The Landsat-8 Thermal Infrared Sensor (TIRS) has been acquiring two-band thermal infrared images of the Earth’s surface since 2013. The calibration of the two-band system has been monitored using the on-board calibrator and validated with vicarious calibration performed by NASA/Jet Propulsion Laboratory and Rochester Institute of Technology since launch. An update to the radiometric calibration was introduced into the Collection-2 processing system in late 2020 to correct for signal-dependent and time-dependent calibration errors. In November 2020, the Landsat-8 spacecraft experienced two safeholds and, while TIRS seemingly recovered nominally, there were slowly developing changes as a result. By December 2020, the TIRS Band 11 responsivity had decreased by as much as 2%. It was determined that a contaminant has been slowly depositing on a component in the optical path since the safehold and continues as of this writing. As of late June 2021, the responsivity is still decreasing in both spectral bands; Band 11 band-average responsivity has dropped by 3.7% and Band 10 band-average responsivity has dropped by 2.0%, though the decrease in responsivity is not uniform across the focal plane. Since March 2021, the TIRS products have been processed with calibration gains that account for the changing responsivity.
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