Multiangle SpectroPolarimetric Imager (MSPI).

摘要

Substantial impacts of aerosols on climate and public health underscore the need for accurate characterization of atmospheric aerosol distributions and microphysical properties. The Multiangle SpectroPolarimetric Imager (MSPI) combines accurate multispectral, multiangle, and polarimetric technologies in a single instrument that images a wide swath on the Earth's surface to advance aerosol remote sensing capabilities. MSPI is required to have 3% radiometric uncertainty and 0.005 degree of linear polarization (DoLP) uncertainty. These are difficult requirements that push the limits of available technologies needed to perform space-based polarimetric imaging. This work examines three topics related to MSPI fabrication and calibration: polarization errors and their correction, achromatic, athermal, quarter wave retarder fabrication, and analysis of a polarization state generator (PSG) for MSPI polarization calibration confirmation.;MSPI polarization errors may arise from surface geometry of the optical components, coatings, and quarter wave plates (QWPs). Static polarization errors can be calibrated out, but result in decreased SNR. Polarization errors that drift following calibration cannot be corrected, so a sensitivity analysis is used to set time-varying diattenuation and retardance magnitude tolerances. QWPs are required to work in concert with the PEMs to modulate the linear component of the Stokes vector. A three-material achromatic, athermalized QWP was designed, fabricated and its performance validated. Analysis indicated that the compound QWP was unlikely to meet the requirements if plates were specified by thickness. To address this, a method for QWP fabrication was developed that involves monitoring retardance during polishing. To verify MSPI performance, a PSG was built and calibrated which outputs weakly linearly polarized light with DoLPs varying from 0.0005 to 0.4 with 0.0005 uncertainty by passing nearly unpolarized light through a tilted plane parallel plate. The PSG was intended to act as a calibration standard based on calculated DoLP, but proved difficult to model. Therefore, the DoLP was instead measured to repeatability of 0.0005. Finally, example spectropolarimetric image data taken with MSPI was presented. Work on a follow-on prototype continues that will advance the technologies needed to realize the space-based, fully capable MSPI.