In this paper, a robust control based on disturbance observer is proposed to improve the tracking accuracy of the Risley prism system (RPS). Applying the flexible thin-wall ring mechanism in the RPS causes a series of tracking and pointing challenges. Disturbances such as friction, shaft deformation, and model perturbation significantly deteriorate the tracking and pointing accuracy of the RPS. Two different observer-based control methods are proposed to guarantee the tracking precision of the RPS. Moreover, the disturbance observation and compensation (DOC) performance of the proposed methods is analyzed and compared. Finally, simulation and experiment results indicate that the proposed control methods, especially the DOC-expanded state observer control mode, obtain the best performance for disturbance rejection in the RPS.

Vibration rejection is one of the key techniques to stabilize the line of sight (LOS) for phased array telescope systems. Conventionally, feedback control based on image sensors is mainly used to correct the tip/tilt errors caused by disturbances and to keep the LOS stable. However, it is restricted by the sampling rate and time delay of image sensors, leading to a limited closed-loop bandwidth. Disturbances in the middle and high frequencies are hard to suppress. In this paper, disturbance-propagation-characteristics-based feedforward control is proposed to overcome these problems. A theoretical imaging model of the phased array telescope is developed to analyze the LOS disruption caused by disturbance. In addition, to improve the disturbance suppression bandwidth and correction accuracy of the system, the disturbance propagation characteristics of the phased array telescope system are analyzed. Combined with the disturbance feedforward, targeted compensation is achieved for the sub-apertures. Finally, a comparative experiment is carried out based on the self-developed Fizeau phased array telescope system to verify the superiority of the proposed method.

In the satellite-borne Fizeau phased array telescope (FPAT), the imaging quality is significantly deteriorated by tip-tilt disturbances induced by carrier maneuvering. The long exposure time of the image sensor is usually required for a good signal-to-noise ratio, which brings the time delays into the control loop of the fast-steering mirror. As a result, a low control bandwidth is insufficient to compensate for FPAT disturbances. In this paper, a time delay compensation-based disturbance observer (TDC-DOB) is proposed to reject carrier disturbances. This new TDC-DOB is plugged into the original feedback control loop, so the disturbance rejection can be optimized by designing an appropriate delay compensation controller. An improved TDC-DOB controller is presented to reject the widespread carrier disturbances, and also reduce the amplifications induced by the waterbed effect. This proposed controller can achieve a satisfying disturbance rejection beyond the closed-loop bandwidth which breaks the limitation of the existing DOB method. Both simulations and extensive experiments are carried out to demonstrate that the TDC-DOB method can earn a significant improvement for the disturbance rejection in comparison with the conventional feedback controller.

At present, the majority of sparse-aperture telescopes (SATs) are unable to observe moving targets. In this paper, we describe the construction of and present the results obtained using a Fizeau directly-imaging sparse-aperture telescope (FDISAT) that permits pointing and the tracking of moving targets. The telescope comprises three sub-apertures, each of which is equipped with a Risley prism system that permits a maximum tracking range of 5° and has independent boresight adjustment capability. On targets in various positions, experiments with pointing and tracking are conducted. The maximum root-mean-square error (RMSE) of pointing in the sub-apertures was found to be 8.22 arcsec. When considering a target moving at 0.01°/s for approximately 320 s, the maximum RMSE of tracking in the sub-apertures was found to be 4.23 arcsec. The images obtained from the focal plane detector exhibit clear interference fringes while tracking. The experimental results demonstrate that the system can effectively track moving targets, providing a method for SAT observation of moving targets.

Maintaining stable line-of-sight (LOS) for free-space optical (FSO) communication terminals on mobile platforms is a critical challenge due to platform-induced jitter. Conventional inertial sensor-based feedforward control (FFC) employs a high-pass filter (HPF) to decouple commanded tracking motion from disturbance measurements, but this sacrifices the system’s ability to reject crucial low-frequency disturbances. This paper proposes a synergistic control architecture that combines a disturbance observer (DOB) for low-frequency disturbance estimation with a gyro-based FFC for mid- and high-frequency rejection. The two signals are seamlessly fused by a complementary filter pair. This band combination strategy relaxes the conflict between wideband disturbance rejection and agile target tracking, eliminating the destructive effects of motion coupling without performance-degrading. The parameter tuning is systematically formulated as an optimization problem for the complementary filter pair, constrained by the target’s maneuvering dynamics. Furthermore, the closed-loop stability and robustness are analyzed based on the sensitivity function and the small-gain theorem. Using this approach, satisfactory control performance is achieved for a telescope mounted on a 6-degree-of-freedom swing platform. Comparative simulations and experiments are conducted to validate the effectiveness of the proposed approach.