Effect of cloud polarization properties on target discrimination

The strong polarization signatures of many man-made surface targets suggest their use for detection and discrimination in satellite reconnaissance and surveillance. Since thin clouds on average cover more than 40% of the Earth's surface, they are likely to contribute frequently to the obscuration and apparent polarization of such targets. We have carried out theoretical calculations of cloud particle scattering functions in combination with Monte Carlo calculations of radiative transfer in clouds, in order to determine the polarization signatures of thin clouds as a function of the relative spatial orientations of the Sun, target, and detector, the cloud optical depth, the cloud particle phase (water or ice), and the electromagnetic wavelength. Our calculations of the scattering properties of individual cloud particles revealed situations in which the polarization of scattered, initially unpolarized, sunlight is very high. The theoretical cloud signatures were then combined with image polarization data for man-made surface targets and natural backgrounds in order to determine the consequences for the composite image with clouds of various optical depths. For the case investigated here we find that polarization signatures can be a useful discriminant for a surface target even when viewed through a cloud at optical depths as high as two.