Study On Light Scattering Characteristics Of Bare Surface Over Rugged Terrain

The study on the optical properties of targets and background environment plays an instructive role in the areas of target detection and recognition as well as guidance. So the research on the characteristics of bidirectional reflectance distribution functions(BRDF) and the establishment of database and model of bare surface over rugged terrains have significance on the characteristics of background light radiation. In this paper, the BRDF models of typical objects are firstly introduced and the analysis of distinguishing features as well as their merits and drawbacks is also done. Then a three-dimensional mesh model of rugged terrains is built using 3DS MAX software based on digital elevation model data. And the average normal vector of each slope element is derived from its slope angle and aspect. Given the sun-view geometry in global system, the sun-view geometry of each slope element in local coordinate system can be calculated through coordinate conversion. Considering the shadowing effect, a BRDF model of bare surface over rugged terrains is established in visible and near-infrared band. The validation of the model is conducted, taking Badain Jaran desert as an example. The effect of degree of surface relief with different objects and solar zenith angle on the BRDF model is studied. The results show that both terrain fluctuation and changes of solar and viewing angles would result in the changes in illumination and shading. And the tendency of BRDF varying with viewing azimuth angle of different rugged terrains has characteristic asymmetric distribution in the given sun geometry. BRDF in near perpendicular plane is larger than that in near principal plane. BRDF of the same rugged terrain decreases with the increase of solar zenith angle. BRDF tends to be larger as the wavelength increases. Moreover, BRDF of snow tends to be larger at the same wavelength, followed by soil and desert. Then the influencing factors of surface BRDF are analyzed using the Monte Carlo method to simulate the complex background as random rough surface. What's more, a 3D model of target and background is built. Combined with BRDF measurement result of target samples, the light scattering intensity of target and rugged terrain is calculated. Then the influence of surface relief, land cover types and target attitude on target and background contrast is conducted. The results indicate that light intensity of land surface covered with snow is extremely strong. In sharp contrast, soil and desert tend to be weaker and similar. So land cover types is one of the main factors that leads to the changes of target and background contrast in addition to degree of surface relief. Meanwhile, target attitude has an effect on the contrast to some degree. Finally, it would lead considerable error regarding the land surface as a Lambertian surface, which would result in considerable difference in intensity between calculated and theoretical values.