High-altitude demonstration of LWIR polarimetry using uncooled microbolometers

An Infrared Channeled Spectro-Polarimeter (IRCSP) was successfully demonstrated in a near-space environment on a high-altitude balloon mission out of the NASA Columbia Scientific Balloon Facility in August 2021. The compact IRCSP is sensitive to linearly polarized thermal radiation between 7-12 μm and targets polarized cloud top scattering. The IRCSP comprises a series of polarization modulation components followed by a diffraction grating and lens that images a polarization modulated spectrum at the focal plane. At altitudes greater than 30 km, the IRCSP reported brightness temperatures (𝑇𝑏 ) between 250–285 K with an uncertainty of less than 1.5K. These IRCSP 𝑇𝑏 values overlapped the collated GOES-16 cloud top temperatures within the pointing uncertainty of the instrument. IRCSP’s inaugural high-altitude deployment demonstrated the operation of uncooled microbolometers in the low-pressure environment and resulted in the first known observations of downward-viewing thermal polarized radiation from Earth’s atmosphere. Using the Lomb–Scargle periodogram with a maximum acceptable false alarm probability of 1%, the IRCSP detected statistically significant polarized signals with average degrees of linear polarization (DoLP) ranging from 1% to 20%. The top-of-atmosphere simulations that motivated IRCSP’s development predict that LWIR polarization is highest for ice clouds of moderate optical thickness. IRCSP observed cloud-top DoLP trends with brightness temperature that are consistent with these simulations. The highest and lowest 𝑇𝑏 values occur for no clouds and optically thick clouds, respectively. The lowest DoLP measurements by IRCSP were at these extreme conditions. The DoLP peaked at intermediate 𝑇𝑏 values. This preliminary work motivates IRCSP’s participation in future deployments that provide independent data products of cloud microphysical properties.