  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.
