Madilynn Compean, Air Force Institute of Technology; Todd Small, Air Force Institute of Technology
Keywords: remote sensing, geostationary non-resolved imagery, anisotropic materials, solar panels, Bi-directional Reflectance Distribution Function (BRDF)
Abstract:
Light curve analysis is an alternative method used to investigate satellite activity, particularly in situations where high-resolution imagery from ground-based optical systems is impossible, such as geosynchronous satellite observations. Glinting features found in reflection patterns from satellites provide important distinguishing information, but large errors have been documented when trying to model such features. The bidirectional reflectance distribution function (BRDF) describes the spatial distribution of a materials reflectance as a ratio of the incident irradiance to scattered radiance, and BRDFs play an essential role in simulating light curves and interpreting light curve observations. BRDFs are a function of five variables, which includes two angles to describe the incident ray direction, two angles to describe the scattered ray direction, and wavelength. Due to their dependence on angles, BRDFs can be highly dependent on illumination and observation geometry. A common category of BRDF – microfacet models – uses a combination of geometric optics with a stochastic description of surface roughness. Microfacet models typically make simplifying assumptions which trade simulation accuracy for improved computing speed in scene generation applications. Another class of BRDF – physical optics models – are typically more radiometrically accurate but have greater complexity and increased needs for computing power. Due to the geometric optics basis of microfacet models, they fail to account for wave optics effects such as diffraction. Physical optics models easily account for wave effects, but there is an underlying assumption that the surface is completely characterized.
Previous work combined a microfacet model with a closed-form wave optics solution to account for distinct solar cell diffraction features, capitalizing on better accuracy with reduced computing power needs. This work presents in-plane, off-specular BRDF measurements for an updated multi-slit, duel diffraction pattern model for reflection from a solar panel while under laser observed in the near- and far-field. The duel diffraction pattern accounts for the reflection off the metal conducting grids on the panels surface as well as the absorptive InGaP layer between the metal grids. The theoretical modification to the solar cell model is analyzed using high fidelity, low density measurements gained from a modified Complete Angle Scatter Instrument (CASI).
In BRDF coordinates, the origin is located in the solar panel plane, and each location on the Earth is then associated with a particular scatter direction relative to that origin. In certain geometries, the solar cells diffraction pattern may scatter across the Earths surface, so that peaks in brightness may be observed in situations where the typical law of reflection would be not satisfied for purely specular reflection. The diffraction pattern peaks which contribute to off-specular signals on the Earths surface are not currently modeled using typical microfacet models, which typically assume simple, isotropic surfaces. Failing to account for conical, multi-slit and shadowing diffraction effects from a solar panel leaves open the possibility of mis-modeling and mis-interpreting such reflection within light curves.
Date of Conference: September 19-22, 2023
Track: Satellite Characterization