Detection of a poorly resolved airplane using SWIR polarization imaging

Polarization can be used to detect manmade objects on the ground and in the air, as it provides additional information beyond intensity and color. Skylight can be strongly polarized, so the detection of airplanes in flight requires careful consideration of the skylight degree and angle of polarization (DoLP, AoP). In this study, we detect poorly resolved airplanes (>= 4 pixels on target) in flight during daytime partly cloudy and smoky conditions in Bozeman, Montana. We used a Polaris Sensor Technologies SWIR-MWIR rotating imaging polarimeter to measure the polarization signatures of airplanes and the surrounding skylight from 1.5 to 1.8 mu m in the short-wave infrared (SWIR). An airplane flying in a clear region of partly cloudy sky was found to be 69% polarized at an elevation angle of 13 degrees with respect to the horizon and the surrounding skylight was 4-8% polarized (maximum skylight DoLP was found to be 7-14% at an elevation angle of 50 degrees). As the airplane increased in altitude, the DoLP for both airplane and surrounding sky pixels increased as the airplane neared the band of maximum sky polarization. We also observed that an airplane can be less polarized than its surrounding skylight when there is heavy smoke present. In such a case, the airplane was 30-38% polarized at an elevation angle of 17 degrees, while the surrounding skylight was approximately 40% polarized (maximum skylight DoLP was 40-55% at an elevation angle of 34 degrees). In both situations the airplane was most consistently observed in DoLP images rather than S-0 or AoP images. In this paper, we describe the results in detail and discuss how this phenomenology could detect barely resolved aircrafts.