Shipborne Acquisition, Tracking, and Pointing Experimental Verifications Towards Satellite-to-Sea Laser Communication

Fast and high-precision tracking technology for image-based closed-loop cascaded control system with a Risley prism and fast steering mirror

The Risley prism's compact structure, dynamic responsiveness, and high tracking accuracy make it ideal for photoelectric image tracking. To realize fast and high-precision tracking of the target, we propose an image-based closed-loop tracking cascade control (IBCLTCR-F) system using a single image detector that integrates the Risley prism and fast steering mirror (FSM). Firstly, We propose a cascade control input-decoupling method (CCIDM) for the IBCLTCR-F system to solve the complex problem of coarse-fine control input decoupling in traditional single detector cascaded control systems. Moreover, the CCIDM method ensures that the FSM deflection angle is small and does not exceed its range during the fine tracking process, by using the Risley prism to compensate for the FSM deflection angle. Next, we design the image-based closed-loop tracking controllers of the Risley prism system and FSM system and analyze the stability of the IBCLTCR-F system. Finally, we track static and moving targets through experiments. The experimental results verify the feasibility of the IBCLTCR-F system, the effectiveness of the decoupling method, and the fast and high-precision tracking of the targets.

Pointing-error correction of optical communication terminals on motion platforms using a parameter model and kernel weight function estimation

Optical communication terminals (OCTs) with high pointing accuracy on motion platforms are highly important for establishing a global communication network. The pointing accuracy of such OCTs is seriously affected by linear and nonlinear errors generated by various sources. A method based on a parameter model and kernel weight function estimation (KWFE) is proposed to correct the pointing errors of an OCT on a motion platform. Initially, a parameter model, which has a physical meaning, is established to reduce the linear pointing errors. Then, the nonlinear pointing errors are corrected using the proposed KWFE method. Tracking star experiments are conducted to verify the efficiency of the proposed method. The parameter model reduces the initial pointing error associated with the stars used for calibration from 1311.5 µrad to 87.0 µrad. After applying parameter model correction, the KWFE method is applied to further reduce the modified pointing error associated with the stars used for calibration from 87.0 µrad to 70.5 µrad. Additionally, based on the parameter model, the KWFE method reduces the actual open-loop pointing error associated with the target stars from 93.7 µrad to 73.3 µrad. The sequential correction using the parameter model and KWFE can gradually and effectively improve the pointing accuracy of an OCT on a motion platform.

Polarization characteristic analysis of orthogonal reflectors with a side-scanning galvanometer in a laser communication terminal system

An alignment tolerance allocation and alignment method of orthogonal reflectors with high polarization-maintaining characteristics is presented. A geometric optics vector model of orthogonal reflectors is established to derive the propagation vector of the output beam, including pitch angle error of the first mirror, the side-scanning angle of the galvanometer, and rotation angle error of the secondary mirror. The polarization transmission matrix of the orthogonal reflectors is derived based on the 3D polarization ray-tracing algorithm to present the relationship between assembly errors and diattenuation or retardance of the orthogonal reflectors. A calculation example of certain polarization ellipticity error is given to quantitatively distribute the orthogonal mirror assembly error. The optimal side-sway angle selection method of the galvanometer side-sway angle in a laser communication system is presented; further, effective measures to minimize the polarization ellipticity error is put forward.