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Satellite Multiangle Spectropolarimetric Imaging of Aerosols

The instrument (see block diagram) would acquire data in push-broom fashion in nine cross-ground-track swaths, each aimed at a different along-ground-track look angle. A separate camera would be used for each look angle, but all nine cameras could be of the same design. Reflective telescope and camera optics would be used because they offer significant advantages over refractive optics, including high transmittance over the broad wavelength range of interest, absence of chromatic aberration, the possibility of achieving the desired effects by use of fewer optical elements, less susceptibility to formation of ghost images, and better and more stable polarization performance. The instrument would measure intensity at wavelengths of 380, 412, 446, 558, 866, 1,375, and 2,130 nm. Polarization plus intensity would be measured at 670 and 1,630 nm.

The design of the instrument would be dominated by the requirement for polarimetric accuracy. Of several polarimetric approaches that were considered, the one selected to satisfy this requirement involves adaptation of advanced techniques of high-precision imaging polarimetry used in ground-based solar astronomy. The approach involves the use of (1) a photoelastic modulator to rapidly rotate the plane of polarization, (2) one or two analyzers and corresponding linear arrays of photodetectors, and (3) demodulation of the photodetector outputs to obtain data from which one can compute the Stokes parameters Q and U (pertaining to the predominance of horizontal over vertical linear polarization and the predominance of 45° over 135° polarization, respectively). In order to implement the specific detection scheme, it would be necessary to design and build flight-qualified modulators, as well as a custom complementary metal oxide/semiconductor integrated circuit for the low-noise, rapidreadout photodetector array.

This work was done by David Diner, Steven Macenka, Lawrence Scherr, and Suresh Seshadri of Caltech; Russell Chipman of the University of Arizona; and Christoph Keller of the National Solar Observatory for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-40936

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