JPSS-1 VIIRS solar diffuser stability monitor response versus sun angle of incidence

Abstract

The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite in orbit as well as for the upcoming Joint Polar Satellite System (JPSS). VIIRS collects Earth radiometry and imagery in 22 spectral from 0.4 to 12.5 μm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 μm wavelength 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+ μm) as seen by laboratory tests as well as on orbit data from MODIS and NPP. VIIRS uses a Solar Diffuser Stability Monitor (SDSM) to monitor the change in the Solar Diffuser reflectance in the 0.4 – 0.94 μm wavelength range to correct the calibration constants. The SDSM measures the ratio of sun light reflecting from the Solar Diffuser to a direct view of the sun. As 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 (AOI), the SDSM response to sun AOI has to be characterized. This paper presents details of the test setup including an extended collimated source simulating the sun across all SDSM bands. The prelaunch characterization results for the JPSS-1 (J1) VIIRS SDSM are presented. Comparison with NPP on orbit yaw maneuver SDSM results shows similar behavior demonstrating that the J1 test successfully characterized the SDSM response to sun AOI.