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SAR satellites use side-looking radar systems to generate radar images. Unlike needle-looking radar systems, side-looking radar systems can distinguish signals within the footprint based on the time of receipt. In the needle-looking radar system, signals from different objects cannot be distinguished because the distance to the satellite is the same. However, in the side-looking radar system, objects at different distances from the satellite have different returned times, allowing us to determine which signals belong to each object. This information is used to generate radar images in the azimuth direction. Some of you may wonder why SAR satellites need to be side-looking. The primary reason is that the side-looking radar system can generate the radar images. Unlike side-looking radar system, the needle-looking radar system can only generate point light measurements, because the signals within the footprint cannot be distinguished. The key to distinguishing the signals in the same footprint is the time of receipt of the signals. The returned time is used to index the signals. Looking at the left figure, there are two objects, object A and B. Because the distance from A and B to the needle direction of the satellites are the same, the returned signals from A and B are also the same. The returned times are all t0. With this, we cannot know whether the received signals come from object A or B. Actually, we can achieve this goal by the method of INSAR. But this is beyond this course, so I won't talk about this. In terms of the side-looking radar system, here we can see object A and B have different distance from the satellite. Hence, the returned time from object A and B are different. Object B is closer to the satellite than object A, so the returned time from object B is also shorter than that from object A. The returned time of object B is t0 and length of object A is t0 plus delta t. Now, we can know what signals belong to what objects in the range direction based on the returned time. The returned time provides the information to put the received signals in the range coordinate. Across the flight direction, which we call is the azimuth direction, we are able to generate the radar image.