  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 by
these tiny satellites. To this end, first, a comprehensive channel modeling is
conducted considering key real channel parameters including mechanical gimbal
error, fast steering mirror angle error, laser beamwidth, MRR area, atmospheric
turbulence, and channel coherence time. Accordingly, a closed-form expression
for the distribution of the received signal is derived and utilized to propose
a maximum likelihood based method to sense and estimate the initial position of
the satellite. Subsequently, the distribution of the distance estimation error
during the sensing phase is formulated as a function of the laser beamwidth and
the gimbal error, which enables us to fine-tune the optimal laser beamwidth to
minimize sensing time. Moreover, using the sensing and initial satellite
distance estimation, two positioning algorithms are proposed. To compare the
performance of the proposed positioning method, we obtain the lower bound of
the positioning error as a benchmark. Finally, by providing comprehensive
simulations, we evaluate the effect of different parameters on the performance
of the considered MRR-based system in both the sensing and positioning phases.
