Enabling technologies for high-accuracy multiangle spectropolarimetric imaging from space

Satellite remote sensing plays a major role in measuring the optical and radiative properties, environmental impact, and spatial and temporal distribution of tropospheric aerosols. In this paper, we envision a new generation of spaceborne imager that integrates the unique strengths of multispectral, multiangle, and polarimetric approaches, thereby achieving better accuracies in aerosol optical depth and particle properties than can be achieved using any one method by itself. Design goals include spectral coverage from the near-UV to the shortwave infrared; global coverage within a few days; intensity and polarimetric imaging simultaneously at multiple view angles; kilometer to sub-kilometer spatial resolution; and measurement of the degree of linear polarization for a subset of the spectral complement with an uncertainty of 0.5% or less. The latter requirement is technically the most challenging. In particular, an approach for dealing with inter-detector gain variations is essential to avoid false polarization signals. We propose using rapid modulation of the input polarization state to overcome this problem, using a high-speed variable retarder in the camera design. Technologies for rapid retardance modulation include mechanically rotating retarders, liquid crystals, and photoelastic modulators (PEMs). We conclude that the latter are the most suitable. Two approaches for using a PEM to achieve high polarimetric accuracy are presented. In the first approach, amplitude modulation, the device is used intermittently to modify the incoming polarization state so that different detectors-those with polarizing filters in different orientations-can be accurately cross-calibrated. In the other approach, synchronous demodulation, signals accumulated during sub-cycles of the modulation are sorted and stored using a high-speed electronic charge-caching circuit built into the detector array.