1. Different link types for different applications

Two main categories of links can be distinguished in the field of optical space communications:

• between two terminals on two satellites (regardless of their respective orbits);

• between a terminal on board a satellite (or other spacecraft) and a terrestrial terminal (ground, sea, airplane, etc.).

The major difference between these two links is the propagation channel. The first link is established between two terminals separated by a vacuum, while the second takes place partly in the earth's atmosphere. We'll refer to the former as an optical inter-satellite link (OISL), and the latter as an optical cross-atmospheric link (OCAL). Crossing the atmosphere brings with it additional constraints compared with crossing a vacuum, particularly in terms of atmospheric turbulence....