Multi-angle polarimeter inter-comparison: the PODEX and ACEPOL field campaigns

A multi-angle polarimeter has been proposed for the NASA Aerosol-Cloud-Ecosystem (ACE) mission, recommended by the National Research Council's Decadal Survey. Such instruments are uncommon in orbit, and there is a great diversity of prototype instrument characteristics. For that reason, NASA funded two field campaigns where airborne polarimeter prototypes were deployed on the high altitude ER-2 aircraft. The first field campaign, POlarimeter DEfinition EXperiment (PODEX), was carried out in southern California in early 2013. Three polarimeters participated: the Airborne Multi-angle SpectroPolarimeter Imager (AirMSPI), the Passive Aerosol and Cloud Suite (PACS) and the Research Scanning Polarimeter (RSP). PACS, on its first deployment, suffered detector problems, while AirMSPI and RSP performed within expectations. Initial comparisons of AirMSPI and RSP observations found Degree of Linear Polarization (DoLP) biases. Following corrections to both instrument's calibration and/or geolocation techniques, these issues have improved. We will present the details of this comparison. The recent ACEPOL mission returned to southern California in October-November with a larger compliment of multi-angle polarimeters. This included AirMSPI and RSP, like in PODEX. Additional polarimetric instruments included AirHARP (Airborne HyperAngular Rainbow Polarimeter, a successor to PACS) and SPEX Airborne (SPectropolarimeter for Planetary Exploration). Two Lidars were also deployed: The High Spectral Resolution Lidar -2 (HSRL-2) and the Cloud Physics Lidar (CPL). While data processing is still underway, we will describe the objectives of this campaign and give a preview of what to expect in subsequent analysis.