Surface Material Characterization from Multi-band Optical Observations

Abstract

Abstract : Ground-based optical and radar sites routinely acquire resolved images of satellites. The images provide means to construct accurate wire-frame models of the observed body, as well as understanding of its orientation as a function of time. Because such images are typically acquired in a single spectral band, they provide little information on the types of materials covering the various surfaces. Detailed surface material characterization requires spectrometric and/or multi-band photometric measurements. Many instruments provide such multi-band information. However, the sensors often measure brightness of the entire satellite, with no spatial resolution. Because such whole-body measurements represent a summation of contributions from many reflecting surfaces, an un-mixing or inversion process must be employed. First section of this paper describes the inversion theory required to retrieve satellite surface material properties from temporal sequences of whole-body multi-band brightness measurements. The inversion requires the following input: 1) multi-band measurements of a satellite's reflected brightness, 2) satellite's wire-frame model, including each major component capable of reflecting sunlight, 3) satellite's attitude, specifying the orientation at the time of each multiband measurement, and 4) database of bi-directional reflection distribution functions for candidate surface materials. As output, the inversion process yields estimates of the fraction of each major satellite component covered by each candidate material. Second section describes tests of the method by applying it to simulated multi-band observations of a cubical satellite with different materials. Tests indicate the inversion method successfully retrieves the six known materials when provided a noise-free scan of the cube.