We model the effects of the leaves of mature broadleaf (deciduous) trees on air-to-ground free-space optical communication systems operating through the leaf canopy. The concept of leaf area index (LAI) is reviewed and related to a probabilistic model of foliage consisting of obscuring leaves randomly distributed throughout a treetop layer. Individual leaves are opaque. The expected fractional unobscured area statistic is derived as well as the variance around the expected value. Monte Carlo simulation results confirm the predictions of this probabilistic model. To verify the predictions of the statistical model experimentally, a passive optical technique has been used to make measurements of observed sky illumination in a mature broadleaf environment. The results of the measurements, as a function of zenith angle, provide strong evidence for the applicability of the model, and a single parameter fit to the data reinforces a natural connection to LAI. Specific simulations of signal-to-noise ratio degradation as a function of zenith angle in a specific ground-to-unmanned aerial vehicle communication situation have demonstrated the effect of obscuration on performance.
The effects of clouds on optical down-links (satellite to ground) have been recognized by a variety of authors [7,12]. However, the typical approach has been simply to explore the probability of a "cloud-free line of sight." This somewhat ambiguous criterion does not acknowledge that a communication link, albeit at a degraded data rate, could still be established in the presence of clouds of certain optical thicknesses [1,2]. Moreover, it's not obvious what "cloud-free" means. Is there some implied threshold below which the path is considered to be free of clouds?To realistically model the effects of clouds on the performance of an optical communication system, we make use of existing databases that contain cloud statistics parameterized on optical density [9,10]. In conjunction with these statistics, we use a radiation transport model parameterized on optical thickness that describes the spatio-temporal spreading effects of multiple scatter [4,5]. Scatter characteristics (scatter and extinction cross-sections, asymmetry parameters) used in this model are determined from particle size distributions by Mie theory [6,11]. Together, these databases and this model allow derivation of link availability probabilities. Results are presented in terms of geographic probability maps of communication at or above prescribed bandwidths. ⇒ Mie Theory ⇒ β s , β a , 〈cosθ〉, 〈cos 2 θ〉 Proc. of SPIE Vol. 4821 321 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 4821 323 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 4821 325 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms
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