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Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center (DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2° and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the experiment and highlights the results of the measurement trials.

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The choice of wavelength is essential for the variety of different communication scenarios in the field of free space optics (FSO). Possibilities are Satellite and HAP (High Altitude Platform) Downlinks, HAP-HAP links, HAP-Satellite links and all kinds of links involving aeronautical vehicles. This paper addresses the influence of the wavelength dependent attenuation of clouds, the atmospheric transmission in the NIR and MIR and a statistical analysis of cloud coverage data for an estimation of link availability. Regarding the calculation of atmospheric transmission the free available simulation tools libRadtran and GENLN2 have been used. To identify advantageous wavelengths to increase link availability, cloud attenuation is determined by Mie scattering calculations of particle size distributions of various cloud types. Here the MIR wavelength interval between 10 µm and 12 µm has been found to give the lowest attenuation in clouds. However in most cases clouds will block the optical link. For that matter a statistical analysis of satellite based data from the European Cloud Climatology (ECC) is done to reveal favorable places with high availability in Europe. The improvement of link availability when a concept of ground station diversity is applied has also been investigated. An availability of almost 99 % is reached with four hypothetical stations in southern Europe. Further the difference between availability values of single years decreases with multiple stations.

SUMMARYOptical backhaul downlinks from high-altitude platforms (HAPs) are investigated. An experiment demonstrated the advantages of optical links: a small and lightweight terminal with low power consumption was launched to the stratosphere and data transmitted down to a ground station at a rate of 1:25 Gbit=s: Owing to the chosen system parameters and the high budget margin, disturbing turbulence effects did not decrease the link performance.The scientific aspect of the experiment was to study turbulence effects in order to design future systems with higher transmission performance. On the day of the experiment, measured scintillation and wavefront distortions were minimal in the morning. The best atmospheric conditions were observed about 3 h after sunrise with a peak of the atmospheric coherence length r 0 at 16 cm: An r 0 of 4 cm was measured as the worst case before sunrise and later during the day. This trend could also be observed for power-ðs ; a lognormal intensity probability density function was measured.Apart from the robust intensity modulation scheme with direct detection which was used for the trial, future improved systems could benefit from a coherent transmission scheme. According to the r 0 measurements and further simulations on heterodyne efficiency it turned out that the aperture size can be decreased from 40 to 10 cm without any significant change in the link margin.Future stratospheric optical links between HAPs or links from platforms to satellites will not suffer from cloud blockage but it remains an issue for up/downlinks to a ground station. This can be mitigated by ground-station diversity. Four optical ground stations in the southern part of Europe can lead to an availability of over 98%. The separation distance of the ground stations is about 900 km with a negligible correlation of cloud cover. A change of wavelength from the employed 1.55 to a wavelength around 11-mm with minimum cloud attenuation would increase the link availability for thin clouds.

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.

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The increasing resolution of earth observation sensors will require much higher data rates for the data downlink in future than is feasible with conventional RF-technology. This applies for earth observation satellites as well as for aeronautic observation platforms, such as aircraft or stratospheric high altitude platforms. The most promising solution for this data downlink bottleneck is the application of optical free space transmission technologies. DLR has built diverse atmospheric flight terminals and performed several trials of optical downlinks from space (together with partnering organizations) as well as from atmospheric carriers in recent years. Here we present and compare results of such communication system trials.