In-flight validation of SPEX airborne spectro-polarimeter onboard NASA's research aircraft ER-2

High accuracy multi-angle polarimetry is of crucial importance for remote sensing of aerosol and cloud properties with accuracies demanded by climate and air quality studies. In this contribution, we provide an detailed description of the multi-angle spectro-polarimetric instrument “SPEX airborne” that was developed to operate from NASA’s high altitude research aircraft ER-2. SPEX airborne delivers measurements of radiance and linear polarization at nine fixed viewports with angles equally distributed over at total angular range of 112°, at visual wavelength in the range 400-760nm. Each viewport acts as a pushbroom spectrometer with a swath of 6°. SPEX airborne participated in the recent the ACEPOL campaign in October-November 2017 when it flew together with NASA’s Research Scanning Polarimeter (RSP), the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI), and the Airborne Hyper-Angular Rainbow Polarimeter (AirHARP). We compare polarimetric and radiometric measurements from SPEX airborne with those collected by RSP at four visible wavelength bands. Simultaneous measurements were made while flying over widely different scenes, under different illumination and meteorological conditions. This provided a large dynamic range in radiometric and polarimetric values. We find that the Degree of Linear Polarization (DoLP) measured by both instruments agrees well with a RMS differences of ~0.005 as the best result for 555nm. For radiance measurements excellent agreement is obtained with a RMS difference of ~4%. The in-flight comparison results provide verification of SPEX airborne’s capability to deliver high-quality data.