Analysis of the CanX-1 Engineering Model Spectral Reflectance Measurements

Abstract

This paper presents a detailed analysis of the CanX-1 engineering model spectral reflectance measurements that were collected during the model’s spectrometric characterization. Over 1000 low-resolution spectra spanning a range between 350 and 2500 nm were gathered over a wide range of observational geometries in a controlled laboratory environment. As part of the data analysis, a physics-based reflectance model was implemented in a numerical computing environment to allow for a better understanding of the measurements. In the end, the experiment led to two unexpected results. First, the spectrometric measurements of the triple-junction photovoltaic cells showed significant interference fringes at wavelengths greater than 800 nm, which, at the time of this experiment, did not correspond to any previously published spectra for this type of material. Second, these empirical results also showed that the spectral reflectance of photovoltaic cells varied significantly as a function of the illumination geometry. The modeling results confirmed this finding and showed that this was also the case for an aluminum surface with an rms surface roughness different from zero. These results invalidate a widely held assumption among those who study the spectrometric characterization of artificial space objects that the spectral reflectance of homogeneous materials is invariant. The outcomes of this study not only serve to improve the laboratory characterization of spacecraft in controlled environments before their launch but also provide new insight in the interpretation of spectrometric measurements of artificial space objects, namely spacecraft and space debris, in Earth orbit.