The First Year of HARP2/PACE Performance Based on its Ground and On-Orbit Characterization

Abstract

The Hyper-Angular Rainbow Polarimeter-2 (HARP2) was launched on board the Plankton, Aerosol, Cloud and ocean Ecosystem (PACE) mission, in February 2024, for the global measurement of aerosol and cloud properties as well as to provide atmospheric correction over the footprint of the Ocean Color Instrument (OCI). HARP2 is designed to collect data over a wide field of view in the cross-track direction (+/-47deg) allowing for global coverage in about two days, as well as an even wider field of view in the along-track direction (+/-54 deg) providing measurements over a wide range of scattering angles. HARP2 samples 10 angles at 440, 550, and 870nm focusing on aerosol and surface retrievals, and up to 60 angles at 670nm for the hyper-angular retrieval of cloud microphysical properties. The HARP2 instrument collects three nearly identical images with linear polarizers aligned at 0°, 45°, and 90° that can be converted to push-broom images of the I, Q, and U Stokes parameters for each angle, and each wavelength. The HARP2 technology was first demonstrated with the HARP CubeSat satellite which collected a limited dataset for 2 years from 2020 to 2022. HARP2 extends these measurements to a full global coverage in two days, seven days a week.This talk will show a brief description of the instrument and will focus on the calibration and characterization of the HARP2 instrument at the ground and on-orbit. The calibration and characterization of such a wide FOV instrument with polarization sensitivity produces particular challenges that are not common on other classes of instrument with narrower FOV. HARP2 was characterized at the ground from the spectral, radiometric, and polarimetric perspectives, as function of temperature, and aiming for high accuracy across the whole field of view of the instrument. Multiple techniques have been developed to address this characterization, which has resulted in about 1TByte of calibration data.Once in orbit, the HARP2 team has also developed special strategies for the continued monitoring of the calibration parameters as well as for tracking the absolute calibration of the system. These strategies include lunar and solar calibration, flat field, dark current monitoring, vicarious calibrations looking at well know ground target, intercomparison between sensors, etc. Results from the ground and on-orbit calibration will be presented and discussed in detail.