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资源三号是我国首个高分辨率民用立体测图卫星星座,01星、02星分别于2012年、2016年成功发射,资源三号03星于2020年7月25日在太原卫星发射中心成功发射。除继承资源三号02星几何分辨率分别为2.1 m正视、2.7 m前后视的三线阵相机和5.8 m多光谱相机载荷外,资源三号03星还实现了国产星敏感器以及激光测高仪的业务化,同时将卫星设计寿命从原来的5年提高为8年。卫星运行于回归周期为59天的太阳同步回归圆轨道,通过三星组网,重访周期从3天缩短到1天,保证我国高分辨率立体测绘数据的长期稳定获取,形成全球领先的业务化立体观测能力[8]。
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资源三号03星激光测高仪在02星试验性载荷的基础上做了适当的优化改进,如表1所示。具体包括:设计寿命从试验性载荷变为8年,同时增加了一个备份激光器,激光发散角从150 μrad缩小为90 μrad,对应地面光斑大小约45 m,采用前后沿鉴别的方式,具有多阈值探测能力。
表 1 资源三号02/03星激光测高仪参数对比表
Table 1. Parameters comparison between ZY3-02 and ZY3-03 satellite laser altimeter
Parameter ZY3-02[15] ZY3-03 Number of beams Only one Two, one as backup Frequency/Hz 2 2 Central wavelength/nm 1 064.4 1 064.4 Pulse energy/mJ 200 150 Pulse width/ns 7 5 Divergence angle/μrad 150 90 Ground footprint size 75 m@500 km 45 m@500 km Pointing change/μrad ≤10 ≤5 Receiving aperture/mm 220 260 Total transmitted pulse >1×108 Lifetime 8 years Power consumption/W ≤50 ≤80 Weight/kg ≤40 ≤95 Terrain adaptability/(°) ≤2 ≤5 Ranging accuracy/m ≤1 ≤0.3 资源三号03星激光测高仪的测距原理如图1所示。受硬件资源等限制,03星激光测高仪的硬件误差主要包括发射波形时刻鉴别误差、回波时刻鉴别误差、晶振计时误差和TDC (Time Distance Calculator)时间间隔测量误差,平地理论测距精度优于0.3 m。
图 1 资源三号03星激光测高仪阈值探测测距原理图
Figure 1. Diagram of ZY3-03 satellite laser altimeter based on the threshold detection and ranging
资源三号03星激光测高仪回波有四个通道,包括三个宽带通道和一个窄带通道。其中宽带通道又分为三个子通道,分别为高阈值TH1子通道,低阈值TH2子通道和噪声监测阈值TH4子通道。窄带通道为TH3通道。噪声监测阈值TH4子通道类似于资源三号02星设计状态,仅记录门内第一个回波的前沿,输出测距值,同时可以输出距离内的过阈值脉冲个数,用于在轨检测噪声,指导阈值设置;其他TH1,TH2和TH3三个通道均记录门内回波脉冲的前沿和后沿,并进行输出,每个通道均具有记录四次回波脉冲的能力。
根据主波时刻和回波时刻计算激光测高仪的飞行时间为:
$${T_{tof}} = {T_{echo}} - {T_{tx}} - \Delta {T_{THi}}$$ (1) 式中:
$\Delta {T_{THi}}$ 为各通道延迟常数。对大气延迟影响
$\Delta R$ 等进行改正后,激光的测距值为:$$R = \frac{c}{2}{T_{tof}}{\rm{ - }}\Delta R$$ (2) -
对地观测卫星激光测高数据在获取过程中,受到激光指向、卫星轨道和姿态、大气、地球物理潮汐、大光斑内复杂地形地物等多种因素的影响,标准化的测绘处理是确保资源三号03星高精度激光测高的重要前提。
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如图2所示,根据卫星激光测高严密几何模型进行激光地面足印几何定位,结合卫星上搭载的GPS定位仪器和星敏感器获得的卫星位置和姿态信息,解算激光足印精确的三维坐标[16]。卫星激光测高严密几何定位模型如下式所示:
$$\begin{split}& \left( {\begin{array}{*{20}{c}} {{X_P}} \\ {{Y_P}} \\ {{Z_P}} \end{array}} \right) = \left( {\begin{array}{*{20}{c}} {{X_S}} \\ {{Y_S}} \\ {{Z_S}} \end{array}} \right){\rm{ + }}\;{{R}}_{J2000}^{WGS84}{{R}}_{Body}^{J2000}\\& \left[ {\left( {\begin{array}{*{20}{c}} {Lx} \\ {Ly} \\ {L{\textit{z}}} \end{array}} \right) - \left( {\begin{array}{*{20}{c}} {Dx} \\ {Dy} \\ {D{\textit{z}}} \end{array}} \right) + \left. {\left( {\begin{array}{*{20}{c}} {\rho \sin \theta \cos \alpha } \\ {\rho \sin \theta \sin \alpha } \\ { - \rho \cos \theta } \end{array}} \right)} \right]} \right.\end{split}$$ (3) 式中:
