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The functional requirements and design drivers for an Optical Communications subsystem are assessed based on the system requirements imposed by a proposed Europa Orbiter mission. Unlike near-Earth optical communications systems, deep space missions impose a unique set of requirements that drives the subsystem design. Significant challenges on laser efficiency, thermal control, pointing and tracking, stray/scatter light control, and subsystem mass/power need to be addressed for a successful subsystem implementation. The baseline design concept for a lasercom subsystem for the Europa orbiter mission employs a 30-cm diameter, diffraction-limited telescope, and a diode pumped solid state laser operating at 1 .06 pin to support downlink communications. The baseline pointing and tracking approach is to perform Earth Image tracking with occasional calibration using the Earth-moon or Earth-star images. At high phase angles when the Earth image does not provide sufficient brightness for high rate tracking, inertial sensors (accelerometers) measurements are used to propagate the knowledge of the optical boresight at a higher rate in between celestial reference updates. Additionally, uplink beacon tracking will be used to support pointing at short range and near solar opposition when Earth image along does not provide sufficient signal power for tracking.

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Analyses of uplink and downlink data from recent free-space optical communications experiments carried out between Table Mountain Facility (near Pasadena, CA) and the Japanese ETS-VI satellite are presented. Fluctuations in signal power collected by the satellite's laser communication experiment (LCE) due to atmospheric scintillation and its amelioration using multiple uplink beams are analyzed and compared to experimental data. Downlink data was analyzed to determine the cause of a larger than expected variation in signal strength. In spite of the difficulty in deconvolving atmospheric effects from pointing errors and spacecraft vibration, experimental data clearly indicate significant improvement in signal reception on the uplink with multiple beams, and the need for stable pointing to establish high data rate optical communications.