Optical Satellite Downlinks to Optical Ground Stations and High-Altitude Platforms

Earth-observation (EO) satellite missions using high-resolution optical or radar sensors are producing an immense amount of data which needs to be send down to earth. The fraction of satellite operational time in future missions is therefore clearly limited by the downlink-capability. The current X-band architecture is facing its technological limitations in terms of data rate while causing increased demand on antenna-sizes and transmit power. This bottleneck can be overcome by direct optical downlinks from EO-satellites to the ground with multi-gigabit data rates. According optical satellite terminals will be extremely small and light-weight and will require few transmit power, but one drawback is the link blockage by thick clouds. This can be overcome either by ground station diversity and careful site selection or by using optical terminals onboard high altitude platforms which serve as relais-stations for the satellite. Here we present feasibility and expected performance of these two optical scenarios and propose according space and ground station architectures.