# Line of Sight and Image Motion Compensation for Step and Stare Imaging System
## 4 Experiments and Results  
To verify the method of LOS and image motion compensation, a series of flight tests were performed. The flight altitude t of the aircraft is about 7400m, and the photographing distance is from 22 km to 24 km. There are two kinds of imaging methods: fixed-point mode and search mode. Fixed-point mode imaging refers to the continuous imaging of fixed target points on the ground. And search mode refers to seamless coverage imaging of an area on the ground.  
In the fixed-point mode, the coordinate of the target is fixed and is located in (0,0,0). With the aircraft flying, the gimbaled command angles and velocities of IMC are calculated in real time. The gimbal subsystem adjusts the pointing angles to keep the LOS point to the fixed target. At the same time, according to the flight information of the aircraft, the actual gimbaled angles and the position of the compensation mirrors, the LOS and the corresponding ground target coordinates can be calculated reversely. And the position deviation between the planned imaging target and the actual target point can be obtained. Figure 8 shows the data that the system performs fixed-point imaging in two different directions. Although the position of the aircraft changes greatly, LOS always points to the fixed target and images the target from all angles. Due to the measurement errors of various sensors, the errors of the control system and other reasons, there will be position deviations in the imaging projection area on the ground. The position deviation of roll and pitch directions are within 15 m, as shown in Figure 9. In the exposure process, FSMs adjust the position in real time according to the given speeds to keep the image clear. Figure 10 is photographed images of the two fixed-point imaging in the mid wave infrared (MWIR) band. The tri-bar can be clearly distinguished from the detailed enlarged pictures.  
Figure 8: Aircraft flight path and ground imaging projection when the system performs fixed-point imaging in two different directions.  
In the search mode, the target coordinates are variable. In the case of ensuring the overlap, different targets are imaged in sequence, to perform search imaging on an area. The command and algorithm unit calculates the gimbaled command angles of the current target and sends it to the gimbal subsystem before exposure. The position of the target is always maintained during the exposure, and it switches to the command angles of the next target after the exposure is completed. To improve efficiency, the system step and start back and forth. The first row is from left to right, the second one is the opposite. The roll gimbaled angle changes greatly with each frame step, while pitch gimbaled angle changes greatly with each row step. Figure 11 is the ground imaging projection map in search mode. It can be seen that it can achieve the coverage imaging of the area and meet the overlapping requirements. The real-time changes of the gimbaled angles are shown in Figure 12. During each frame, the gimbaled angles approach gradually the command angles and keep stable until the next command angles are received. Figure 13 shows the positions of the FSMs at the exposure time in the search process. Since the step direction is opposite after each row, the compensation direction of the next row is reversed.  
Figure 10: Photographed images of the two fixed-point modes (**a**) image obtained from the first fixed-point imaging; (**b**) image obtained from the second fixed-point imaging.  
Figure 9: Target positioning accuracy in X and Z directions for the second fixed-point imaging.  
Figure 11: The ground imaging projection map in search mode.  
Figure 12: The gimbaled angles in search mode.