Analysis of power scintillation and fading margin in the LEO-ground downlink with the OSIRISv1 laser terminal on Flying Laptop and the DLR optical ground station Oberpfaffenhofen

Abstract

Free-space optical communications is an emerging field for a variety of use cases in the domain of satellite communications. It is deployed in Earth observation missions to downlink payload data, in telecom missions (GEO or LEO based) to up- and downlink data streams and in quantum communication missions to implement the needed quantum and classical channels. Part of the propagation path goes through the atmosphere where the propagating optical wave experiences wave-front distortions and consequently distortions of the irradiance field. These distortions lead to scintillation of the optical power in the focal plane of the optical ground stations receiver front-end which can cause signal outages. Characterization of power scintillation is very important to assess the needed fading margin in the communication link design. Thus, these power scintillations are matter of investigation in this paper. Measurement campaigns were conducted to experimentally characterize the power scintillation. Received power was measured with a 40 cm telescope located at the DLR site Oberpfaffenhofen, i.e. in a suburban area. Satellite source is the OSIRISv1 laser terminal on the LEO satellite Flying Laptop. The campaigns were conducted in the years 2018, 2019 and 2020 where data of 15 passes could successfully be recorded. The power scintillation statistics are analyzed over elevation. The power scintillation index shows different behavior from pass to pass which is due to the different environmental conditions during the individual passes. Median values and quartiles of power scintillation index over elevation are given. Furthermore, statistics of fading margin for link budget calculation are derived. Both can be used to define best, nominal and worst cases in the design of the LEO-ground communication link.