Assessment of Signal Losses in LEO Satellite-to-Ground Optical Communication Links

We present a model and a computation of signal losses in satellite-to-ground optical communication links for an low Earth orbit (LEO) satellite operating in the C-band, with an optical ground station (OGS) located in Zagreb, Croatia. The total loss estimate includes contributions from satellite and OGS telescope gains, free-space loss, atmospheric attenuation, pointing error, power scintillation loss, optical transmission in the Coudé-path, losses due to the optical elements in the OGS, single- mode fiber loss, and quantum efficiency of the photodetector. Understanding these losses is crucial for estimating secure key rates and distilled keys for the quantum key distribution (QKD) protocol that will be used between planned European satellite for quantum communication and the optical ground station in Zagreb. We show that the total loss ranges from 29.91 dB to 56.71 dB, depending on the elevation angle.