,采用全波形探测体制,可以获取陆地表面植被的垂直分层信息,实现树冠和地表高程的同时探测,借助于高精度激光指向测量技术,GLAS可以实现在地面坡度小于3°的情况下10 cm的测距精度。GLAS计划利用532 nm激光开展气溶胶、云垂直分布廓线的探测,但是由于倍频模块的不稳定性,其532 nm激光能量迅速降低,因此其针对大气的探测任务并未顺利实施。
图2展示了一个观测周期内,GLAS观测的南北极高程结果,利用GLAS的长期测量数据,研究人员展示了全球变暖导致的北极海冰消融趋势[8]。随着最后一台激光器停止工作,GLAS载荷的任务在2009年10月终止。GLAS载荷为美国后续空间激光任务积累了大量经验,NASA后续空间激光探测任务中,激光器的稳定性得到大幅度提升,例如,2006年发射的CALIPSO激光大气探测卫星,其激光器与ICESat卫星均由Fibertek公司提供,CALIPSO卫星激光雷达的寿命远超GLAS,从发射在轨运行至今仍在正常采集数据。
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ATLAS是世界上首个基于单光子探测技术的空间激光测高载荷,于2018年9月搭载于ICESat-2卫星发射升空,卫星工作于500 km公里圆形轨道,ATLAS工作于Nd:YAG激光器的二倍频(532 nm),这样可以用更为成熟的光电倍增管开展单光子探测,激光重复频率也提高到10 kHz,这样相邻激光足印的间距不超过1 m,可以实现近乎连续的星下激光足印测量,提高地形、植被、树冠点云数据的密度[7, 9]。ATLAS的激光器脉冲宽度大约1.5 ns,对单光子探测体制下测距精度的提高具有重要意义,采用了大量先进的技术,倍频效率高达70%,整体电光转换效率也高达8.2%,有利于降低激光器功耗,减小卫星资源需求,实现多波束对地探测。如图3所示。
ATLAS由于采用光子探测体制,单波束能量仅40~170 μJ即实现500 km距离下的对地测距。载荷设计了6个波束(图4),分为三个强能量波束和三个弱能量波束,以适应不同反射率目标的测量,提高反射率的定量反演精度。
图 4 ICESAT-2卫星激光载荷地面激光足印分布
Figure 4. Ground laser foorprint distribution of laser payload on ICESat-2
ATLAS所工作的532 nm波段还具备一定的水体穿透能力,能对部分浅水区域水底高程进行探测。图5为星下足印从山坡到水下的ATLAS原始单光子点云数据,该数据清晰地展示了被植被覆盖的陆地表面、海面、浅滩水底的地形变化,还展现了海洋波浪的周期性形态特征。ATLAS的水深测量能力受限于水体衰减及散射特征、水底地物反射率,从数据上来看,ATLAS最大实现了超过30 m水深的测量[9]。
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LOLA是世界上第一个在月球轨道上开展多波束月表高程探测的载荷,它于2009年6月18日搭载于美国发射的月球勘测轨道飞行器(LRO)上升空[5],LOLA同样工作于1 064 nm波段,轨道高度50 km,利用衍射光学器件将激光分为5个不同方向的波束,在提高点云数据获取率的同时,还可以实现月表斜率、粗糙度等信息的获取。LOLA在接收系统焦平面采用阵列探测器接收回波信号。图6展示了LOLA的主体结构、衍射光学器件及月表激光足印分布,LOLA所获取的高程、反射率、粗糙度、斜率等信息,可以反映不一样的月面特征,LOLA获得了迄今为止最清晰的月面三维高程图。图7为月球勘测轨道飞行器上搭载的月球激光测高仪测得的不同参数的分布。
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嫦娥一号和二号上均搭载了激光高度计[10](图8),实现了我国空间激光测高载荷的首次空间应用。嫦娥1/2号激光高度计由中国科学院上海技术物理研究所研制,工作于Nd:YAG激光器的1064 nm基频光,单脉冲能量150 mJ,月表光斑直径大约600 m。月面回波由直径134 mm望远镜接收,经窄带滤光片滤除大部分太阳背景辐射后,汇聚至雪崩光电二极管,再经阈值检测电路实现月面高程直接获取,嫦娥1/2号搭载的激光高度计具备以5 m精度(3σ置信度)实现月球表面高程探测的能力。利用嫦娥1号激光高度计的数据,我国首次实现了月表DEM的高精度获取(图9),为后续嫦娥系列着陆任务的选址提供了基本数据。
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资源三号02星是资源三号系列卫星的第二颗,于2016年5月发射,搭载我国首个对地激光测高试验载荷[11],开展线性探测体制下的对地测距应用。载荷工作于1064 nm,激光脉冲宽度大约6.5 ns,单脉冲能量175 mJ,有效接收孔径210 mm,卫星轨道高度505 km,与嫦娥1/2号激光测高仪一样,采用阈值检测方式实现地球表面高程探测。资源三号02星激光测高载荷经在轨检校后,在小于2°坡度情况下,平面定位精度优于15 m,高程精度优于1.0 m[12]。
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高分七号是我国第一颗应用于开展天基对地立体测绘的卫星,它的主载荷包括双线阵立体相机和激光测高仪,中国科学院上海技术物理研究所负责开展激光测高仪的研制,该载荷具备2个波束的激光测高能力,其获取的地表高程数据,能为双线阵立体相机提供高精度控制点,为基于摄影测量学的立体测绘提供绝对高程参考。图10为高分七号激光测高仪的整体结构,它的发射系统包括4台配置激光扩束系统的Nd:YAG激光器,采用2主2备的形式,在任意时刻均有2台激光器同时工作,搭配接收系统中的两个方向信号采集系统,实现双波束对地激光测高。
