Full sky imaging polarimetry for initial polarized modtran validation

Abstract

Although military studies of the last ten years have shown that visible polarimetry supplies supplemental surveillance information, the polarimetric signatures of ground-based objects greatly depend on the illuminating skylight polarization. The polarization of a pure molecular atmosphere is easily modeled, but aerosols and clouds modify clear-sky polarization substantially. The Air Force has developed a polarimetric atmospheric radiative transfer model (MODTRAN-P) to simulate atmospheric effects. To assist MODTRAN-P code validation, a full-sky visible polarimeter has been developed using liquid crystal variable retarders (LCVRs). Unique calibration issues of LCVR instruments are addressed. A fisheye lens can be exchanged for a telephoto lens to provide system flexibility. This allows comparison between changing sky and changing target signatures.Calibration accuracy is within ±3% Degree of Linear Polarization (DoLP). Comparison of measured data with MODTRAN-P calculations shows that single-scatter models over-predict the sky polarization, while the improperly implemented multiple-scatter models under-predict it. Furthermore, a model comprising only one scattering path looking directly at a cloud insufficiently predicts polarization. The polarization is dependent upon whether or not there are clouds in surrounding areas. Similarly, clouds affect adjacent clear sky polarization, but further instrumentation is needed to understand whether this is caused by sub-visual cloud layers in these clear-sky areas or by illumination from neighboring clouds. Halo and cloud polarizations are also treated briefly.