Optical Communication from HAPs - Overview of the Stratospheric Optical Payload Experiment (STROPEX)

Abstract

This paper describes the STRatospheric OPtical Downlink EXperiment (STROPEX) which is part of a large European Union funded research project called “CAPANINA” (Communications from Aerial Platform Networks delivering Broadband Communications for All). The aim of STROPEX is to demonstrate a high-bitrate optical downlink from a stratospheric balloon or High Altitude Platform and perform measurements on the optical channel. At first, an overview of the CAPANINA project is given. Then the system approaches for the optical trials are presented. Atmospheric attenuation effects on optical waves are described and calculation results of total attenuation for the downlink and used wavelengths are presented. Index-of-refraction effects and their impact on the intensity distribution at the receiver are introduced. The spatial covariance function, the probability density functions (PDF), derived from numerical simulations are discussed for two different turbulence conditions. In the last part, instruments to measure atmospheric parameters are described. These instruments are used to characterize the optical atmospheric channel during the campaigns. The first instrument measures the deterioration of the beam wavefront caused by atmospheric turbulence. The second instrument is a real-time turbulence profiler. The atmospheric turbulent layers are derived from measurements of the spatial distribution of the intensity at the receiver. A brief introduction to the working principles of these two instruments and the favored approach for STROPEX is given.