Polarimetric calibration of a spectro-polarimeter for remote sensing and characterization of aerosols

High accuracy multi-angle polarimetry is of crucial importance for remote sensing of aerosol properties with accuracies demanded by climate and air quality studies. In this contribution, we discuss the polarimetric calibration of the multi-angle polarimeter "SPEX airborne". SPEX airborne is a multi-angle viewing instrument providing snapshot measurements of spectral radiance and degree of linear polarization at fixed viewing angles. Radiance and polarization are measured as a continuous function of wavelength in the 400-760nm range, at nine viewing angles equally distributed over an angular range of -56° to +56°. Each viewing-angle aperture has a swath of 7° with an instantaneous field of view of 0.5°l° (cross- times along-track). SPEX airborne measures the degree and angle of linear polarization of scattered sunlight by means of spectral modulation. For each field of view, the instrument records two modulated spectra. Ideally, these are perfectly in anti-phase, such that the sum of the modulated spectra is modulation free and gives the spectral radiance. The state of linear polarization is derived from the scaled difference of the modulated spectra. As a result of finite image quality at the focal plane, any spectropolarimeter using spectral modulation will show different polarimetric responses for the two modulated spectra, which breaks the anti-phase symmetry. For SPEX airborne this is indeed the case, and special care is taken both in the calibration and in the data processing. Ignoring this can lead to errors both in polarimetric and radiometric measurements. It is shown however that these errors are quite small. In this contribution, we will outline the spectral modulation technique employed by SPEX airborne to measure the state of linear polarization, explain which instrumental parameters are to be determined by calibration and how they feature in the data processing chain. We discuss the polarization calibration setup and how polarization calibration measurements are processed into useful calibration data. Taking a Mueller matrix approach, we show how differences in polarimetric responses can be mitigated, while also other imperfections like telescope polarization are accounted for without extra calibration effort. We also present a scheme to correct for sharp features in the solar spectrum, which would otherwise mix into the modulation spectra via the finite slit-width of the spectrometer and result in polarimetric errors. The effect of telescope polarization is also discussed Polarimetric calibration of SPEX airborne is relevant for SPEXone, a compact multi-angle polarimeter that builds on SPEX airborne heritage and space-borne spectrometer heritage within the Netherlands. SPEXone has been developed for NASA's PACE mission, which has a planned launch date in 2023.