Characterization of the HARP2 Instrument and its Influences on the Polarimetric Retrieval of Aerosol Particles

Abstract

The HyperAngular Rainbow Polarimeter (HARP) is a small form-factor passive remote sensing imager which can measure the linear Stokes parameters via the “division of amplitude” scheme. Incoming telecentric light is split via an optical prism element into three beams, with each beam having a preferential polarization axis which is preserved through the splitting process in a Pickering (0◦ ,45◦ ,90◦ ) configuration. The front lens interface of HARP is a wide field-of-view (FOV) lens which spans 114◦ along its orbital track, and 94◦ cross-track. There exists AirHARP, HARP CubeSat, HARP2, and AirHARP2 (in chronological order), built via the Earth and Space Institute (ESI) at the University of Maryland Baltimore County (UMBC), all of which share these characteristics as well as others: HARP-class instruments have four spectral channels nominally referred to as blue, green, red, and near-infrared (NIR) which correspond to 440, 550, 665, and 865 nm, respectively. All HARP spectral channels are determined via the spectral “stripe filter” placed at the end of the optical train which divides rows of the light sensitive pixels on the HARP sensor by spectral channel and by “view sector“ corresponding to the angular information of the front lens. In this way HARP contains up to 60 red view sectors, and 20 in the remaining bands with pattern following: green, red, NIR, red, blue, red, green... though HARP retains the ability to selectively turn off individual view sectors to customize this structure via a “line table.” HARP2 in particular is of significance for it being aboard a major NASA mission platform: the Plankton Aerosol Cloud and ocean Ecosystem (PACE) satellite launched in early 2024. Aboard PACE, HARP2 collects full global coverage data of the linear Stokes parameters every two days, and is the first polarimeter to do so in over a decade. Therefore it is of tantamount importance to properly characterize HARP2 and to use that information to determine how best to use HARP2 instrument data for the retrieval of atmospheric aerosols, which remain a major uncertainty in understanding climate change. The thesis herein focuses on the wide FOV characterization of HARP2 and on the retrieval of atmospheric aerosols using an FOV-aware instrument error model. This includes a review of the HARP Cubesat technical demonstration operations and evaluations which led to the experimental design and analysis of the HARP2 calibration across its FOV. Noted are the deviations in polarimetric accuracy as a function of the FOV coordinates found during the calibration campaign performed at Goddard Space Flight Facility (GSFC) in 2022. Informed by these experiments (HARP and HARP2), we evaluate the influence of FOV-dependent error on optimal estimation (OE) aerosol retrievals, as performed by the Generalized Retrieval of Aerosol and Surface Properties (GRASP) retrieval algorithm.