SPEX airborne spectropolarimeter calibration and performance
2019
To improve our understanding of the complex role of aerosols in the climate system and on air quality, measurements are needed of optical and microphysical aerosol. From many studies, it has become evident that a satellite-based multiangle, multiwavelength
polarimeterwill be essential to provide such measurements. Here, high accuracy (∼0.003) on the degree of linear polarization (DoLP) measurements is important to retrieve aerosol properties with an accuracy needed to advance our understanding of the aerosol effect on climate. SPEX airborne, a multiangle hyperspectral
polarimeter, has been developed for observing and characterizing aerosols from NASA’s
high-altitude researchaircraft ER-2. It delivers measurements of
radianceand DoLP at visual wavelengths with a spectral resolution of 3 and 7–30 nm, respectively, for
radianceand polarization, at nine fixed
equidistantviewing angles from −56° to +56° oriented along the
ground track, and a swath of 7° oriented across-track. SPEX airborne uses spectral
polarization modulationto determine the state of linear polarization of scattered sunlight. This technique has been developed in the Netherlands and has been demonstrated with ground-based instruments. SPEX airborne serves as a demonstrator for a family of space-based SPEX instruments that have the ability to measure and characterize atmospheric aerosol by multiangle hyperspectral polarimetric imaging remotely from a satellite platform. SPEX airborne was calibrated radiometrically and polarimetrically using
Jet PropulsionLaboratory (JPL) facilities including the Polarization Stage Generator-2 (PSG-2), which is designed for polarimetric calibration and validation of the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI). Using the PSG-2, the accuracy of the SPEX airborne DoLP measurements in the laboratory setup is found to be 0.002–0.004.
Radiometric calibrationis realized with an estimated accuracy of 4%. In 2017, SPEX airborne took part in the “Aerosol Characterization from
Polarimetersand Lidar” campaign on the ER-2 that included four
polarimetersand two lidars. Polarization measurements of SPEX airborne and the coflying Research Scanning
Polarimeter(RSP), recorded during the campaign, were compared and display root-mean-square (RMS) differences ranging from 0.004 (at 555 nm) up to 0.02 (at 410 nm). For
radiancemeasurements, excellent agreement between SPEX airborne and RSP is obtained with an RMS difference of ∼4%. The lab- and flight-performance values for polarization are similar to those recently published for AirMSPI, where also an intercomparison with RSP was made using data from field campaigns in 2013. The intercomparison of radiometric and polarimetric data both display negligible bias. The in-flight comparison results provide verification of SPEX airborne’s capability to deliver high-quality data.
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