David Strong, Strong EO Imaging, Inc.; Charles J. Wetterer, KBR; Timothy Giblin, i2 Strategic Services LLC; Matthew Fitzgerald, USAF Academy; Francis Chun, USAF Academy
Keywords: Polarimetry, Hyperspectral, SSA, SDA, Geosynchronous, Non-Resolved Objects, Characterization
Abstract:
Man-made satellites in geosynchronous orbital regimes cannot be resolved by imaging conducted through small aperture telescopes which are typically 1-meter diameter or smaller. In order to utilize these telescopes in space domain awareness (SDA), previous efforts have focused on the disjoint use of astronomical techniques such as multi- and hyper-spectral analysis as well as polarimetry. Recent polarimetric analysis of geosynchronous satellite observation conducted by the United States Air Force Academy’s (USAFA) 16-inch aperture telescope suggests the polarization of light can discriminate between signals from the solar panels and different features on the bus of a satellite. This, in itself, is not enough to reveal specific details about the composition of a satellite. In order to better extrapolate features of satellites from a photometric signature, it may prove useful to combine polarimetric information with spectral information. At USAFA, a new 1-meter aperture f/6 telescope features two separate filter wheels mounted between the telescope and the 9216×9232 pixel CCD. The first filter wheel serves as the polarimeter. The polarimeter is a set of four polaroid filters at orientations corresponding to angles 0 degrees, 45 degrees, 90 degrees, and 135 degrees with respect to the camera’s focal plane. After passing one of the filters in the polarimeter, light is then passed through a diffraction grating on the second filter wheel, creating a diffraction pattern. The diffraction grating has 200 lines/mm. The first order diffraction pattern for different wavelengths of light will occur at different pixels on the camera. The pixels corresponding to each wavelength vary based on the geometry of the imaging system but can be determined by observing stars with known spectra. By collecting the spectra of an object through each of the four polarization filters, it is possible to compute Stoke’s parameters (S0, S1, and S2) for each measured wavelength. With this, a degree and angle of linear polarization can be assigned to each color of light. Based on previous analysis of satellite polarization signatures, it is expected that the polarization of blue light (primarily reflected from solar panels) will increase proportionally with the phase angle. In addition to confirming previous results, this spectral polarimetry offers an opportunity to better characterize the material properties of observed satellites. The polarization of reflected light is determined by the index of refraction of a material while the wavelength is determined by its color. By combining wavelengths with matching polarimetric signatures, the true color of a specific material on a satellite may be recovered. With both color and polarimetric information, it is possible to compare specific aspects of a satellite’s photometric signature to known spacecraft materials, allowing the satellite’s composition to be identified through this form of spectral analysis. The polarization of reflected light is also determined by its angle of reflection, permitting its use in determining the geometry of specific materials on the target satellite. Ultimately, by unifying spectral and polarimetric analysis, small-aperture telescopes, incapable of resolving a satellite, may still provide utility in SDA by providing insight into the materials and geometry of a satellite.
Date of Conference: September 19-22, 2023
Track: Satellite Characterization