Abstract
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite launched on October 28, 2011 as well as the upcoming Joint Polar Satellite System (JPSS). VIIRS collects radiometric and imagery data of the Earth’s atmosphere, oceans, and land surfaces in 22 spectral bands spanning the visible and infrared spectrum from 0.4 to 12.5 mm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 mm wavelength range (M1 – M11, I1 – I3) range is performed by measuring the sunlight reflectance from Solar Diffuser Assembly (diffuser is Spectralon™). Spectralon™ is known to solarize due to sun UV exposure at the blue end of the spectrum (~0.4 – 0.6+ mm) as seen by laboratory tests as well as on orbit data from MODIS. VIIRS uses a SDSM (Solar Diffuser Stability Monitor) to monitor the change in the Solar Diffuser reflectance in the 0.4 – 0.94 mm wavelength range and provide a correction to the calibration constants. The SDSM measures the ratio of sun light reflecting from the Solar Diffuser to a direct view of the sun. The intensity of the light reaching the SDSM in both Solar Diffuser view and sun view is a function of the sun’s angle of incidence. The error in the Solar Diffuser to sun view in the SDSM ratio due to response due to sun angle of incidence translates into an error in the Solar Diffuser reflectance correction. Thus the SDSM response variation with sun angle of incidence has to be characterized. The sun angle of incidence of variation for the Solar Diffuser view is measured by measuring the BRDF of the Solar Diffuser as a function of angle of incidence. This paper presents the test equipment developed and the results for the complete SDSM relative response to sun angle of incidence in sun view for Flight J1 for JPSS. The relative response to the sun angle of incidence was performed with an uncertainty better than 0.5%. The test required developing a source to simulate the sun including parameters such as a 0.5° extended collimated source with sufficient intensity for adequate signal to noise on the SDSM detectors. The source also needed to have adequate stability to characterize the angular response with acceptable repeatability. Details of the equipment as well as typical results for Flight J1 for JPSS will be presented.
Characterization of JPSS Solar Diffuser Stability Monitor Response to Sun Angle of Incidence
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite launched on October 28, 2011 as well as the upcoming Joint Polar Satellite System (JPSS). VIIRS collects radiometric and imagery data of the Earth’s atmosphere, oceans, and land surfaces in 22 spectral bands spanning the visible and infrared spectrum from 0.4 to 12.5 mm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 mm wavelength range (M1 – M11, I1 – I3) range is performed by measuring the sunlight reflectance from Solar Diffuser Assembly (diffuser is Spectralon™). Spectralon™ is known to solarize due to sun UV exposure at the blue end of the spectrum (~0.4 – 0.6+ mm) as seen by laboratory tests as well as on orbit data from MODIS. VIIRS uses a SDSM (Solar Diffuser Stability Monitor) to monitor the change in the Solar Diffuser reflectance in the 0.4 – 0.94 mm wavelength range and provide a correction to the calibration constants. The SDSM measures the ratio of sun light reflecting from the Solar Diffuser to a direct view of the sun. The intensity of the light reaching the SDSM in both Solar Diffuser view and sun view is a function of the sun’s angle of incidence. The error in the Solar Diffuser to sun view in the SDSM ratio due to response due to sun angle of incidence translates into an error in the Solar Diffuser reflectance correction. Thus the SDSM response variation with sun angle of incidence has to be characterized. The sun angle of incidence of variation for the Solar Diffuser view is measured by measuring the BRDF of the Solar Diffuser as a function of angle of incidence. This paper presents the test equipment developed and the results for the complete SDSM relative response to sun angle of incidence in sun view for Flight J1 for JPSS. The relative response to the sun angle of incidence was performed with an uncertainty better than 0.5%. The test required developing a source to simulate the sun including parameters such as a 0.5° extended collimated source with sufficient intensity for adequate signal to noise on the SDSM detectors. The source also needed to have adequate stability to characterize the angular response with acceptable repeatability. Details of the equipment as well as typical results for Flight J1 for JPSS will be presented.