A mountain-top-to-valley optical link demonstration was performed in Switzerland between Säntis mountain, 2’502m
altitude, and Dübendorf airfield, 448m altitude. The link distance at very low elevation angle of 2° was 55km. Main goal
was to evaluate an optical communication system for LEO-to-Ground links in realistic atmospheric conditions, though
worst case, comprising the impact on data throughput and on pointing acquisition and tracking performance. Three wavelengths were tested simultaneously, a downlink at both, 1550nm and 808nm together with a 1064nm uplink,
thus allowing for comparison of atmospheric transmission impact over a wide wavelength range. Alongside, all
transmitters were designed to be eye-safe. The mountain top transmitter was installed inside a service building and the 60cm receiver telescope on the airfield was placed in an open stand. The link demonstration forms part of an on-going development activity started at RUAG Space with support from ESA in 2010. This activity is currently in the Engineering Model phase and aims at the Flight Model to be ready in 2016. Goal is to develop an optical downlink terminal that primarily addresses the needs of the emerging market of small satellites, the optical ground terminal and the ground network topology. The overall test approach is presented and explained together with a summary of all activities performed. Test results are presented and the discovered issues are addressed. Furthermore, a general overview is provided on the development activity and its current status.