${\left[ {{X_P}\;{Y_P}\;{Z_P}} \right]^{\rm{T}}}$ 为激光足印点在WGS84坐标系下的三维坐标;${\left[ {{X_S}\;{Y_S}\;{Z_S}} \right]^{\rm{T}}}$ 为该激光足印点对应时刻卫星在WGS84坐标系的三维坐标;${{R}}_{Body}^{J2000}$ 为星敏感器测定的激光足印点对应时刻卫星本体坐标系与J2000坐标系之间的旋转矩阵;${{R}}_{WGS84}^{J2000}$ 为激光足印点对应时刻WGS84坐标系与J2000坐标系之间的旋转矩阵;${\left[ {Dx\;Dy\;Dz} \right]^{\rm{T}}}$ 为GPS的相位中心在卫星本体坐标系下的坐标;${\left[ {Lx\;Ly\;Lz} \right]^{\rm{T} }}$ 为激光参考中心在卫星本体坐标系下的坐标;$\; \rho $ 为激光所测距离值;$\theta $ 为激光指向与本体坐标系Z轴的负向夹角;$\alpha $ 为激光指向在XOY平面上的投影与X轴正向夹角。激光指向角也可以采用与卫星本体系三个轴的三个夹角进行描述,两者属于等价关系[1]。截止到2021年4月30日,资源三号03星激光发射次数超过125万次,有效点数约72万个,占比约57.6%,全球分布情况如图3所示。资源三号03星激光测高标准产品目前已在自然资源部国土卫星遥感应用中心实现业务化处理。
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高程控制点提取是复合测绘应用的重要基础,具备全波形采样的激光高程控制点提取方法已得到有效应用[11,17],但资源三号03星不具备全波形采样功能,文中在借鉴资源三号02星试验性激光测高载荷处理经验的基础上[5],结合资源三号03星激光测高仪阈值探测的特点,针对性地提出了高程控制点ECP (Elevation control Point)提取及质量标记的方法,具体流程如下:
(1)确保大气、潮汐、精密轨道和姿态等参数的有效性,对激光测高标准产品的完备性进行质量控制;
(2)利用全球30 m的地表覆盖数据GLC30,将落在水面的激光点标记为10;
(3)结合阈值探测和地形地物影响,结合波形阈值计算的脉宽,对没有落水的激光点,设定回波脉宽阈值为
${\tau _s}$ ≤20 ns时,默认光斑内坡度小于2°,此时激光点标记为1;若20 ns≤${\tau _s}$ ≤40 ns,默认坡度介于2°~5°,质量标记为2,其余的标记为3;(4)进一步地,对标记为1和2的激光点,借鉴资源三号02星激光点的提取思想,采用全球30 m格网的AW3D30产品,计算激光点高程与光斑范围内AW3D30平均高的差值dh。如果高差|dh|≤2 m,则原标记不变;若高差2 m<|dh|≤5 m,则原标记为1的点变为2;若|dh|>5 m,则原标记为1和2的点均变为3。
经筛选提取后,激光点标记为1的能用作高程控制点使用,标记为2的点仅推荐在标记为1的点较少的情况下参考使用,标记为3的误差较大不推荐使用,标记为10的代表该点落水。
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利用地面红外探测器是实现激光点绝对几何精度验证的最直观方法[18],但该方法成本较大。文中在实现激光点标准化测绘处理后,选择内蒙古苏尼特右旗和江苏苏州两个验证区,结合外业RTK-GPS和高精度机载LiDAR点云数据,对资源三号03星激光点精度进行验证和分析。
(1)在轨定标区附近验证结果
在内蒙古苏尼特右旗外场定标区附近,针对2020年9月8日、9月23日的资源三号03星实际激光落点位置,选择地形平坦、没有植被和地物影响的7个激光点,采用高精度的RTK-GPS测量方式,对每个激光点以约5 m的间隔测量其附近地面点的三维坐标,形成RTK-GPS点阵,如图4所示。
以激光点平面位置为中心,外扩25 m统计光斑范围内RTK-GPS的平均高作为参考,统计激光点的高程误差,结果如表2所示。该区域激光测高精度较高,高程精度为(0.051±0.232) m。
表 2 内蒙古苏尼特右旗资源三号03星激光点实地验证结果表(单位:m)
Table 2. Validation result of ZY3-03 laser points on Sunid Right Banner in Inner Mongolia (Unit: m)
Date ID Elevation Reference elevation Error 2020-09-08 86912108 1 007.364 1 007.218 0.146 86912110 1 052.404 1 052.183 0.221 2020-09-23 92095212 1 073.461 1 073.316 0.145 92095214 1 079.237 1 079.174 0.063 92095216 1 074.751 1 074.405 0.346 92095220 1 086.253 1 086.500 −0.247 92095222 1 094.673 1 094.992 −0.319 Statistical result Max 0.346 Min 0.051 RMSE 0.232 (2)附加ECP标记的江苏苏州验证结果
2020年10月10日资源三号03星第1183轨经过江苏苏州,有16个激光点位于验证区内。参考数据为高精度的机载LiDAR点云数据,获取时间为2018年,高程精度优于0.15 m,高程基准统一为WGS84大地高。针对进行了高程控制点ECP标记的资源三号03星激光测高标准产品,以激光点平面位置为中心,外扩25 m统计光斑范围内LiDAR点云数据平均高,计算其与激光点高程误差,结果如表3所示。
表 3 2020年10月10日的1183轨激光测高精度(单位:m)
Table 3. Laser elevation accuracy of the No.1183 orbit on 10th Oct, 2020 (Unit: m)
