假設地表物不變，則衛星觀測地表的變化量可視為大氣變化所引起的，基於這前 提下，Tanre 等人於 1988 年定義一結構函數，並提出由Landsat TM多時影像求取大氣氣溶 膠光學厚度的計算方法。1992年Holben等人將此法應用於AVHRR資料亦獲得不錯之結果。但 Tanre所定義的結構函數僅考慮單一方向，無法完全描述地表的特性，尤其應用於地面解析 很好的資料時。本文則將考慮多方向的結構函數並應用於地面解析較佳之SPOT的資料。應用 結構函數法求取氣溶膠光學厚度過程中，結構函數分布型態的差異將會造成誤差，而地形和 地表覆蓋物的特性以及高地面解析資料則是造成結構函數分布型態上差異的主要因素。因此 本研究由結構函數之分布特性，分析選擇一最佳位置數，以消除因結構函數分佈型態的變異 所造成的誤差。本研究方法和暗密植被的方法比較，結果非常合理，具極高之可行性。

　　　　　 By assuming that the ground reflectance is constant, variations of satellite signal may be attributed to variations of the atmospheric optical properties. Based upon this, Tanre et al, 1988 has defined the structure function for deriving aerosol optical depth from TM images. Holben (1992) also has gotten the similar results using AVHRR data. Basically, the structure function is defined by single direction only. It can't represent the complete properties of the surface structure. To overcome this, a multi directional structure function, which should include more information than the single directional from the surface structure, is used in this paper to improve the structure function method for SPOT data. In general, the error of aerosol optical depth determination could be enlarged by the different pattern of structure function distribution which is caused by the characteristics of local landcovers, terrain and the high resolution property of SPOT data. In order to reduce such error, it is necessary to determine an optimum number of distance to truncate the different pattern of structure function distribution. After these improvements, the results of this study are reasonable when compared with the Dense Dark Vegetation method's results.