Temporal and spatial impulse response of a water cloud channel for free space optical communication

Free space optical communication between spacecraft and earth in the presence of an overcast sky is essentially the transmission of optical signals through multiply scattering media. The multiple scattering involved in the process may be due to cloud, haze or fog. The present study considers the scattering due to cloud only. The parameters of the cloud (i.e. propagation model and microphysical characteristic) are chosen to approximate the low altitude cumulus cloud. Cloud, as a part of the optical communication channel, causes temporal and spatial widening of the optical signal. Here, based upon Monte Carlo model, numerical results of the impulse response of the cloud are obtained at certain wavelength and with a specified field of view of the optical receiver. This model was based on the forward-propagating photon transport algorithm; where it was designed to trace the trajectories of photon packets from radiation sources (delta laser beams) to termination due to absorption or escape from the top of the cloud. The response of the cloud is calculated by using the concept of the local estimation method. The results demonstrate the response from two optical thicknesses tau = 6, 8 and show the estimation of the Monte Carlo error.