Development of forward motion compensation method with tilt mirror for high resolution earth observation satellite cameras

The most important difficulty encountered in obtaining high quality images from high-resolution satellite cameras is the image motion occurring in the optical system focal plane due to the orbital motion of the satellite. The orbital motion of the satellites requires the optical systems to perform their duties on a platform moving at high speeds. This results in shifts in the resulting ground images, degrading image quality and limiting exposure times. This problem can be solved by using Forward Motion Compensation methods that reduce image movement in the focal plane, enabling the acquisition of high-quality images. Unlike existing methods in the literature, this study comprehensively investigates the use of the tilted secondary mirror method as an opto-mechanical method. The forward motion resulting from satellite orbital motion is compensated based on the principle of deliberately misaligning the secondary mirror of a Ritchey-Chretien (RC) Cassegrain telescope by applying a controlled tilt motion, thereby creating a reverse image motion. This method eliminates the relative motion in the focal plane, thus achieving image stability throughout the exposure time. Theoretical and experimental studies have demonstrated that a tilt angle as small as 0.020° applied to the secondary mirror has a negligible effect on image sharpness. The proposed method can increase exposure time by a factor of 52 without the need for TDI sensors, and compensates for focal plane image shifts due to orbital motion with a residual error of 0.62 pixels. This has been verified in a laboratory environment by using a laboratory-installed motion simulator and the developed camera system.