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This paper focuses on the optimal design of a modulated retroreflector (MRR)laser link to establish a high-speed downlink for cube satellites (CubeSats),taking into account the weight and power limitations commonly encountered bythese tiny satellites. To this end, first, a comprehensive channel modeling isconducted considering key real channel parameters including mechanical gimbalerror, fast steering mirror angle error, laser beamwidth, MRR area, atmosphericturbulence, and channel coherence time. Accordingly, a closed-form expressionfor the distribution of the received signal is derived and utilized to proposea maximum likelihood based method to sense and estimate the initial position ofthe satellite. Subsequently, the distribution of the distance estimation errorduring the sensing phase is formulated as a function of the laser beamwidth andthe gimbal error, which enables us to fine-tune the optimal laser beamwidth tominimize sensing time. Moreover, using the sensing and initial satellitedistance estimation, two positioning algorithms are proposed. To compare theperformance of the proposed positioning method, we obtain the lower bound ofthe positioning error as a benchmark. Finally, by providing comprehensivesimulations, we evaluate the effect of different parameters on the performanceof the considered MRR-based system in both the sensing and positioning phases.

Modulating Retroreflector-based Satellite-to-Ground Optical Communications: Sensing and Positioning