Use of Radiative Transfer Equation (RTE) for Estimating Optical Signal Attenuation through Inhomogeneous Clouds

High data rate requirement has necessitated the use of optical wireless links from satellite-to-ground that invariably pass through the inhomogeneous cloud layers. It has thus become necessary to accurately model the scattering channel and estimate attenuation suffered by the information bearing optical signal traversing the multiple scattering inhomogeneous clouds. This paper introduces the utilization of three dimensional radiative transfer equation RTE for prediction of attenuation in the optical signal traversing the cloud layers. The three dimensional radiative transfer equation explicitly handles the spatial complexity and multiple scattering effects of inhomogeneous clouds and provides accurate propagation channel estimation without invoking mathematical assumptions. Cloud droplets mainly composed of liquid water attenuates the beam of light through absorption and scattering in forward direction. Therefore, it is necessary to generate three dimensional cloud fields that exhibit realistic spatial distribution of the cloud structure. This statistically generated cloud field from experimental data is used as an input to three dimensional radiative transfer model to calculate transmitted irradiance at the bottom of cloud to predict attenuation in cumulus cloud scene. This paper employs the numerical solution of three dimensional radiative transfer equation using Monte Carlo simulation that uses stochastic methods to simulate physical processes of scattering and absorption within inhomogeneous cloud layers. The open source I3RC Monte Carlo code has been chosen to perform simulations at commonly employed optical wavelengths of 850 nm and 1550 nm. The 10 μm has been specifically analyzed to gain insight of wavelength dependence in optical propagation through clouds.