ID ECP flag Elevation Reference elevation Error Remarks 97960914 1 12.453 23.52 −11.067 Roof 97960916 1 12.054 12.97 −0.916 Near the building 97960920 2 20.576 14.66 5.916 Road with vegetation 97960924 1 12.677 13.36 −0.683 Cropland 97960926 1 10.524 10.69 −0.166 Cropland 97960928 2 14.608 19.92 −5.312 Building and tree 97960930 1 11.460 12.37 −0.910 Cropland 97960932 1 9.756 9.83 −0.074 Paddy field 97960934 2 30.476 31.19 −0.714 Roof 97960936 1 9.995 10.74 −0.745 Open space 97960938 3 14.560 11.75 2.810 Vegetation 97960940 2 11.699 9.97 1.729 Trees 97960942 1 10.525 11.15 −0.625 Cropland 97960944 2 31.953 12.52 19.433 Construction site 97960950 2 116.315 118.29 −1.975 Building 97960952 1 30.225 30.31 −0.085 Grassland 从表3中可以看出,在农田、草地、裸地等区域,03星激光点高程精度较高,在居民区、林地等有建筑、树木的区域,因光斑内地形地物复杂,激光点的高程精度相对较差,这些区域的激光点应该尽量避免使用。该验证区16个资源三号03星激光点的总体高程精度为(0.414±6.213) m,经过高程控制点自动提取后,控制点标记为1的有9个,占比约52.9%,除其中点号为97960914的激光点因位于居民区的建筑物附近,高程误差达到11 m外,剩下的8个点高程误差为(−0.526±0.624) m,满足1:50 000立体测图高程控制点精度要求[19]。
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为了提升资源三号03星三线阵立体影像的高程精度,利用资源三号03星光学立体影像和激光测高数据同平台获取的特点,开展了激光测高点与光学立体影像条带模式的联合测绘处理,评价利用激光测高点提升立体影像高程精度是否满足1∶50 000测图精度要求。选择了黑龙江试验区、河北太行山两个实验区,其中黑龙江实验区位于东北平原,面积约3万 km2,整个区域的地形起伏从海拔120~550 m,其中南部是平地地形,北部是丘陵和山地地形。河北太行山实验区位于河北省太行山地区,整个区域的地形起伏从海拔40~220 m,属于山地地形。
2020年9月4日和11月4日资源三号03星第663轨和1552轨分别经过河北太行山、黑龙江试验区,同步获取了三线阵立体影像和激光点数据。为了验证试验精度,分别采用分布于试验区内的RTK-GPS测量的115个和270个高精度外业点作为影像检查点、并取其中的9个作为平面或平高控制点,所有点的大地基准为WGS84坐标系,高程基准为WGS84椭球高,平面和高程精度均优于0.2 m,点位分布如图5所示。
图 5 实验区复合测绘数据和外业检查点分布示意图
Figure 5. Distribution of combined surveying data and field check points in experimental regions
在区域网平差中,自动匹配的连接点中误差优于0.3 pixel。如表4所示,为了有效验证和对比激光与影像复合测绘的实验效果,每个试验区均开展了如下4组区域网平差实验:
表 4 实验区不同平差实验结果统计表
Table 4. Statistics result of different adjustment in the experimental regions
Region Group Laser elevation control point Horizontal control point Ground control point Check point RMS of check point/m Horizontal Elevation Taihang mountain A 0 0 0 115 26.99 11.25 B 0 0 9 106 5.98 3.32 C 6 0 0 115 27.43 4.45 D 6 9 0 106 5.97 4.31 Heilongjiang flat A 0 0 0 270 15.26 5.27 B 0 0 9 256 4.99 2.41 C 91 0 0 270 15.35 2.58 D 91 9 0 256 5.04 2.56 A: 无任何控制条件下光学立体影像自由网平差,验证立体影像的原始几何精度;
B: 选取少量RTK-GPS点作为平高控制点,针对立体影像开展区域网平差,验证常规控制条件下立体影像的几何精度。
C: 利用激光点作为高程控制点,开展立体影像与激光点联合区域网平差,验证利用激光测高数据提升光学立体影像的效果;
D: 利用激光点作为高程控制,选取少量RTK-GPS点作为平面控制点,开展联合区域网平差,验证引入平面控制时,利用激光测高数据提升光学立体影像的效果。
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我国1∶50 000立体测图成果的平面精度要求为平地优于25 m、山地优于37.5 m,高程精度要求为平地优于3 m、山地优于5 m。
对比实验组A和B可以发现,两个验证区在完全无控制的情况下,平面精度能达标,但高程无法直接满足要求,利用外业控制点后均能满足相关测图规范要求。实验组C结果表明,单独采用激光点作为高程控制,在没有其他控制数据的情况下,对立体影像的平面精度几乎没有影响,但高程精度有较大提升,其中河北太行山区从11.25 m提高到4.45 m,黑龙江地区从5.27 m提高到2.58 m,均能满足1∶50 000测图要求。
