## 3 Optimizing In-Orbit SAR Operational Parameters for Altitude Variation
### Definition of Swath Angle for Image Acquisition  
As a reference angle, an angle of the swath within a near to far range should be defined to generate the access angle range for a SAR system. The reference angle affects the SAR operational parameter generation and the antenna pattern generation, and finally, determines the SAR performance. In case of the previous heritage KOMPSAT-5, the reference angle was defined as the incidence angle corresponding to the middle of the swaths. The angle range for image acquisition was calculated based on the required ground swath width and incidence angle. The center of the angle range was set to the same as the center between the near and far incidence angles of all the swaths at the nominal altitude. This angle was constant regardless of the altitude variation. The near and far incidence angles were then calculated to comply with the required ground swath width. For KOMPSAT-6, however, the reference angle is defined in a straightforward way. The look angle can be used as the reference angle to calculate the angle range of the near and far swaths at the predicted altitude. Image acquisition is strongly affected by which one between the incidence and look angles is used for the reference angle. During the in-orbit operation, beam pointing of the SAR antenna is dependent on the satellite altitude where the image acquisition is executed. Therefore, the deviation of the beam pointing should be reflected to the swath design considering the altitude variation. In the previous KOMPSAT-5, the incidence angle was used for the reference angle. Figure 11 shows examples of a SAR image that was acquired during the early operation phase of KOMPSAT-5. Each figure was acquired at a different altitude. As shown in Figure 11, the acquired images have an asymmetric property even though the image was acquired for Greenland, in which is supposed to produce an image having uniform intensity. This is because the incidence angle at the nominal altitude was used for the one at the predicted altitude without considering the altitude variation. The incidence angle was then used to acquire an image even though the actual altitude  
Figure 10: Flow chart for generating in-orbit SAR operational parameters. was changed to 566.285 and 569.907 km, respectively, from 550 km, which is the nominal altitude of KOMPSAT-5. The altitude variation causes the beam pointing error of about 0.07\({}^{\circ}\) in the antenna pattern. Figure 12 shows the look angle variation at near, center and far position, respectively, across the swaths when the altitude changes between 509 and 540 km. Figure 12a shows the look angle variation when the incidence angle is used for the reference angle without any correction as done in KOMPSAT-5. When the altitude changes between the minimum and maximum ones, the look angle also changes significantly for all the positions of the swaths. As shown in Figure 12b, however, the change in the look angle becomes minimal when the look angle is used for the reference angle. The same altitude variation changes the look angle much less than before. As a result, utilization of the look angle results in a smaller change in the beam error value for each altitude. For SAR application, an antenna pattern affecting the RAR performance is designed based on the look angle. Figure 13 compares the RAR performances when the incidence and the look angles, respectively, are used for the reference angle in the two swaths. The RAR performance significantly changes according to the altitude variation. The RAR performance is much better when the reference angle is based on the look angle instead of  
Figure 11: Examples of SAR image: (**a**) actual altitude is 566.285 km in ascending orbit; (**b**) actual altitude is 569.907 km in descending orbit. Figure 12: Look angle variation between minimum and maximum altitude when (**a**) incidence angle; (**b**) look angle at center of swath, respectively, are used for the reference angle. the incidence angle.