A multipurpose scanning spectral polarimeter (SSP): Instrument descriptionand sample results

This paper describes the design and characteristics of a scanning spectral polarimeter designed to measure spectral radiances and fluxes in the range between 0.4 and 4.0 mu m. The instrument characteristics are described, and the procedures to calibrate the unpolarized radiances and fluxes in the sp ectral range from 0.4 to 1.1 mu m are discussed along with detailed error a nalyses of this calibration. The accuracy of the radiance calibration was d etermined to be approximately 3%. The calibration of fluxes based on two di fferent procedures is estimated to be accurate to 3%-6%. Detailed calibrati on of fluxes was performed using a standard lamp that is not an isotropic s ource. For this type of calibration the angular response of the flux channe l deviates from a pure cosine function at wavelengths longer than 0.74 mu m , thus forming a frontal lobe. A less detailed calibration using a 40-in. i ntegrated sphere was also performed. In that case the light source is isotr opic and the frontal lobe does not appear. Calibration factors are derived by combining data from bath calibration procedures. A comparison with spect ral flux measurements obtained from different instruments with different an gular response properties is presented with agreement that is within the qu oted calibration accuracy. Measurements obtained from two different aircraf t flights are presented to illustrate the types of application of the data. Data analyses shows that the reflected (unpolarized) fluxes measured above a cirrus cloud can be reasonably matched to modeled fluxes using optical p roperties retrieved from the measured (unpolarized) radiances.