Performance Estimation of Optical LEO Downlinks

Optical satellite communications have continually gained attention throughout previous years. Optical downlinks to transmit data from Earth observation satellites to ground may be an attractive solution to overcome the bottleneck caused by the limited spectrum and data rates of radio frequency systems. Due to the significant attenuation introduced by clouds, it is generally considered that optical communication systems cannot penetrate clouds. Therefore, the optimization and design of optical ground station (OGS) networks enabling a high combined availability is crucial for the application of optical downlinks. Within the ONUBLA 1 study funded under the European Space Agency’s General Study Programme, a simulation software has been developed. It includes a global cloud database, which was generated from various satellite sources, and a simulation tool, which allows for assessing the system performance of optical satellite links in terms of availability, achievable throughput, latency, and other characteristics. Different system parameters, such as ground station network topologies, sensor- and communication-link-rates, memory sizes, and many more, can be taken into account in the simulation. In this paper, we focused on the assessment of optical low Earth orbit (LEO) downlinks. A large number of system configurations and parameters have been compared and will be presented in this paper. Three OGS network configurations with sites at European, polar, and worldwide locations have been compared. We could confirm that large, worldwide OGS networks perform better and enable the transmission of more data than small, regional networks. We particularly found that the buffer size on board the satellite plays a crucial role for the system design. A further key result is that latency-critical applications require large OGS networks to meet the requirements. Finally, a comparison with potential RF systems shows that optical LEO downlinks can compete with RF solutions and even outperform them if a reasonably sized buffer is available on board the satellite. 1 ONUBLA is an anagram of the Spanish word nublado (≙ cloudy).