Satellite based optical communications in space is rapidly progressing, involving an increasing number of flight validations in LEO, GEO and beyond. Various space agencies successfully demonstrated both, optical Inter-Satellite communication Links (ISL's) which have been validated in flight experiments since 2001 and direct-to-Earth (DTE) optical communications.
Most prominently, NASA's lunar lasercom demonstration mission LLCD in 2013 established optical DTE communications at 622 Mbps from the Moon to Earth. European industry developed in short time a PPM modulation receiver that was successfully used during the LLCD demonstration in frame of the Lunar Optical Communications Link cross-support activity between ESA and NASA. This paved the way for inter-operability especially for deep space optical communications.
Europe has been able to maintain a strong position with so far the only commercially available and validated ISL product worldwide: the LCT (Laser Communication Terminal) has reached TRL9 in IOD experiments and in Europe's EDRS program. This provides a sound basis on space qualifications and operations heritage when going for laser communications for deep space.
ESA launched in 2010 the development of a micro-LCT (together with the corresponding ground- station) for high data rate LEO to Earth optical communications (Optel- ) for small Earth observation satellites. The system achieves TRL6 in these days and the development provides interruption-tolerant transmission of on-board user data by a dedicated buffer memory system.
Several inter-island horizontal links (145 km distance through the atmosphere) using PPM modulation successfully demonstrated a simulation of a Deep-Space link from L2 and from Mars.
Given the solidly maturing optical space communication technology and the clearly increasing demand on downlink data rate to increase science return, a technology enhancement in communication technology (particularly from deep space) is called for. The Deep Space Optical Communications Architecture Study (DOCOMAS) focuses on future evolutions of deep space communication architectures for optical downlinks from a deep space probe directly to Earth. The emphasis hereby is on the ground segment, including cloud mitigation strategies.
The key enabling technologies that have been identified are dedicated optical ground antennae, free space coupled single photon counting detectors and a generic design approach to an optical payload terminal that can be applied to more than 3 orders of magnitude in link distance, starting from some ‰ of an A.U. up to several A.U. A genuine example for optical direct-to-Earth communications in a NEO mission context was derived in frame of a CCN to this study. That concept design was tailored for the asteroid investigation mission named "AIM" in frame of the AIDA activity together with NASA.