Gao Bo-Cai, Ward Evan, Bowles Jeffrey, Yingling Adam
Remote Sensing Division, Code 7230, Naval Research Laboratory, Washington, DC 20375, USA.
Space Sciences Division, Code 8000, Naval Research Laboratory, Washington, DC 20375, USA.
Sensors (Basel). 2020 Dec 1;20(23):6874. doi: 10.3390/s20236874.
When viewing Earth's surfaces from a low Earth orbiting (LEO) satellite platform with an optical sensor, the upward light propagation path from the ground to the satellite is affected by atmospheric refraction. For imaging sensors with a spatial resolution of about one km on the ground, atmospheric refraction is typically neglected during geo-registration of the satellite images. However, for high spatial resolution imaging systems with surface pixel sizes of approximately one meter or finer, the neglect of atmospheric refraction effects can typically introduce errors of a few meters in the spatially registered images. The atmospheric refraction effects need to be properly taken into consideration during the spatial registration of high spatial resolution satellite images. We have found that, with minor modifications, the ray tracing models implemented inside the LOWTRAN series of atmospheric radiative transfer codes developed in the 1970s and 1980s, in particular LOWTRAN7 in late 1980s, can be used for modeling the pixel displacement resulting from atmospheric refraction for satellite observations. The LOWTRAN series models were originally designed for calculating atmospheric transmittances and radiances for radiation going through long paths of the Earth's atmosphere. In the ray tracing portions of the codes, a spherical model atmosphere from the ground to 100 km is finely divided into about 30 thin atmospheric layers. The refraction angles for ray paths between consecutive layer boundaries are accurately calculated. We make a new use of the refraction angles calculated by the LOWTRAN7 code to study the surface pixel shift resulting from atmospheric refraction for satellite observations. In this letter, we report the modeling results on surface pixel displacements for different satellite altitudes and downward view zenith angles, several atmospheric temperature and pressure profiles, a few surface elevations, and wavelength dependencies from blue (450 nm) to near-IR (865 nm). These results can have reference values for researchers to estimate refraction-induced pixel displacements in their high spatial resolution satellite images. The results may also potentially help in designing spacecraft algorithms for accurate instrument pointing and mission tasking to automatically capture short-lived science events.
当使用光学传感器从低地球轨道（LEO）卫星平台观测地球表面时，从地面到卫星的向上光传播路径会受到大气折射的影响。对于地面空间分辨率约为1公里的成像传感器，在卫星图像地理配准过程中通常忽略大气折射。然而，对于表面像素尺寸约为1米或更小的高空间分辨率成像系统，忽略大气折射效应通常会在空间配准图像中引入几米的误差。在高空间分辨率卫星图像的空间配准过程中，需要适当考虑大气折射效应。我们发现，经过微小修改，20世纪70年代和80年代开发的LOWTRAN系列大气辐射传输代码（特别是20世纪80年代后期的LOWTRAN7）中实现的光线追踪模型，可用于模拟卫星观测中大气折射导致的像素位移。LOWTRAN系列模型最初设计用于计算辐射穿过地球大气长路径时的大气透过率和辐射亮度。在代码的光线追踪部分，从地面到100公里的球形模型大气被精细地划分为约30个薄大气层。精确计算了连续层边界之间光线路径的折射角。我们对LOWTRAN7代码计算的折射角进行了新的应用，以研究卫星观测中大气折射导致的表面像素偏移。在这封信中，我们报告了不同卫星高度、向下观测天顶角、几种大气温度和压力剖面、一些地表海拔以及从蓝光（450纳米）到近红外（865纳米）的波长依赖性下的表面像素位移建模结果。这些结果可为研究人员估计其高空间分辨率卫星图像中折射引起的像素位移提供参考值。这些结果还可能有助于设计航天器算法，以实现精确的仪器指向和任务分配，从而自动捕捉短暂存在的科学事件。
