Modelling optical refraction and turbulence effects within the MSBL (Marine Surface Boundary Layer)

摘要

PIRAM is a French bulk model that computes, within the MSBL, vertical refractivity profiles and refractivity gradients calculated for each optical transmission window as well as for radar bands. PIRAM also computes the C_n~2 vertical profiles. Under unstable situatiosn, subrefraction occurs and reduces the optical horizon. Near the horizon an intervisibility zone may be observed: any source (target) located in this zone will be seen by an EO sensor under 2 distinct apparent elevation angles. We develoepd, a few years ago, a simple ray-tracing algorithm using PIRAM refractivity outputs, to compute optical horizons and intervisibility ranges for a given atmospheric situation. More recently, we have added new capabilities to our ray-tracing program; now, it also computes the "refractance" parameter and the optical path between a given optical source or target and the considered EO sensor. Atmospheric turbulence effects are quantified by several parameters such as the scintillation variance, the atmospheric coherence length or the standard deviation of the angular displacement. All these parameters are computed by taking into account the exact optical path and the complete C_n~2 vertical profile. Our first comparisons with the values provided by the Canadian IRBLEM softwre lead to promising results.