Progress in Developing a Multiangle SpectroPolarimetric Imager (MSPI) for Aerosol Remote Sensing from Space

The National Research Council's Earth Sciences Decadal Survey identifies a multiangle, multispectral, high- accuracy polarization imager as one component of its notional Aerosol-Cloud-Ecosytem (ACE) mission. Under NASA's Instrument Incubator Program (IIP) and internal JPL funding, we have been developing a candidate instrument approach, the Multiangle SpectroPolarimetric Imager (MSPI). The MSPI architecture is conceptually similar to the Terra Multi-angle Imaging SpectroRadiometer (MISR), but the new camera design incorporates features of other aerosol instruments by extending the spectral range to the ultraviolet and shortwave infrared, increasing the image swath to achieve more rapid global coverage, and adding high-accuracy polarimetry in selected spectral bands. UV intensity observations are sensitive to aerosol absorption and height; the longer wavelengths provide improved particle size discrimination; and multiangle acquisition provides sensitivity to particle shape and helps separate aerosol backscatter and extinction from surface reflectance. The 0.5% DOLP uncertainty specification allows for the simultaneous retrieval of aerosol optical depth and particle size when combined with accurate radiance measurements, and provides sensitivity to the real part of the aerosol refractive index, thus providing unique information related to particle composition. Many factors can affect polarimetric accuracy for an imager, including polarization sensitivity of the optics, gain differences among the different detectors whose signals are combined to measure polarization, and spatial displacements on the ground of the locations where different polarization orientations are measured. The MSPI camera design deals with these issues by: (a) using a reflective optical design with optimized mirror coatings to minimize instrument-induced polarization, (b) introducing a rapid, time-variable retardance into the optical path, which has the effect of modulating the polarized component of the incoming light so that DOLP is recovered using relative measurements from each detector, and (c) accounting for scene gradients in the measurement approach. We report on experimental and theoretical results that establish detailed camera design requirements, the status of enabling technology developments, and progress in constructing and testing a prototype spectropolarimetric camera.