The Multi-angle Imaging SpectroRadiometer (MISR) has been acquiring global cloud and aerosol data from polar orbit
since February 2000. MISR acquires moderately high-resolution imagery at nine view angles from nadir to 70.5°, in four
visible/near-infrared spectral bands. Stereoscopic parallax, time lapse among the nine views, and the variation of
radiance with angle and wavelength enable retrieval of geometric cloud and aerosol plume heights, height-resolved
cloud-tracked winds, and aerosol optical depth and particle property information. Two instrument concepts based upon
MISR heritage are in development. The Cloud Motion Vector Camera, or WindCam, is a simplified version comprised
of a lightweight, compact, wide-angle camera to acquire multiangle stereo imagery at a single visible wavelength. A
constellation of three WindCam instruments in polar Earth orbit would obtain height-resolved cloud-motion winds with
daily global coverage, making it a low-cost complement to a spaceborne lidar wind measurement system. The
Multiangle SpectroPolarimetric Imager (MSPI) is aimed at aerosol and cloud microphysical properties, and is a
candidate for the National Research Council Decadal Survey's Aerosol-Cloud-Ecosystem (ACE) mission. MSPI
combines the capabilities of MISR with those of other aerosol sensors, extending the spectral coverage to the ultraviolet
and shortwave infrared and incorporating high-accuracy polarimetric imaging. Based on requirements for the nonimaging
Aerosol Polarimeter Sensor on NASA's Glory mission, a degree of linear polarization uncertainty of 0.5% is
specified within a subset of the MSPI bands. We are developing a polarization imaging approach using photoelastic
modulators (PEMs) to accomplish this objective.
Through acquisition of well-calibrated near-nadir and oblique-angle imagery (0° - 70° zenith angles) at moderately high
spatial resolution (275 m - 1.1 km), the Multi-angle Imaging SpectroRadiometer (MISR) experiment aboard NASA's
Terra satellite has taken atmospheric remote sensing in new directions. Retrieval algorithms that were largely conceptual
prior to Terra launch in 1999 have led to publicly available aerosol and cloud products with direct application to global
climate and particulate air quality research. Automated algorithms making use of stereoscopic parallax, time lapse
among the nine angular views, and the variation in radiance with view angle, scattering angle, and wavelength (446-866
nm) make possible unique data sets including geometric cloud and aerosol plume heights derived independently of
emissivity or temperature assumptions; height-resolved cloud-tracked winds; and aerosol optical depth and particle type
over a wide variety of surfaces including bright desert source regions. To illustrate these capabilities, examples of
regional and global MISR data products, quantitative evaluations of product accuracies based on comparisons with
independent data sources, and time series showing seasonal and interannual variations are presented here. Future sensor
improvements aimed at building upon MISR heritage, including expanding the spectral coverage to ultraviolet and
shortwave infrared wavelengths, adding polarization channels, and widening the sensor swath, are also discussed.
The importance of measuring the complete global range of cloud optical depth distribution is reviewed. While current
techniques do a fairly good job of measuring small and moderate thicknesses, the difficulty of measuring large optical
depths is noted. The use of multiangle views of cloud reflectivity offers a possible solution to this difficulty, at least for
those clouds that have unobscured sides. Preliminary work with Multiangle Imaging SpectroRadiometer (MISR) data
shows the ability to constrain the retrieved optical depths using a combination of geometrical reconstruction and three-dimensional
radiative transfer modeling.
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