Free-space laser communications for satellite downlinks: Measurements of the atmospheric channel

Future satellite communications can benefit greatly from the usage of frequencies in the optical spectrum. These allow data-rates comparable to fiber optical communications and provide additional advantages over traditional radio-frequency carriers like more power efficiency, small weight and size, and usage of frequency bands without ITU regulation. However, optical satellite downlinks are affected by the degradation of signal quality due to atmospheric turbulence. In fact, a profoundly verified channel model for this scenario does not exist yet, as there is no satisfying database of channel measurements so far. To contribute in disposing this lack of data, the Optical Communications Group at DLR performed laser downlinks from the Japanese OICETS to the DLR Optical Ground Station in Oberpfaffenhofen near Munich, Germany, in 2006 and 2009. The trials in 2006 were in cooperation with JAXA, NICT and DLR, whereas in 2009 they were part of a downlink-campaign comprising JAXA, NICT, ESA, NASA/JPL, and DLR. Measurements of intensity scintillations and wave-front distortions were undertaken that show the development of atmospheric turbulence over the elevation angle. Measurement devices like the differential image motion monitor, pupil camera, Shack-Hartmann wave-front sensor, and power meters were applied. Metrics that may be derived from the measurements are Fried parameter, scintillation index, intensity correlation length, and complex wave-front. The measurements may support the refinement of the common channel models. The applied communication scheme was a 50-Mbit/s on-off keying at a wavelength of 847 nm. Bit-error rates were recorded in parallel to the atmospheric measurements. In ten trials out of eighteen, signals were received while the others were hindered by cloud blockage. Seven of these trials were found to have useful data for analysis. The elevation angle of the analyzed links ranges between 4 and 55 deg. The present paper contains a description of the experiments in 2006 and 2009 and highlights the results so far. In this paper, the setup of the optical ground station is outlined as well as the measurement devices and results. Emphasis is put on analysis of the measurements of received optical power and complex wave-fronts