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LEO-to-GEO intersatellite links using laser communications bring important benefits to greatly enhance applications such as downloading big amounts of data from LEO satellites by using the GEO satellite as a relay. By using this strategy, the total availability of the LEO satellite increases from less than 1% if the data is downloaded directly to the ground up to about 60% if the data is relayed through GEO. The main drawback of using a GEO relay is that link budget is much more difficult to close due to the much larger distance. However, this can be partially compensated by transmitting at a lower data rate, and still benefiting from the much-higher link availability when compared to LEO-to-ground downlinks, which additionally are more limited by the clouds than the relay option. After carrying out a feasibility study, NICT and the University of Tokyo started preparing a mission to demonstrate the technologies needed to perform these challenging lasercom links. Furthermore, to demonstrate the feasibility of this technique, an extremely-small satellite, i.e. a 6U CubeSat, will be used to achieve data rates as high as 10 Gbit/s between LEO and GEO. Some of the biggest challenges of this mission are the extremely low size, weight and power available in the CubeSat, the accurate pointing precision required for the lasercom link, and the difficulties of closing the link at such a high speed as 10 Gbit/s., Comment: 5 pages, 10 figures, 3 tables

NICT initiated R & D activities of Small Optical Transponder (SOTA) for micro-satellites to demonstrate attractive features of optical technology. Development of the SOTA started based on the bread board model (BBM). Design review of BBM revealed the necessity of engineering model (EM) for critical technologies of 2-axes gimbal and receiving optics. Evaluation of the EM carried out and final design of the SOTA was fixed. This paper describes design progress of the SOTA.

This paper explains a plan of research and development of high speed optical communication technology for observed data transmission from satellites/airplanes. The amounts of data acquired on observation satellites or airplanes have become larger with the rapid improvements of resolutions of sensors. In order to transmit the vast amounts of data, a communication system that combines free space optical communication system and terrestrial optical networks is proposed. Particular communication protocol is planned to be developed to realize efficient and stable link quality under the special characteristics of free space optical link such as interruption by rain or cloud. Also digital coherent detection scheme is proposed to be implemented to the system to overcome the degradation by atmospheric scintillation. The plan includes the development of mobile onboard laser communication terminal and the demonstration experiments of the total system.

The new tracking control system has been developed for Free-Space Optical Communications. It consists of the agile two-axis gimbals, the high performance fine-pointing-mechanism, the optical coarse acquisition sensor, and the optical fine tracking sensor. In the conventional tracking control systems, the two-axis gimbals is combined with the narrow range actuator for the fine pointing control, and it have the role of the coarse pointing control based on the GPS position and the body attitude. The each motion of the tracking mechanisms, therefore, is mutually disturbed by the individual control algorithms, and the obtained tracking accuracy is restricted by each motion. To improve the control performance of this two-stage type, the synchronized tracking-control system is proposed. The features of the system are the cooperative/predictive control using the each control/detective signals, and the improvement of the performance for disturbances, with the hollow-structured and rapid two-axis mirror gimbals. This control scheme is applied for the developed tracking control system, and makes it possible to achieve Free-Space Optical Communications. The paper shows the system configuration, the control algorithms and strategy for Free-Space Optical Communications, and the effect of the proposed control system by the several test results.1