地表反照光对天基空间目标的成像影响

天基空间目标观测时, 在对空间目标的可见光特性提取的过程中, 随着其姿态的不断变化, 存在太阳光照射不到目标表面、或是在某一特定位置由于目标的强烈反射导致太阳光照度不均匀的情况. 针对这些问题, 分析了在可见光和近红外波段空间目标表面的光照特性, 提出了利用地表反照光作为天基空间目标成像辅助照明光源的思想, 给出了一种精确的建模方法. 基于漫反射模型建立了地表反照光在空间目标表面的照度计算方法, 借助satellite tool kit卫星工具软件获取太阳、目标的坐标, 省去了以往方法中烦琐的矩阵相乘和坐标转换过程; 对任意时刻的地表有效反照区域给出了判断, 引入对地球表面进行网格划分的方法, 对划分后的每一个面元均匀采样, 通过数值积分可计算出整个有效地表反照区域的地表反照辐射. 以某天基成像任务中的太阳同步轨道卫星为例, 就地表反照光对目标的光照情况进行仿真, 计算得出卫星在经过北极上空时可以利用地表反照光作为辅助光源这一重要结论. 仿真结果验证了建模方法的正确性.

Influence of earth's reflective radiation on space target for space based imaging

The space-based surveillance, which would mainly use the space-based visible, has great value for civil and military applications currently and for a fairly long future period. In space-based surveillance, the visible and near-infrared radiation characteristics of the space target are influenced by its attitude variation. This influence is especially prominent in space-based imaging. In some ways, solar radiation cannot arrive at the surface of the space target, or the arriving radiation is not uniformly distributed because of the space target's strong reflection at a particular position. In order to solve these problems, the visible and near-infrared illumination characteristics of the space target surface are analyzed. Moreover, a notion that earth's reflective radiation can be used as illumination for space target imaging is given, and an accurate modeling method is proposed. Firstly, based on diffuse reflectance model, a method of mathematically calculating the illumination at space target's position from earth's reflective radiation is established. And a formula for calculating illumination is derived. Secondly, the coordinates of sun and space target at any time can be obtained by the Satellite tool kit software, in which the complicated multiplying matrix and coordinate transformation algorithm introduced in some references are avoided. Thirdly, the method of estimating earth's reflective radiation region at arbitrary moment is introduced. The grid division method is generated and the uniform sampling is used in each small area. Fourthly, the position of a surface cell is transformed from the sphere reference frame into the J2000.0 inertial frame. The earth's reflective radiation can be calculated through numerical integration. Finally, the illumination from earth's reflective radiation to a sun synchronous orbit satellite in an imaging mission based on space is calculated by the given parameters. The results show that the earth's reflective radiation is luminous enough for space target imaging when the satellite passes through arctic. When the satellite moves on the orbit, we can obtain more detailed information about target satellites' bottom then the ground simulation imaging. The on-orbit imaging results demonstrate the validity of the modeling method, which could support the foundation of our space-based surveillance system theoretically and technically and could be used as a reference of space-based orbit measurement and determination in deep space exploration.