The Hyperspectral Polarimeter for Aerosol Retrievals (HySPAR): a new instrument for passive remote sensing of aerosols

Aerodyne Research has developed and built a snap-shot imaging spectropolarimeter for aerosol retrievals, based on our patented polarimetric spectral intensity modulation (PSIM) technique. The resulting sensor, the Hyperspectral Polarimeter for Aerosol Retrievals (HySPAR), is capable of capturing, in a single snapshot, a full elliptical Stokes spectrum over 450-900 nm and over a 120 degree field of view. The single beam, single snapshot optical design possesses inherent spectral and polarimetric channel coregistration. HySPAR is equally suitable for ground-based uplooking measurements as well as downlooking measurements from an airborne platform. In this paper, we describe the principle of operation, present the performance specifications, give an overview of the instrument design, and present validation results. Validation results compare HySPAR measurements to sources of known polarization states. In many cases, HySPAR achieves better than 1% degree of polarization accuracy with further improvements planned. We present reflection measurements from a titanium plate which demonstrates HySPAR's circular polarization measurement capability. This is of particular relevance to enhanced parameter retrievals for non-spherical and optically thick aerosols. A principal advantage of the HySPAR sensor is that it is able to capture a full Stokes (i.e. 4 component) spectrum over a 120 degree swath of the solar principal plane in a single snapshot of the order of 1 second, thus eliminating the possibility of moving clouds contaminating the polarimetry. Many conventional polarimeters compute the polarimetry by assembling multiple time separated images and are thus quite susceptible to yielding incorrect results for such a time varying scene.