Multi-angle polarimetry

Multi-angle polarimetry measures the polarization state of sunlight reflected from Earth's atmosphere and surface across multiple viewing angles, using linear-polarization channels (Degree of Linear Polarization, DoLP) to retrieve aerosol and cloud microphysical properties, including particle size, shape and refractive index, that intensity-only radiometry cannot resolve. Because a single-angle intensity measurement is ambiguous for these properties, retrieval depends on collecting polarized radiance at several fixed or scanned view angles along and across the satellite ground track. Swath width and pixel size vary by an order of magnitude between implementations: NASA's SPEXone instrument, also flown on the PACE mission, uses a compact hyperspectral-spot design with a 100 km swath,^[1] while HARP2's wide multi-angle imaging design covers a swath of about 1,550 km,^[2] reflecting a tradeoff between area coverage and spectral detail.

NASA's HARP2 instrument, flown on the PACE mission, illustrates the wide-coverage end of this range. It measures linear polarization at 0, 45 and 90 degrees across up to 60 along-track viewing angles at 669 nm (10 angles at 441, 549 and 873 nm), spanning plus or minus 57 degrees along-track with a 94-degree cross-track field of view and a swath of about 1,550 km, to derive aerosol and cloud particle size, refractive index and shape with polarization accuracy under 1 percent.^[2]

The method is demonstrated on several current-generation satellite sensors beyond HARP2, spanning both wide-swath and narrower hyperspectral implementations, and is applied to climate radiative-forcing and cloud-aerosol-interaction questions rather than a single instrument design.