高分七号激光测高仪2个波束角度大约1.4°,对应地面激光足印间距大约12.5 km,激光重复频率3 Hz,采用0.6 m的望远镜接收地球表面回波信号,图11展示了地面激光足印分布。高分七号采用全波形探测体制,相比我国之前空间激光测高载荷开展的阈值检测测距方式,它能对地表附近的回波进行高速采样,采样率达到2 GSps,可以分辨激光落点区域的树冠、地面、建筑等信息,全波形探测获取的树高数据,能为森林生态系统研究提供数据。全波形探测在数据后处理上更加灵活,利用波形分解技术,可以实现足印下多目标的同时探测。高分七号激光测高仪,对我国开展全球范围内1∶1万比例尺测绘奠定基础。
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表2列出了目前成功实施的空间激光测高任务的主要技术指标,从探测目标来看,目前的空间激光测高载荷主要针对地球于太阳系其他星体表面高程测量任务开展,利用卫星对地形的不断重复探测,通过长时间的数据累积来实现全星球尺度的高程获取,或者通过对卫星的高精度轨道控制,来实现高精度的足印重返,为长时间尺度全球变化研究提供可对比的长期观测数据。
表 2 国内外已成功实施的部分空间激光测高载荷主要技术指标
Table 2. Major technical specifications of some laser altimetry payloads that have been successfully implemented at home and abroad
Payload GLAS Laser altimeter LOLA Laser altimeter ATLAS Laser altimeter Launch time 2003 2007/2010 2009 2016 2018 2019 Platform ICESat Chang’e-1/2 LRO Ziyuan-3(02) ICESat-2 GF-7 Observation Earth surface elevation Lunar surface elevation Lunar surface elevation Earth surface elevation Earth surface elevation Earth surface elevation Orbit height 600 50 50 505 500 500 Telescope diameter/m 1.0 0.13 0.15 0.21 0.8 0.6 Laser wavelength/nm 1 064/532 1 064 1 064 1 064 532 1 064 Laser repetition rate/Hz 40 1-5 28 2 10 3 Pulse energy 35-75 mJ 150 mJ 2.7 mJ 175 mJ 48-170 μJ 180 mJ Beam number 1 1 5 1 6 2 Detection method Linear detection Linear detection Linear detection Linear detection Photon counting Linear detection 从技术体制上来看,空间激光测高载荷主要采用线性探测体制,利用阈值检测或者全波形采集方式,实现距离的准确测量。线性探测体制对单波束激光能量要求较高,在卫星资源受限情况下,这种依赖于强目标散射信号的探测体制很难实现多波束探测,且很难提高激光重频以增加点云沿轨密度。随着2018年ICESAT-2上搭载的ATLAS载荷的成功开始数据采集,单光子探测技术在空间激光测高中开始成功应用,这种对单波束资源需求更少的技术可以提高激光测高载荷的波束数,实现高密度的数据获取。
Development and review of space-based laser altimetry technology
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摘要: 空间激光测高技术通过在天基平台搭载脉冲激光测距仪,实现行星表面形貌信息获取,是开展对地和深空探测的重要手段。系统梳理了空间激光测高技术的发展历史和现状,提出应向多波束观测、近海及岛礁水下地形探测、激光主动多光谱探测这三个方向发展,建议开展近红外波段单光子探测、超窄带光学滤波、多光子分辨探测、单光子数据星上预处理、高转换效率激光及非线性光频变换关键技术研究,促进我国空间激光测高技术的进一步发展。Abstract: Space-based laser altimetry, which measures the planetary surface topography based on time-of-flight ranging, is an important technology for earth and deep space exploration. The development history and current status were sorted out systematically and development in three directions was proposed, including multi-beam observation, underwater terrain observation of offshore and islands and laser active multispectral detection. It was suggested that some key technologies should be developed, including infrared single photon detection, ultra-narrowband optical filtering, multiphoton resolution detection, single-photon data on-board preprocessing and high conversion efficiency laser and nonlinear optical frequency conversion, to promote the further development of Chinese space laser altimetry technology.