对比实验组B和C,黑龙江地区基于激光点和高精度地面控制点的高程精度结果分别为2.58 m和2.41 m,河北太行山区两者的高程精度提升对比结果为4.45 m和3.32 m,说明平原地区激光点的效果更明显,基本可相当于外业控制点的水平。山区则因目前的阈值探测模式及地形限制,点的分布如图5(a)所示可能有一定欠缺,导致结果比外业控制点稍差,但是高程精度也已经满足我国1∶50 000山地区域高程精度要求。
实验组D的结果则表明,在激光与光学立体影像联合区域网平差中加入适量的平面控制点后,在不影响激光点对立体影像高程精度提升效果的同时,可进一步提升试验影像的平面精度,使之完全满足我国1∶50 000比例尺立体测图精度要求。
Laser altimetry data processing and combined surveying application of ZY3-03 satellite
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摘要: 资源三号03星是自然资源部主持建造的用于1∶50 000立体测图的陆地遥感业务卫星,该星装备了业务化的激光测高仪,主要用于获取高精度高程控制点。论文针对资源三号03星激光测高数据,研究了标准化测绘处理流程和高程控制点提取方法,在内蒙古苏尼特右旗和江苏苏州开展了精度验证,并选择黑龙江和河北两个实验区开展了复合测绘应用验证。精度验证结果表明,资源三号03星激光点在内蒙古苏尼特右旗平坦区域高程精度为(0.051±0.232) m,在江苏苏州城市建成区的激光点总体精度为(0.414±6.213) m,经高程控制点提取和质量标记后的激光点高程误差为(−0.526±0.624) m,能满足1∶50 000测图高程控制需求。复合测绘应用表明,利用资源三号03星激光高程控制点,立体影像高程精度在黑龙江平坦地区能从5.27 m提高到2.58 m,河北太行山区能从11.25 m提高到4.45 m;无论是平地还是山区,资源三号03星激光高程控制点均能有效提高立体影像的高程精度并满足1∶50 000测图需求。Abstract: ZY3-03 is a land remote sensing satellite for 1: 50 000 stereo mapping built by the Ministry of Natural Resources. It is equipped with operational laser altimeter, which is mainly used to obtain high-precision elevation control points. In this paper, aiming at the laser altimetry data of ZY3-03 satellite, the standardized surveying processing flow and the method of extracting elevation control points is studied. Moreover, the accuracy verification in Sunid Right Banner of Inner Mongolia and Suzhou of Jiangsu Province is implemented, and the combined surveying and mapping application in Heilongjiang and Hebei areas is experimented and validated. The accuracy verification results show that the elevation accuracy of ZY3-03 laser points in the flat area of Sunid Right Banner in Inner Mongolia is (0.051±0.232) m, and the overall accuracy of the laser points in the urban area of Suzhou, Jiangsu is (0.414±6.213) m, and the elevation accuracy after elevation control points extraction is (−0.526±0.624) m, which can meet the elevation control requirement of 1:50 000 mapping; The application of combined surveying and mapping shows that the elevation accuracy of stereo images can be improved from 5.27 m to 2.58 m in flat area of Heilongjiang and from 11.25 m to 4.45 m in Taihang mountain area of Hebei by using laser elevation control points derived from the ZY3-03 satellite. It is concluded that the elevation accuracy of stereo images can be effectively improved by using laser elevation control points of ZY3-03 satellite in both flat and mountainous areas, and the requirement of 1:50 000 mapping can be met.
