| 摘要: | 合成孔徑雷達(Synthetic Aperture Radar, SAR)可應用於環境與災害觀測上，透過雷達差分干涉技術可得到地表的位移量，其精度更可達到公分以上等級。但其在山區與城市區域中，會有前坡縮短、疊置效應、陰影效應等問題，這些效應會使雷達在觀測地表時獲得錯誤的坐標。將合成孔徑雷達影像與經反投影後的數值地表模型(Digital Surface Model, DSM)影像結合，是一個讓雷達影像擁有正確的空間位置的有效方法。
本研究的主要目的是解決合成孔徑雷達影像的坐標不正確的問題，其原理是將DSM影像利用光線追蹤法的方式，模擬衛星掃描此地形的SAR影像，生成模擬的SAR影像。再將其與雷達影像連結，由模擬的SAR影像提供真實SAR影像中每個像素的三維空間坐標。
本研究的測試範例包括大壩、橋梁、超高建物等，在這些區域中疊置效應會使連續高強度的雷達信號顯示於SAR影像上的河流與水庫水面等不合理的位置。訊號應是源於壩體、橋梁與建物本身的結構反射，而合理坐標位置應要在這些人造建物的牆面或頂部。但合成孔徑雷達僅具有二維坐標，無法表示這類信號的位置，透過本研究所開發的方法，將二維坐標轉換成三維坐標可以解決這類問題。在本研究的案例中，這些錯誤位置的雷達信號，如房屋、橋梁訊號等，在經修正後，水平方向上可修正20~40公尺長的距離，可改善影像坐標的正確性；而其他區域的雷達信號，如平原等，相較於疊置效應區域，平原區域的原始坐標誤差較低，經修正後，水平方向上仍可修正0~10公尺，而且正確地反推回三維模型表面。SAR影像經過此研究校正後可有利於後續的處理與分析，以及將分析成果正確套疊其他圖資。
;Synthetic Aperture Radar (SAR) data are often used in environmental and disaster observations and monitoring. Using the Differential SAR Interferometry (D-InSAR) technique, the displacement of the surface can be obtained. However, in mountainous and urban areas, there may be serious problems such as foreshortening, layover effect and shadow. This will lead to errors in calculating coordinates and removing terrain effects. Combining synthetic aperture radar imagery and digital surface model (DSM) is a potential solution to reduce these artifacts.
The objective of this study is to solve the problem of incorrect coordinates of SAR images. The idea is to use the DSM with ray tracing method to simulate the SAR image of the terrain scanned by a satellite to generate a simulated SAR image. Then, combining the simulated SAR image with the radar image, so that the simulated SAR image provides the three-dimensional coordinates of each pixel in the real SAR image.
The developed method is applied to examples of correcting the coordinates of dams, bridges or tall building in SAR images. Layover effect will cause continuous high-intensity radar signals to be located at wrong positions. These signals should come from the structural reflection of the dam body, bridges, and buildings. The correctly derived positions should be on the wall or top of these artificial structures, but SAR images have only two-dimensional coordinates and cannot correctly represent the positions of the signals. After applying the method developed in this study, converting the two-dimensional coordinates into three-dimensional coordinates can solve these problems. In most cases of this study, the radar signal at the wrong position can be corrected by 20~40 meters in the horizontal direction, and it can improve the accuracy of the image coordinates. However, the radar signals in other areas, such as plains, the coordinate offsets of these positions are less substantial. However, after applying the developed method, the radar signal can still be corrected by 0~10 meters in the horizontal direction and mapped onto 3D surfaces correctly. The outcome of this research can help the subsequent processing and analysis of SAR data, and the results can be correctly overlaid with other spatial datasets. |