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Key words:
- laser altimetry /
- lidar /
- single photon detection /
- multi-beam
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表 1 1970~2019年国内外空间激光测高载荷发展历史
Table 1. Development history of space laser altimetric payloads at home and abroad from 1970 to 2019
Launch time Country Payload Space-based platform Major mission 1971-1972 USA Lunar laser altimetry Apollo 15-17 The world’s first lunar surface elevation measurement mission. Limited by the laser technology, only several thousand measurements were taken during the three missions 1992 USA Mars observer laser altimeter Mars observer This mission was planned to measure the Mars surface elevation. But the satellite was failed to enter the Mars orbit 1994 USA Lunar laser altimetry Mentine orbiter The world’s first lunar surface elevation measurement mission 1996-1997 USA Shuttle laser altimeter STS-72 The SLA mission carried out twice space laser altimetry experiments, acquired about 80 hours data, and established a global elevation control point database for the first time 1996 USA Mars orbiter laser altimeter Mars Global Surveyor The world's first successfully Martian elevation measurement mission 2003 USA Geoscience Laser Altimeter System Ice, cloud,and land elevation satellite World's first laser altimetry system for long-term continuous observation of the earth 2003 Japan Laser altimetry Hayabusa spacecraft The system successfully detected the three-dimensional shape of the Itokawa asteroid at 2005 2004 USA Mercury laser altimeter MESSENGER spacecraft The spacecraft entered the orbit of Mercury at 2011 and carried out three-dimensional survey of the northern hemisphere of Mercury during the observation period of one earth year 2007 China Laser altimeter Chang’e-1 China firstly carried out the lunar elevation observation. The payload acquired more than 9 million measurements until 2009 2007 Japan Laser altimeter SELENE satellite Japan firstly carried out the lunar elevation observation. 2008 India Lunar Lnging Instrument Chandrayaan-1 India firstly carried out the lunar elevation measurement. 2009 USA Lunar orbiter laser altimeter Lunar reconnaissance orbiter The 5-beam design is used to obtain the highest resolution 3D topographic map of the lunar surface so far. 2010 China Laser altimeter Chang’e-2 The second lunar elevation measurement program by China. 2013 China Laser three-dimensional imaging sensor Chang’e-3 The payload quickly scanned the lunar terrain at the final stage of Chang’e-3 soft landing. 2016 China Laser altimter Ziyuan-3 (02) satellite The first space-based lidar observation of earth surface elevation by China. 2018 USA Advanced topographic laser altimeter system Ice, cloud,and land elevation satellite-2 The world’s first photon counting lidar for earth surface elevation measurement. ATLAS largely improved the elevation measurement efficiency than previous space-based laser altimeter. It also has the ability to measure the atmospheric aerosol and clouds 2018 USA Global ecosystem dynamics investigation International space
stationISS-based lidar altimeter for forest observation with repetition rate of 242 Hz to achieve nearly successive laser footprint observation 2018 China Laser three-dimensional imaging sensor Chang’e-4 The terrain scanning lidar is used for the first Lunar backside soft landing 2019 China Laser altimeter GF-7 The laser altimeter was used for accurate elevation control points measurement of 3D surveying and mapping satellite 表 2 国内外已成功实施的部分空间激光测高载荷主要技术指标
Table 2. Major technical specifications of some laser altimetry payloads that have been successfully implemented at home and abroad
Payload GLAS Laser altimeter LOLA Laser altimeter ATLAS Laser altimeter Launch time 2003 2007/2010 2009 2016 2018 2019 Platform ICESat Chang’e-1/2 LRO Ziyuan-3(02) ICESat-2 GF-7 Observation Earth surface elevation Lunar surface elevation Lunar surface elevation Earth surface elevation Earth surface elevation Earth surface elevation Orbit height 600 50 50 505 500 500 Telescope diameter/m 1.0 0.13 0.15 0.21 0.8 0.6 Laser wavelength/nm 1 064/532 1 064 1 064 1 064 532 1 064 Laser repetition rate/Hz 40 1-5 28 2 10 3 Pulse energy 35-75 mJ 150 mJ 2.7 mJ 175 mJ 48-170 μJ 180 mJ Beam number 1 1 5 1 6 2 Detection method Linear detection Linear detection Linear detection Linear detection Photon counting Linear detection -
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