For space industries, free-space optical communications are becoming a mature technology, but the impact of their use to download observations from spatial imagery systems has still to be evaluated. Unlike current radio-frequency technology, free-space optical communications are strongly impacted by weather conditions, and most notably by clouds. In order to cope with the later, it is necessary to achieve ground station diversity, i.e. having a network of optical ground stations able to receive data from satellites. In this paper, we aim to find a subset of a given number of ground stations maximizing the amount of data that can be downloaded from a low-earth orbiting satellite to the Earth during its missions. We present a Mixed Integer Linear Program model and a hierarchical method based on an exhaustive enumeration of the sets of stations and on a dynamic programming algorithm to solve it. The efficiency of this method is evaluated on several instances based on real ground station networks and on cloud cover throughout the last twenty years.