Optical on–off keying data links for low Earth orbit downlink applications

As Optical Low Earth Orbit Downlinks (OLEODL) serve for sensor data download from earth observation satellites, their link scenario is strongly asymmetric, since the data flow is mostly simplex or at least the downlink datarate (DR) is orders of magnitude higher than the uplink (the later may \nonly serve for tele-command and link protection). Therefore, the antenna gain can be distributed favourably: with small and lightweight transmitters in space and correspondingly moderately sized antennas (i.e., receiver telescope apertures) on the ground. The disturbing atmosphere only affects the lower end of the link close to the receiving ground station, which on the one hand allows for simple techniques for link stabilisation by aperture averaging but on the other hand complicates some of the advanced modulation and detection formats, since these may require ophisticated techniques like adaptive optics for coupling into single-mode fibres. Therefore, data format options for OLEODL focus mainly on rather low complex and robust direct detection (DD) techniques.