The Research Scanning Polarimeter: Calibration and Ground-based Measurements Brian Cairns a , Edgar E. Russell b and Larry D. Travis c a Department of Aplied Physics and Applied Mathematics, Columbia University, New York b SpecTIR Corporation, Santa Barbara, CA. c NASA Goddard Institute for Space Studies, New York ABSTRACT SpecTIR Corporation has recently completed building the Research Scanning Polarimeter (RSP). This instrument was designed to provide highly accurate polarimetric measurements both from aircraft and from the ground. The spectral range of the measurements is from 410nm to 2250nm and the field of view of the instrument is scanned over a 120° swath (+/-60° from nadir/zenith). Here we describe the results of the instrumental calibration and the quantitative interpretation of ground-based measurements. Recently we have acquired data using the RSP on an aircraft and a brief discussion of the information content of this data and some preliminary aerosol retrievals over the Pacific ocean are presented. 1. INSTRUMENT DESCRIPTION The RSP instrument uses a polarization compensated scan mirror assembly to scan the fields of view of six boresighted, refractive telescopes through +/-60° from the normal with respect to the instrument baseplate. The refractive telescopes are paired, with each pair making measurements in three spectral bands. One telescope in each pair makes simultaneous measurements of the linear polarization components of the intensity in orthogonal planes at 0° and 90° to the meridional plane of the instrument, while the other telescope simultaneously measures equivalent intensities in orthogonal planes at 45° and 135°. This approach ensures that the polarization signal is not contaminated by scene intensity variations during the course of the polarization measurements, which could create false polarization. These measurements in each instantaneous field of view in a scan provide the simultaneous determination of the intensity, and the degree and azimuth of linear polarization in all nine spectral bands. The instrument has nine spectral channels that are divided into two groups based on the type of detector used: visible/near infrared (VNIR) bands at 410 (30), 470 (20), 550 (20), 670 (20), 865 (20) and 960 (20) nm and shortwave infrared (SWIR) bands at 1590 (60), 1880 (90), and 2250 (120) nm. The parenthetic figures are the full width at half maximum (FWHM) bandwidths of the spectral bands. These spectral bands sample the spectrum of reflected solar radiation over most of the radiatively significant range, with measurements under typical clear sky conditions ranging from significant Rayleigh scattering (410nm) to single scattering by aerosol (2250nm) within a single measurement set. The desired polarization-insensitive scanning function of the RSP is achieved by the use of a two-mirror system with the mirrors oriented such that any polarization introduced at the first reflection is compensated for by the second reflection. Boresighted refractive telescopes define the 14mrad field of view of the RSP. Dichroic beam splitters are used for spectral selection, interference filters define the spectral bandpasses and Wollaston prisms spatially separate the orthogonal polarizations onto the pairs of detectors. The detectors for the VNIR wavelengths are pairs of UV-enhanced silicon photodiodes. The detectors for the SWIR wavelengths are pairs of HgCdTe photodiodes with a 2.5 μm cutoff cooled to 163K. With the exception of the dual preamplifiers located near each detector pair, the RSP electronics is contained within three stacked, interconnected modules. The electronics provides the amplification of the signals detected by the 36 detector channels, sampling and 14-bit analog-to-digital conversion of the resultant signals, the servo control of the scanner rotation and temperature of the SWIR detectors, and the control logic that formats the instrument signal and housekeeping data and supports transmission of the digital data to a personal computer for storage. A liquid nitrogen dewar is used to cool the SWIR detectors during both ground and airborne operation. To optimize the performance of the SWIR channels the temperature of the detectors is servo controlled at 163K during operation. Digital data from 152 scene sectors (IFOVs) over 121 degrees of scan, dark samples from 10 sectors and instrument status data are formatted by the RSP electronics and transmitted each scan to a personal computer for storage. The average data rate of 110kbps provides readout of the 36 signal channels together with instrument status data at a scan rate of 71.3 rpm. This scan rate results in an IFOV dwell time of