Research activities at the German Aerospace Center (DLR) concerning optical free-space communications have focussed on coherent communication systems for inter-satellite link (ISL) applications for a long time. Under DLR contract Tesat Spacecom has developed the DLR-LCT (laser communications terminal) which relies on coherent technology. This terminal will be verified in space as secondary payload onboard the earth observation satellite TerraSAR-X, to be launched in 2006. In a first step, downlink experiments will be carried out.
The DLR Institute of Communications and Navigation is involved in this ambitious project by assessing the feasibility of the downlink experiment through atmospheric turbulence and by conducting channel measurements. An initial feasibility study shall theoretically investigate the influence of atmospheric turbulence on coherent optical transmission and assess the success probabilities of the particular experiment with regard to the specific ground station conditions. Since theory is always based on arbitrary assumptions on the composition and structure of the atmosphere, measurements at the specific ground station shall be carried out. Measurement results shall enable a refinement of disturbance models in order to predict the condition during the downlink experiments. Relevant atmospheric parameters, such as scintillations, phase-front distortions, atmospheric seeing, angle-of-arrival fluctuations, attenuation, Cn2- and wind profiles will have to be recorded.
To carry out these measurements, DLR will develop an "Atmospheric Turbulence Monitor" (ATM). The ATM mainly consists of a 16-inch telescope and a number of instruments for various measurements. These instruments are based on astronomical devices for use with stars, however have to be modified to be suited for measurements with close objects such as LEO or GEO satellites. The ATM will as well comprise a tracking system, that allows for measurements with LEO satellites such as TerraSAR-X.
This paper presents the outline of the DLR atmospheric turbulence monitoring measurement campaign and describes the preliminary design of the "Atmospheric Turbulence Monitor".