Optimizing the Performance of Space to Ground Optical Communications

Clouds are a key driver in the performance of free-space optical communication (FSOC) systems. Clouds are composed of liquid water and ice crystals, and frequently produce atmospheric fades easily exceeding 10 decibel (dB). In these cases, impacts on FSOC systems may be severe even with efficient codes. On the other hand, there are times when transmission loss due to clouds is less than three dB as a result of thin, ice crystal based cirrus. In these cases, optical transmissions may still be possible, resulting in a high performing network. The ability to characterize the distribution of clouds and their transmission effects are critical in order to quantify the performance of FSOC ground networks. A high-end, state-of-the-art, cloud retrieval system has been developed using geostationary visible and infrared imagery. The database contains global retrievals for a 23-year period (1997 - present) at high spatial (4km) and temporal resolution (15 minute). The Lasercom Network Optimization Tool (LNOT) is used along with a space architecture and the cloud database to optimize the configuration of sites including placement that maximizes performance. LNOT factors in the impacts of mission data collection, onboard storage, data rates and link margin for clouds. Thousands of simulations have been performed that indicate that with just a few sites, FSOC performance, as measured by the percent data transferred metric, meets and exceeds that of radio frequency transmissions. Various studies will be presented at the conference.