FSO Proceedings of EuCAP2011 - Dipartimento di Ingegneria ...

A fast analytical approximated method of solution for RTE,based on the generalization of the Eddington approximationand capable to unfold the azimuthal dependence of theradiance field, has been developed [17], [18]. Performances ofthis new radiative model, named GERM (GeneralizedEddington Radiative Model), in simulating the backscattered(reflected) and forwarded (transmitted) intensity field due to aplane- parallel homogeneous medium, have been evaluated bycomparison with the “exact” DISORT numerical model. Themain theoretical limitations of GERM are related to theassumption of: i) stratified medium geometry; ii) unpolarizedradiation. A way to apply GERM to a 3-D problem is to resortto a 1-D slant geometry along the viewing angle, that is toconstruct an equivalent 1-D problem from the given 3-D one.The limitation due to the hypothesis of unpolarized radiancecan be only removed if the elements of the rotated phasematrix can be approximated by means of a Sobolev phasefunction [16]. The validity of this approximation would, ofcourse, depend on the properties of the particle distributionPreliminary GERM numerical results are shown in Fig. 6 interms of percentage fractional error, with respect to DISORToutputs, as a function of optical thickness and volumetricalbedo for asymmetry scattering factor g. The percentageerror is less than 10% on average, being higher for largeralbedo and optical thickness.IV. PRELIMINARY MEASUREMENTSPreliminary measurements, carried out in Rome using theFSO link illustrated in Sect. II, are shown here. The temporaltrend of temperature and humidity for an event with moderatestratiform rain with some embedded convection is shown inFig. 7. The coupled FSO/meteorological measurements spanbetween December 14 and 15, 2010 and clearly evidences theanti-correlation between thermal and wet characteristics of theatmosphere.For the considered winter event Fig. 8 shows the sametemporal trend of Fig. 7, but for the measured rainfall andreceived power at 1550 nm. The received power is anticorrelatedwith rain intensity: when the latter increases, thereceived power decreases and viceversa. Fig. 9 shows themeasured path attenuation, derived from the received power inFig. 8, and the predicted rain specific attenuation, derivedfrom the semi-empirical model (3). There is a good agreementin terms of value range, even though a certain dispersion isnoted probably due to the raingauge sensitivity (about 0.2mm/h). Further analysis is needed to investigate themeasurement quality and test attenuation prediction models.2520Precipitationrain [mm/h]1510500 5 10 15 20 25 30 35 40 45 50time [h]90Received power [W]80706050403020100 5 10 15 20 25 30 35 40 45 50time [h]Fig. 8. Same as in Fig. 7, but for rain (mm/h) and received power (W).Fig. 7. Temporal trend of temperature (K) and relative humidity (%) overa period of 50 hours on Dec. 14-15, 2010 in Rome (Italy).V. CONCLUSIONSThe activity devoted to the deployment of a new free spaceoptics (FSO) link set up in Rome (Italy) has been illustrated.