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Key words:
- ZY3-03 /
- laser altimetry /
- combined surveying /
- elevation control point /
- global mapping
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表 1 资源三号02/03星激光测高仪参数对比表
Table 1. Parameters comparison between ZY3-02 and ZY3-03 satellite laser altimeter
Parameter ZY3-02[15] ZY3-03 Number of beams Only one Two, one as backup Frequency/Hz 2 2 Central wavelength/nm 1 064.4 1 064.4 Pulse energy/mJ 200 150 Pulse width/ns 7 5 Divergence angle/μrad 150 90 Ground footprint size 75 m@500 km 45 m@500 km Pointing change/μrad ≤10 ≤5 Receiving aperture/mm 220 260 Total transmitted pulse >1×108 Lifetime 8 years Power consumption/W ≤50 ≤80 Weight/kg ≤40 ≤95 Terrain adaptability/(°) ≤2 ≤5 Ranging accuracy/m ≤1 ≤0.3 表 2 内蒙古苏尼特右旗资源三号03星激光点实地验证结果表(单位:m)
Table 2. Validation result of ZY3-03 laser points on Sunid Right Banner in Inner Mongolia (Unit: m)
Date ID Elevation Reference elevation Error 2020-09-08 86912108 1 007.364 1 007.218 0.146 86912110 1 052.404 1 052.183 0.221 2020-09-23 92095212 1 073.461 1 073.316 0.145 92095214 1 079.237 1 079.174 0.063 92095216 1 074.751 1 074.405 0.346 92095220 1 086.253 1 086.500 −0.247 92095222 1 094.673 1 094.992 −0.319 Statistical result Max 0.346 Min 0.051 RMSE 0.232 表 3 2020年10月10日的1183轨激光测高精度(单位:m)
Table 3. Laser elevation accuracy of the No.1183 orbit on 10th Oct, 2020 (Unit: m)
ID ECP flag Elevation Reference elevation Error Remarks 97960914 1 12.453 23.52 −11.067 Roof 97960916 1 12.054 12.97 −0.916 Near the building 97960920 2 20.576 14.66 5.916 Road with vegetation 97960924 1 12.677 13.36 −0.683 Cropland 97960926 1 10.524 10.69 −0.166 Cropland 97960928 2 14.608 19.92 −5.312 Building and tree 97960930 1 11.460 12.37 −0.910 Cropland 97960932 1 9.756 9.83 −0.074 Paddy field 97960934 2 30.476 31.19 −0.714 Roof 97960936 1 9.995 10.74 −0.745 Open space 97960938 3 14.560 11.75 2.810 Vegetation 97960940 2 11.699 9.97 1.729 Trees 97960942 1 10.525 11.15 −0.625 Cropland 97960944 2 31.953 12.52 19.433 Construction site 97960950 2 116.315 118.29 −1.975 Building 97960952 1 30.225 30.31 −0.085 Grassland 表 4 实验区不同平差实验结果统计表
Table 4. Statistics result of different adjustment in the experimental regions
Region Group Laser elevation control point Horizontal control point Ground control point Check point RMS of check point/m Horizontal Elevation Taihang mountain A 0 0 0 115 26.99 11.25 B 0 0 9 106 5.98 3.32 C 6 0 0 115 27.43 4.45 D 6 9 0 106 5.97 4.31 Heilongjiang flat A 0 0 0 270 15.26 5.27 B 0 0 9 256 4.99 2.41 C 91 0 0 270 15.35 2.58 D 91 9 0 256 5.04 2.56 -
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