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Pre-launch characterization of VIIRS straylight

The Visible Infrared Imaging Radiometer Suite (VIIRS) plays an important role in Earth observations and climate studies. Multiple VIIRS instruments have been built, including one onboard the Suomi-NPP spacecraft and another onboard the NOAA-20 spacecraft. These instruments have been extensively tested pre-launch in ambient environment and in a thermal vacuum chamber. One of the important tests in the ambient environment provides the characterization of the straylight response of the instrument. The straylight rejection requirement states that for the spacecraft in an operational, nadir-facing attitude, the VIIRS sensor response to any straylight striking the sensor on any surface (except the entrance aperture and within the sensor field of view) from any angle shall be less than 1% of the response to the given typical spectral radiances. It is challenging to replicate the straylight from the operational environment in a clean room; therefore, some modeling plus a special laboratory setup is necessary. The straylight test is briefly summarized and the test results from five VIIRS instruments built in the past 15 years are compared. It is shown that the straylight performance remained consistent among the VIIRS instruments and they meet the requirement by a healthy margin at the beginning of life, which indicates expected low levels of straylight on-orbit.