I. Introduction
THE resolution of satellite imagery has been largely improved to 1 m or better in recent years. For example, the ground sample distances (GSDs) of IKONOS [1] and QuickBird [2], [28] at nadir have been reported to be 1.0 and 0.61 m, respectively; WorldView-2 [3], [29] has a GSD of 0.46 m (resampled to 0.5 m as required by the government of the United States) at nadir; and GeoEye-1 [4] has a GSD of 0.41 m (resampled to 0.5 m for the same reason above), which is currently the highest resolution among all commercial satellites in the world. All of the above satellites adopted the linear pushbroom imaging mode to acquire high-quality linear-array imagery while integrated with high-accuracy Global Positioning System (GPS) receivers and star trackers to measure its instantaneous position and attitude data at the imaging time. These auxiliary data are transferred to a ground station and are used to perform direct georeferencing. The accuracy of direct georeferencing can achieve 3 m using the auxiliary data of GeoEye-1 [4].