Scintillation reduction using multiple transmitters

Uplink scintillation is a serious issue for laser communication between a satellite and the ground. Fluctuations on the uplinked beacon and communications lasers can be minimized by transmitting multiple independent lasers from separate apertures which then sum incoherently at the satellite. The objective of the experiment described here was to determine the number and spacing required for separate transmitters to reduce fluctuations in the received power due to atmospheric scintillation to acceptable levels. Received power vs. time data were collected for horizontal laser links established between a laser transmitting platform and a receive telescope assembly separated by distances of 1.2 and 10.4 km to mimic the expected atmospheric effects of an uplink slant path to a satellite. Reduction in signal fluctuations was observed as the number of laser transmitters was increased from 1 to 16. A ground terminal design with 16 lasers on an 18 inch diameter circle with a 7 dB fade margin should be adequate to compensate for most scintillation fades, while the remaining deep fades may be corrected by using forward error correction techniques.