Variable Data Rate for Free Space Optical Low Earth Orbit Downlinks (OLEODL)

Low-Earth-orbit (LEO) satellites for Earth observation have typical orbit altitudes of 400 to 600km. Since their on-board sensors resolution is advancing, the need for very high rate satellite downlinks is increasing into the range of several gigabits per second. Free-space optical communications is ideal for such applications, as it offers very high data rate, while allowing smaller satellite terminals and avoiding spectral regulation constraints. However, the key challenges in optical LEO downlinks (OLEODL) are the varying free space loss due to distance changes during a downlink contact, atmospheric attenuation, and the fast signal scintillation due to atmospheric index-of-refraction turbulence (IRT). This paper will briefly explain OLEODL technology, its challenges, and suitable mitigation techniques. The paper will focus on why varying the data rate according to channel state is important for OLEODL and how it can be achieved.