Experimental study of non-resolved active polarimetry for space surveillance

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2016.Cataloged from PDF version of thesis.Includes bibliographical references (pages 119-124).Man-made space debris constitutes a major threat to the future of the space enterprise. The space surveillance community continually seeks more efficient and robust techniques for detecting and characterizing on-orbit debris. This thesis investigates the potential utility of a technique known as active polarimetry, by which a laser radar would illuminate a space object with polarized light and measure the polarization state of the reflected light. A debris fragment's polarimetric signature can help identify its material, shape, and orientation, and, by inference, its mass, origin, and other characteristics. The research takes both an experimental and modeling and simulation approach A bench-top polarimeter ([lambda] = 1064 nm) is used to determine the polarimetric Bidirectional Reflectance Distribution Function (BRDF) of several common spacecraft materials and coatings, including glossy white paint, matte black paint, black Kapton®, silver Teflon®, aluminum, and titanium. Measurements are made in both bistatic (in-plane scans for incident angles of 15°, 30°, 45°, 60°, and 75°) and monostatic (incident angles from 0° to 90°) geometries. The Mueller matrix of each material is then estimated as a function of the illumination and viewing angles. The results reveal notable trends in the materials' geometry-dependent polarimetric properties, particularly diattenuation (D), retardance (R), and depolarization power ([delta]). At specular points, metallic surfaces (i.e., aluminum and titanium) exhibit mirror-like behavior (D = 0, R = 180°, [delta] = 0), while paints and thin films (e.g., Kapton®) are diattenuating (D > 0). All the materials tend to be more depolarizing in the monostatic diffuse regime. Silver Teflon® follows the trends of a metallic surface, with the exception of its distinct retardance at the specular point (R = 115°) and range of retardance values in the bistatic diffuse region (R = 70° to 120°). Since measurements of on-orbit space debris will nominally be non-resolved (in angle), a simulation is also developed (and validated by experiments) to predict the polarimetric signature of non-resolved objects, given the measured polarimetric BRDFs of their constituent materials. The simulation is used to explore object signatures in a variety of engagement scenarios, including monostatic interrogations of stationary and tumbling objects with representative shapes (i.e., panels, spheres, and cylinders), as well as bistatic interrogations of objects with strong specular reflections. The results demonstrate that the signature of a non-resolved object is complex, but can be described as the weighted sum of the geometry-dependent polarimetric behaviors of its facets. In some cases, the signature bears a close resemblance to the behavior of the constituent material, e.g., a white-painted sphere exhibits D = 0, R = 180°, and [delta] = 0.88 in a monostatic geometry, which matches the behavior of glossy white paint in the monostatic diffuse regime. In other cases, the signature is unlike the behavior of any individual facet due to the way the facets' behaviors combine geometrically, e.g., a black-painted sphere exhibits A = 0.67, unlike the behavior of matte black paint at any angle ([delta] 0.5 (or polarizance P > 0.5) cannot be metallic based on the distribution of possible signatures of metal surfaces. A fast-tumbling panel-shaped object covered in silver Teflon® exhibits the characteristic retardance (R = 115°) of silver Teflon* in a monostatic geometry. The monostatic signature of a fast tumbling object can still be indicative of its shape, e.g., a white-painted sphere exhibits a distinctly high depolarization power ([delta] = 0.88) compared to the low depolarization power ([delta] < 0.12) of a fast tumbling panel-shaped object or cylinder with the same coating. Since a passive system can only estimate an object's polarizance (P), current optical telescopes would not be able to determine and exploit many of these distinguishing features such as retardance and depolarization power. Several operational schemes for interrogating space objects with a ground-based polarimetric laser radar are proposed, including short- and long-duration interrogations and interrogations whose measurements are synchronized with the tumbling period of the object. The utility of polarimetric features is discussed in terms of their ability to discriminate between objects with different materials, shapes, and orientations, as well as to obtain fingerprints that can be used to identify objects in the future and monitor their changes. A look-up table is proposed to determine the number and types of measurements required for estimating different polarimetric properties. The table may be referenced to optimally plan a measurement campaign in the field that maximizes the number of objects measurable in a given period of time. The simulation tools and experimental configuration developed for this research are generally useful for assessing the utility of active polarimetry for other applications.by Michael C. Pasqual.Ph. D