<title>Measurements at a 61-km near-ground optical transmission channel</title>

Abstract

An optical ground-to-ground direct-detection transmission experiment over 61 km is being performed by the German Aerospace Center (DLR) in cooperation with the European Aeronautic Defence and Space Company (EADS) and Contraves Space AG, Switzerland. Transmission direction is from the mountain Wallberg in the German Alps down to Oberpfaffenhofen (west of Munich). This beam path suffers strongly from optical turbulence especially at the near-ground part along the last kilometers before the receiver. This causes a very demanding situation regarding received-power scintillations. Transmit power from one data source is 1W at 980 nm. Of special interest is the effect of secondary transmitter apertures with 4m lateral offset to the first. Under strong turbulence conditions this provides statistically independent speckle patterns at the receiver thus improving system performance dramatically. This paper presents measurements at the transmission channel, with emphasise on statistical parameters of the scintillations and angle-of-arrival variations with one and two transmitter sources.