DIRECT OPTICAL HIGH SPEED DOWNLINKS AND GROUND STATION NETWORKS FOR SMALL LEO MISSIONS

Abstract

Direct microwave data downlink capacity of today’s earth observation satellites is limited in terms of available spectrum, transmission data rate, and power consumption. One elegant solution is the use of geostationary data relays satellites which connect to the low flying observation satellite by optical or Ka-band links and do the downlink to the ground station by another Ka-band link. This scenario also allows near real-time data access to the EO-sensors for at least half of the LEO’s orbit. However it also implies efforts in terms of the number of long-range communication terminals employed by one EO-mission. Compact LEOs often cannot afford such a connection due to financial- but also mass- and power-constraints. To enable also these kinds of missions with high rate downlink capacity, small optical terminals can be used that boost the data rate by orders of magnitude compared to today’s RF downlinks. The feasibility of direct optical downlinks from LEO satellites has been demonstrated recently with the laser terminals onboard JAXA’s OICETS and onboard the German TerraSAR-X. As such direct optical downlinks are hindered by cloud cover, a ground station network is required to ensure a certain required average downlink capacity, independent from the cloud situation. For one thing each optical ground station should be situated in a region with low cloud cover in general. But also the different ground stations of one network should be spaced far apart as to avoid correlation of their cloud cover statistics. Furthermore in a global ground station network, sites north and south of the equator are seasonal de-correlated. DLR’s Optical Communications Group is investigating the feasibility of direct optical LEO downlinks theoretically and practically together with partners [1], [2], [3]. The performance of optical downlinks has been evaluated by long-term global cloud statistics with different sets of ground station combinations. Furthermore, downlink campaigns shall be performed in near future with simple optical downlink sources on small LEO missions, verifying the theoretical findings. A Transportable Optical Ground Station (TOGS) will be used to carry out these trial campaigns.