Pre-launch calibration and validation of the Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) instrument

The Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) is a new imaging polarimeter instrument, capable of sampling a single Earth target from up to 120 viewing angles, in four spectral channels, and three linear polarization states across a 114° field of view. AirHARP is telecentric in the image space and simultaneously images three linear polarization states with no moving parts. These two aspects of the design allow for a simple and efficient quantitative calibration. Using coefficients derived at the center of the lens and the detector flatfields, we can calibrate the entire AirHARP sensor in a variety of lab, field, and space environments. We describe the calibration process for the HARP family of polarimeters using AirHARP pre-launch data. We show that this telecentric calibration technique yields a 0.25 % RMS degree of linear polarization (DOLP) accuracy in all channels for pixels around the detector center. To validate across the FOV, we compare our multi-angle reflectance and polarization data with the Research Scanning Polarimeter (RSP) over targets sampled during the NASA Aerosol Characterization from Polarimeter and Lidar (ACEPOL) campaign. For the majority of angles and targets used in the intercomparison, RSP and AirHARP agree better than 1 % in reflectance and DOLP. Therefore, our calibration successfully transfers nadir coefficients to different FOVs, given ambient challenges. This calibration technique makes the HARP design attractive for new spaceborne climate missions: HARP CubeSat (2020–2022), HARP2 (2024–) on the NASA Plankton-Aerosol-Cloud-ocean Ecosystem (PACE), Atmosphere Observing System (AOS) and beyond.