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Geolocation Approach for FY-3A MERSI Remote Sensing Image
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摘要: 风云三号A星 (FY-3A) 中分辨率光谱成像仪 (MERSI) 采用45°镜旋转扫描，形成垂直于卫星飞行轨迹的360°连续圆周扫描方式，多元探测器并扫的技术。该研究依据这种扫描特性，给出了适用于FY-3A MERSI遥感图像地理定位的方法；定义了完善的坐标系及坐标系转换关系，根据MERSI观测几何、卫星空间位置和姿态、仪器空间位置和指向建立了探测器像元观测矢量与地面位置之间关系的模型；通过地形校正消除地形起伏带来的定位误差；在FY-3A地面应用系统中业务运行的同时，通过一定数量的地面控制点分析，将定位误差等效为遥感仪器安装误差，修正了MERSI的仪器指向角度。实验结果表明，使用该方法对MERSI遥感图像地理定位精度达到250 m像元级，满足MERSI图像的高精度定位要求。Abstract: Fengyun 3 series are second generation polar-orbiting meteorological satellites in China. The first satellite of Fengyun 3 series, FY-3A, is a research and development satellite and is launched successfully at 1100 BT 27 May 2008. The Medium Resolution Spectral Imager (MERSI) is a main payload of FY-3A spacecraft and since June 2008, it has been acquiring daily global data in 20 spectral bands from the visible to the thermal infrared-15 with 1 km and 5 channels with 250 m spatial resolution at nadir. In order to satisfy the requirements of quantitative application, MERSI data need to be geolocated, which can provide accurate latitude and longitude information for follow-up remote sensing productions. The geolocation algorithm of MERSI data, which is adapted by National Satellite Meteorological Center (NSMC), is introduced in details, as well as the error analysis.MERSI rotates a 45° mirror to get 360° views across the orbit direction. MERSI is a paddle broom electro-optical instrument that uses the forward motion of the satellite to provide the along-track direction of scanning. MERSI simultaneously senses, in each band, 10 rows of 1 km detector pixels and 40 rows of 250 m detector pixels. The MERSI detectors are grouped on four focal planes. Detectors for each band are laid on the focal planes in the along-scan direction. MERSI swath is about 2000 km, and generates about 140 GB data per day.According to the characteristics above, parameter method is used for MERSI remote sensing data geolocation. This approach creates the spatial relationship model between the sensed data and the earth based coordinate system, according to MERSI scanning mode, sensing geometry, satellite/sensor's attitude and position. There are 10 coordinate systems and 9 rotated relationships involved in the model. Since 45° mirror brings image rotation, there is a module removing the rotation in the model.When processing data, the line-of-sight vector from each detector of a band is calculated in the instrument coordinate system first. Then the line-of-sight and satellite position are rotated to the earth centered rotating coordinates. The intersection of the line-of-sight with the WGS-84 ellipsoid is calculated. An iterative search process is used to follow the line-of-sight from the instrument to the intersection of the terrain surface represented by a DEM. This geolocation approach for MERSI has been applied to FY-3A data preprocessing system.Five factors that influence the accuracy of results are analyzed. These factors include satellite position/velocity error, satellite attitude error, satellite-instrument installation error, instrument-inner geometry error and instrument thermal distortion.Compared geolocation results with the true remote sensing image using the land-water mask, it shows that the error alone the orbit direction is about 0.167 km and the error along the scanning direction is about 0.058 km. This geolocation approach for MERSI (250 m) achieves accuracy up to 1 pixel.
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Key words:
- FY-3A;
- MERSI;
- multi-detectors scanning;
- sensing geometry modeling;
- geolocation
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表 1 各种坐标系定义
Table 1 Definition of coordinate systems
坐标系 原点 参考平面 x轴方向 z轴方向 焦平面坐标系 系统光轴与焦平
面的交点焦平面 沿扫描方向 与焦平面垂直指向焦
点方向扫描镜坐标系 45°扫描镜与系统
光轴的交点沿45°扫描镜的旋转轴
方向45°扫描镜中心点指向
星下点的方向望远镜坐标系 系统光轴与焦平
面的交点x轴垂直于扫描镜的旋
转轴，并且沿光线经扫
描镜在0°扫描角时反
射之后的扫描方向平行于系统光轴指向
卫星飞行方向K镜坐标系 K镜的几何中心
与系统光轴的交点沿K镜的旋转轴方向 与扫描镜坐标系z轴方
向一致仪器坐标系 仪器质心 仪器安装平面 与卫星滚动轴指向一致 与卫星偏航轴指向一致 卫星本体
坐标系卫星质心 卫星滚动轴与俯
仰轴所在平面与卫星滚动轴指向一致 与卫星偏航轴指向一致 轨道坐标系 卫星质心 卫星轨道平面 与卫星速度方向一致 从卫星质心指向地心 地心惯性
坐标系地心 J2000.0平赤道面 指向J2000.0的平春分点 指向J2000.0的平北天极 地心旋转
坐标系地心 国际时间局
BIH1984.0时元定
义的协议地球赤
道面指向国际时间局
BIH1984.0时元定义的
零子午面和协议地球
赤道的交点平行于国际时间局
BIH1984.0时元定义的
协议地球极轴方向大地测量
坐标系地心 
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表 2 利用地面控制点的误差分析
Table 2 Ground control point residuals measured before and after updates to the sensor interior orientation parameters
测量 地面控制点个数 扫描方向定位误差/m 跨轨方向定位误差/m 平均值 标准偏差 平均值 标准偏差 首次 60 1670 454 304 316 更新后 60 167 343 58 327
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