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A high bitrate optical downlink was performed by the stratospheric optical payload experiment (STROPEX), a part of the EU CAPANINA project. The STROPEX objectives were to design and build the necessary hardware to demonstrate an optical backhaul downlink from a stratospheric platform to the ground and to carry out channel measurements on the link. A successful measurement campaign at ESRANGE near Kiruna, Sweden achieved all of these objectives. The transportable optical ground station received an almost error free 1.25 Gbit/s data signal from the payload over a distance of 64.3 km with a bit error rate of better than 10 -9 . This paper gives an overview of the stratospheric optical payload experiment, focusing on the airborne free-space experimental laser terminal (FELT). Additionally, the successful measurement campaign is described and the operation of the experiment is outlined.

In the Capanina project, it has been shown that it is possible to use solely GPS positions for coarse-pointing of a stratospheric free-space optical communication terminal. Although this system design has been shown to be functional, it has to deal with acquisition and reacquisition times of up to 20 seconds. Whenever the line of sight is blocked, the Capanina terminal has to do a reacquisition of the partner terminal.In land-mobile systems, e.g. transmission between a moving vehicle and a fixed station, where objects like trees or buildings can frequently block the line of sight, this type of acquisition/reacquisition is not acceptable.In this paper a revised system design will be shown, which reduces the initial acquisition time to below a second by using information from a GPS-Aided Inertial System (GPS-INS). It will also be shown how reacquisition times and coarse-pointing errors can be reduced to a minimum by using feedback from the optical fine-pointing device. Finally a demonstrator setup and some test results will be presented.

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Earth-observation (EO) satellite missions using highresolution 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 antennasizes 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 lightweight 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 relays-stations for the satellite. Here we present feasibility and expected performance of these two optical scenarios and propose according space and ground station architectures.