Deep-space Optical Communication System (DOCS) for ESA's Space Weather mission to Lagrange orbit L5

ESA plans to embark the Deep-space Optical Communications System (DOCS) on the Space WEather (SWE) mission to the Sun-Earth Lagrange point L5 in the frame of its Space Situational Awareness (SSA) program. DOCS is an in-orbit technology demonstration that also serves a scientific objective, namely the transfer of high-resolution solar imagery. The characteristics of the SWE L5 mission provide substantial advantages for deep-space optical communication, because the equidistant triangular orbital geometry between the sun, the Earth, and L5 (all three distances are equal to 1 AU ≈ 150 million km) ensures that the sun is always separated by 60 degrees from both the space and the ground terminal. This allows for very efficient solar stray-light shielding and thermal management. It also reduces the pointing requirements of the DOCS Space Terminal (DST); a coarse pointing mechanism is not required, nor is a point-ahead assembly. The SSA SWE L5 mission Phase A, Phase B1 and B2 studies to be conducted 2017-2019 will take into account the accommodation and interfacing of the DST. DOCS aims to demonstrate a data rate of 10 Mbps over 1 AU to a 4-meter optical ground station. The development of the DST will follow a similar approach as foreseen for the Asteroid Impact Mission (AIM), namely to increase the technology readiness level (TRL) of existing developments wherever possible, or reasonable. The DST will be located on a side panel of the S/C, and will constantly face the Earth. Residual attitude movements (pointing errors) of the S/C will be covered by the field-of view of the DST telescope (an aperture diameter of 200 mm is envisaged) and by a fine pointing assembly. ESA is also embarking on a study to develop a 4-meter class DOCS ground terminal (DGT). Furthermore, the Consultative Committee for Space Data Systems (CCSDS) is currently developing a standard (“Blue Book”) for “High Photon Efficiency Optical Communications”. DOCS will serve as one of two implementations of the standard, which are required for it to become applicable.